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The  D.  Van  NoSlrand  Company 

intend  this  book  to  be  sold  to  the  Public 
at  the  advertised  price,  and  supply  1 
the  Trade  on  terms  which  will  not  allow 
of  reduction. 


A   TEXT-BOOK   OF 

QUANTITATIVE 
CHEMICAL    ANALYSIS 

BY 

GRAVIMETRIC,  ELECTROLYTIC,  VOLUMETRIC 
AND  GASOMETRIC  METHODS 

WITH  SEVENTY-FOUR  LABORATORY  EXERCISES 

GIVING 

THE  ANALYSIS    OF  PURE    SALTS,   ALLOYS,   MINERALS 
AND  TECHNICAL  PRODUCTS 

BY 

J.  C.    OLSEN,   A.M.,   PH.D. 

Professor  of  Analytical  Chemistry  in  the  Polytechnic  Institute  of  Brooklyn 

formerly  Fellow  of  the  Johns  Hopkins  University 
Member  American  Institute  of  Chemical  Engineers 

FOURTH  EDITION,  REVISED  AND  ENLARGED 


NEW    YORK 

D.   VAN   NOSTRAND   COMPANY 

25  PARK  PLACE 
1910. 


Engineering 
Library 


Copyright,  1908 

BY 

D.  VAN  NOSTRAiND  COMPANY 


CAMBLOT    PRESS,    l8-2O    OAK  STREET,    NEW   YORK 


PEEFACE. 


IN  writing  the  present  book  the  author  has  endeavored  in  the 
first  place  to  produce  a  text-book  on  Quantitative  Analysis  which 
shall  meet  his  own  needs  in  presenting  the  subject  to  his  students. 
The  text-books  available  did  not  give  as  thorough  and  at  the 
same  time  as  comprehensive  a  view  of  the  subject  as  seemed 
desirable.  In  order  to  present  the  subject  from  the  theoretical 
as  well  as  from  the  practical  standpoint,  reference  by  the  student 
to  a  very  considerable  number  of  text-books  and  journals  seemed 
necessary.  This  was  largely  due  to  the  fact  that  each  author 
has  given  special  prominence  to  a  particular  branch  of  the  subject, 
such  as  gravimetric,  electrolytic,  volumetric,  or  gas  analysis.  In 
the  present  text-book  the  endeavor  has  been  made  to  accord  each 
of  these  subjects  the  relative  prominence  which  is  justified  by 
the  extent  to  which  the  methods  concerned  are  actually  used. 
Obsolete  methods  and  new  methods  which  have  not  come  into 
general  use  have  generally  been  excluded. 

In  the  arrangement  and  presentation  of  the  subject-matter  the 
needs  of  the  student  rather  than  the  experienced  analyst  have 
been  kept  continually  in  view  The  needs  of  the  student  have 
been  taken  to  be  the  acquisition  of  a  thorough  comprehension 
of  the  reasons  for  each  step  in  an  analysis  as  well  as  the  develop- 
ment of  the  skill  in  manipulation  which  is  necessary  in  rapid  and 
accurate  work.  It  is  believed  that  by  this  method  the  require- 
ments of  the  professional  chemist  will  also  be  best  served  when 
a  reference  book  is  needed. 

The  order  in  which  the  various  general  methods  are  taken  up 
is  that  used  by  the  author  in  instructing  his  classes.  The  instruc- 
tion on  the  manipulation  of  the  balance  and  general  operations 

iii 

258752 


iv  PREFACE. 

is  first  given.  Then  the  determination  of  metals  and  acids  in 
pure  salts  is  taken  up,  each  student  making  one  determination 
under  each  of  the  general  methods  given.  Instruction  is  then 
given  in  the  separation  of  the  elements  in  connection  with  the 
analysis  of  alloys  and  minerals  The  list  of  alloys  and  minerals 
whose  analyses  are  given  was  selected  with  the  object  of  affording 
practice  in  the  separation  and  determination  of  the  metals  and 
acids  which  are  commonly  found  associated  together.  It  was 
thought  undesirable  to  spend  much  time  studying  the  separation 
of  elements  which  rarely  occur  together  either  in  nature  or  in 
the  arts. 

Although  the  chapters  on  Electrolytic  and  Volumetric  Methods 
follow  those  on  the  analysis  of  minerals  and  alloys,  electrolytic 
and  volumetric  methods  have  been  given  in  these  analyses  for 
the  determination  of  individual  elements  when  these  methods 
are  decidedly  better  than  the  gravimetric  methods.  Such  analyses 
are  most  advantageously  made  while  studying  electrolytic  or 
volumetric  methods. 

The  analyses  of  iron  and  steel,  coal,  water,  fats,  and  oils  are 
given  at  the  end  of  the  course  to  afford  practice  in  general  ana- 
lytical work  and  also  to  illustrate  methods  of  determining  elements 
present  in  very  small  amounts. 

The  chapter  on  Stoichiometry  was  written  to  help  overcome 
the  shortcoming  so  often  met  with  in  otherwise  excellent  chemists, 
namely,  the  inability  to  make  the  necessary  analytical  calcula- 
tions rapidly  and  accurately.  The  author  realizes  that  only  a 
large  amount  of  practice  will  give  the  desired  facility,  and  there- 
fore would  urge  the  student  as  well  as  the  instructor  to  pass  over 
no  problem  which  is  not  thoroughly  understood.  Most  of  the 
methods  of  calculation  have  been  given  through  the  book  in  the 
exercises  to  which  they  apply 

Free  use  has  been  made  of  the  text-books  published  on  the 
various  branches  of  analytical  chemistry.  The  author  has  aimed 
to  acknowledge  in  each  case  the  source  from  which  methods 
liave  been  obtained. 

The  author  takes  great  pleasure  in  acknowledging  the  obliga- 
tions he  is  under  to  those  who  have  so  cheerfully  assisted  him  in 
preparing  this  work.  He  is  especially  grateful  to  Dr.  A.  C.  Lang- 


PREFACE.  V 

muir,  who  has  read  the  entire  manuscript  as  well  as  proof,  and  who 
has  made  numerous  valuable  suggestions  at  all  stages  of  the 
work.  The  author  is  also  under  special  obligation  to  Mr.  Albert 
Seeker,  who  read  the  entire  proof  with  great  care  and  calculated 
the  factors  given  in  Table  II,  page  470.  To  Professors  I.  W.  Fay 
and  Samuel  Sheldon  of  the  Polytechnic  Institute,  and  to  Professor 
Chas.  M.  Allen  of  the  Pratt  Institute,  as  well  as  to  Messrs.  Geo.  C. 
Whipple,  Lincoln  Burrows,  and  Geo.  M.  S.  Neustadt,  the  author 
is  under  obligations  for  suggestions  and  corrections  on  portions 
of  the  book. 

The  drawings  for  the  cuts  were  made  under  the  supervision 
of  Professor  Constantine  Hertzberg  of  this  Institute  by  Messrs. 
Walter  Rapelje,  C.  A.  Benoit,  and  Albert  Seeker,  to  whom  the 
author  wishes  to  express  his  obligation.  Several  cuts  were  kindly 
loaned  by  Messrs.  Eimer  &  Amend  and  by  the  Edison  Manu- 
facturing Company. 

J.  0.  OLSEN. 

POLYTECHNIC  INSTITUTE  OF  BROOKLYN, 
May,  1904. 


PREFACE   TO   THIED   EDITION. 


Two  editions  of  this  text-book  having  been  exhausted  since 
its  publication,  four  years  ago,  the  author  has  taken  advantage  of 
the  printing  of  the  third  edition,  to  make  such  corrections  and 
additions  as  his  own  experience  and  that  of  a  number  of  its  users, 
who  have  been  kind  enough  to  suggest  improvements,  has  shown 
to  be  advisable.  The  author  has  endeavored  in  this  revision  to 
adhere  to  the  original  plan  of  the  work,  namely,  to  describe  no 
method  which  has  not  been  thoroughly  tried  in  his  own  as  well  as 
other  laboratories  and  found  to  be  satisfactory,  believing  that  only 
after  continued  use  by  many  workers  are  all  the  conditions  neces- 
sary to  the  successful  carrying  out  of  a  method  discovered,  so  that 
it  can  be  accurately  described. 

The  new  methods  which  have  been  introduced  in  this  edition 
are  principally  the  following.  Penfield's  method  of  determining 
water  has  been  described  because  it  is  believed  that  this  simple 
and  accurate  method  should  be  more  widely  known.  The  precipi- 
tation of  nitric  acid  by  means  of  nitron  is  important  as  the  only 
gravimetric  method  for  determining  this  common  acid.  The  rota- 
tion of  the  anode  or  cathode  in  electrolytic  determinations  has 
proved  so  advantageous  that  its  use  is  fully  described,  as  well  as 
the  use  of  electric  and  water  motors  for  this  purpose.  The 
methods  for  analyzing  water  have  been  revised  and  brought  into 
better  accord  with  the  standard  methods  adopted  by  the  Ameri- 
can Public  Health  Association.  The  simple  bacteria  count  in 
water  and  the  test  for  coli  have  also  been  described,  as  the  prep- 
aration of  the  culture  media  requires  considerable  chemical  skill. 
A  description  of  various  methods  of  determining  specific  gravity 
as  well  as  the  determination  of  the  calorific  value  of  fuels  has  also 

vii 


viii  PREFACE. 

been  introduced,  since  chemists  are  so  often  called  upon  to  make 
such  determinations.  A  number  of  typical  chemical  problems 
have  been  introduced  throughout  the  book  in  order  to  develop 
the  ability  to  make  chemical  calculations,  which  seems  so  difficult 
for  many  students  to  acquire. 

A  number  of  less  important  additions  have  been  made  through- 
out the  book,  among  which  may  be  mentioned :  Rules  for  using 
and  cleaning  platinum  ware;  volumetric  methods  for  determining 
antimony,  copper,  and  testing  shellac  for  rosin.  The  method  of 
determining  the  Polenske  value,  the  flash  point,  and  the  viscosity 
of  fats  and  oils,  as  well  as  the  separation  of  mineral  from  animal 
and  vegetable  fats  and  oils.  All  atomic  and  molecular  weights  as 
well  as  the  table  of  factors  have  been  recalculated  by  the  1908 
table  of  atomic  weights.  The  description  of  methods  has  been 
improved  wherever  experience  has  shown  the  necessity. 

While  the  author  has  given  the  precautions  necessary  to 
obtain  correct  results  with  all  methods  described,  he  has  endeav- 
ored to  avoid  producing  a  book  which  can  be  followed  mechanic- 
ally with  the  production  of  good  analytical  results,  but  poor  chem- 
ists. In  writing  the  exercises  he  has  assumed  that  the  student 
has  studied  the  text-book  and  knows  the  reason  for  each  step  in 
an  analysis.  The  author  is  of  the  opinion  that  the  student  can- 
not successfully  perform  these  exercises  unless  he  has  carefully 
studied  and  understood  the  text.  It  is  undoubtedly  for  this  rea- 
son that  the  book  has  not  been  found  adapted  for  use  in  large 
laboratory  classes  where  a  sufficient  number  of  competent  instruc- 
tors has  not  been  provided. 

The  author  has  been  very  much  encouraged  by  the  favorable 
reception .  accorded  the  book  in  the  past  and  hopes  for  a  still 
larger  field  of  usefulness  in  the  future.  He  takes  this  opportunity 
of  thanking  all  those  who  have  been  kind  enough  to  point  out 
errors  or  suggest  improvements.  He  desires  especially  to  express 
his  obligation  to  Mr.  Albert  Seeker  for  the  revision  of  the  chapter 
on  oils  and  fats  and  to  Mr.  Andrew  Mayer,  Jr.  chemist  with  the 
National  Lead  Co.  for  the  revision  of  the  chapters  on  the  analysis 
of  alloys. 

T.  C.  OLSEN. 

POLYTECHNIC  INSTITUTE  OF  BROOKLYN, 
September  4,  1908. 


CONTENTS. 


INTRODUCTION. 

PAGE 

Definition  and  Comparison  of  Gravimetric  and  Volumetric  Methods 1 

Personal  Qualities  Essential  to  the  Analytical  Chemist 2 

Limit  of  Accuracy 4 

Utility  and  Importance  of  Quantitative  Analysis 6 

CHAPTER  I. 
THE  BALANCE. 

Construction 7 

Weighing 7 

Determination  of  the  Sensibility  of  a  Balance  (Ex.  1) 15 

Determination  of  the  Relative  Length  of  the  Arms  of  a  Balance  (Ex.  2). ...  16 

Calibration  of  Weights  (Ex.  3) 16 

Weighing  Substances  Containing  Water 19 

Rules  to  be  Observed  in  Using  the  Balance 20 

CHAPTER  II. 
GENERAL  OPERATIONS. 

Theory  of  Precipitation 21 

Filtration 24 

Washing  Precipitates 26 

Preparation  of  Pure  Salts 26 

(a)  By  Recrystallization 27 

(6)  Precipitation  by  Change  of  Solvent 30 

(c)  Precipitation  by  Double  Decomposition 30 

Table  of  Compounds  of  the  Metals  and  Acids  which  can  be  Prepared  in  Pure 

Condition 31 

Preparation  of  Pure  Copper  Sulphate  (Ex.  4) 36 

Preparation  of  Potassium  Magnesium  Sulphate  (Ex.  5) 36 

Preparation  of  Pure  Sodium  Chloride  (Ex.  6) 37 

ix 


x  CONTENTS. 

CHAPTER  III. 
DETERMINATION  OF  WATER. 

PAGE 

By  Loss  in  Weight 39 

Determination  of  Water  of  Crystallization  in  Copper  Sulphate  (Ex.  7) 41 

Determination  of  Water  of  Crystallization  in  Barium  Chloride  (Ex.  8) 43 

Penfield's  Method  of  Determining  Water 43 

Absorption  and  Weighing  of  Water 44 

Drying  Properties  of  Fused  Calcium  Chloride 45 

Drying  Properties  of  Concentrated  Sulphuric  Acid 46 

Drying  Properties  of  Phosphorus  Pentoxide 46 

Determination  of  Water  of  Crystallization  in  Magnesium  Sulphate  (Ex.  9).  47 


DETERMINATION   OF  METALS. 

CHAPTER  IV. 
DETERMINATION  OF  METALS  AS  OXIDE. 

General  Properties  of  the  Oxides  of  the  Metals 50 

Precipitation  of  Iron,  Aluminium,  and  Chromium  by  Ammonia  and  Ammo- 
nium Chloride 51 

Contamination  and  Determination  of  Silica  in  the  Precipitates 53 

Determination  of  Aluminium  in  Potash  Alum  (Ex.  10) 54 

Determination  of  Iron  in  Soft-Iron  Wire  (Ex.  11) 57 

Determination  of  Copper,  Manganese,  Nickel,  and  Cobalt  by  Precipitation 

with  Caustic  Alkali 60 

Determination  of  Copper  in  Crystallized  Copper  Sulphate  (Ex.  12) 62 

Determination  of  Nickel  in  Nickel  Ammonium  Sulphate  (Ex.  13) 62 

CHAPTER  V. 
DETERMINATION  OF  METALS  AS  OXIDE.     (Continued.-) 

Precipitation  of  Lead,  Bismuth,  Calcium,  Barium,  and  Strontium  by  Ammo- 
nium Carbonate 64 

Determination  of  Strontium  in  Strontium  Carbonate  (Ex.  14) 66 

Precipitation  of  Calcium 66 

Determination  of  Calcium  as  Oxalate  in  Calcium  Carbonate  (Ex.  15) 67 

Determination  of  Zinc,  Manganese,  and  Cadmium  by  Precipitation  with 

Sodium  Carbonate 68 

Determination  of  Zinc  in  Zinc  Ammonium  Sulphate  (Ex.  16) 70 

Ignition  of  Salts  of  Volatile  Inorganic  Acids 70 

Ignition  of  Salts  of  Organic  Acids 71 

Determination  of  Lead  by  Ignition  of  Lead  Nitrate  (Ex.  17) 72 


CONTENTS.  xi 

CHAPTER  VI. 
DETERMINATION  OF  METALS  AS  SULPHATE  AND  AS  SULPHIDE. 

PAGE 

Precipitation  of  Barium,  Strontium,  Calcium,  and  Lead  as  Sulphate 73 

Determination  of  Barium  in  Barium  Chloride  (Ex.  18) 75 

Determination  of  Metals  by  Evaporation  with  Sulphuric  Acid 76 

Determination  of  Magnesium  in  Magnesium  Carbonate  (Ex.  19) 77 

Determination  of  Metals  as  Sulphide 78 

Determination  of  Copper,  Mercury,  Lead,  Cadmium,  Silver,  Arsenic,  and 

Tin  by  Precipitation  with  Hydrogen  Sulphide 78 

Removal  of  Free  Sulphur  and  Drying  at  100° 79 

Determination  of  Copper  in  Copper  Sulphate  (Ex.  20) 80 

Determination  of  Arsenic  in  Arsenious  Oxide  (Ex.  21) 81 

Precipitation  of  Zinc,  Manganese,  and  Iron  with  Ammonium  Sulphide 81 

Determination  of  Manganese  in  Potassium  Permanganate  (Ex.  22) 83 

CHAPTER  VII. 

DETERMINATION   OF   METALS  AS   PHOSPHATE,   CHROMATE,   AND 

CHLORIDE. 

Determination  of  Manganese,  Zinc,  and  Magnesium  as  Phosphate 84 

Removal  of  Ammonium  Salts 85 

Determination  of  Magnesium  in  Magnesium  Sulphate  (Ex.  23) 87 

Determination  of  Arsenic  as  Magnesium  Pyro-Arsenate 88 

Determination  of  Lead,  Barium,  and  Chromium  as  Chromate 89 

Determination  of  Potassium  as  Plat  ino-Chlo ride 89 

Lindo-Gladding  Method  for  Determining  Potassium 89 

Determination  of  Silver  as  Chloride 90 

Determination  of  Mercury  as  Mercurous  Chloride 92 

Determination  of  Bismuth  as  Oxychloride 92 

Determination  of  Silver  in  Silver  Nitrate  (Ex.  24) 93 


DETERMINATION   OF  ACIDS. 

CHAPTER  VIIT. 
DETERMINATION  OF  THE  HALOGENS,  SULPHUR,  AND  NITROGEN. 

Determination  of  the  Halogens 94 

Separation  of  Iodine  as  Palladous  Iodide  from  Chlorine  and  Bromine 96 

Separation  of  Iodine  from  Chlorine  as  Thallous  Iodide 97 

Separation  of  Chlorine,  Bromine,  and  Iodine  by  the  Method  of  Jannasch 

and  Aschoff 97 

Determination  of  Chloric  Acid 100 


xii  CONTENTS. 

PAGE 

Determination  of  Hydrocyanic  Acid 100 

Separation  of  Hydrocyanic  Acid  from  Chlorides,  Bromides,  and  Iodides. . . .  10Q 
Determination  of  Sulphur.     Oxidation  of  Sulphur  Compounds  by 

(a)  Fusion  with  Alkali  Carbonates  and  Nitrates 101 

(6)  Fusion  with  Sodium  Peroxide 102 

(c)  Digestion  with  Fuming  Nitric  Acid 102 

(</)  Digestion  with  Liquid  Bromine,  Aqua  Regia,  etc 104 

(?)  Digestion  with  Chlorine 104 

Determination  of  Tellurium  and  Selenium 105 

Determination  of  Nitrogen 

(a)  As  Ammonium  Chloride 105 

(6)  As  Ammonium  Platino-Chloride 106 

(c)  As  Nitron  Nitrate 106 

(d)  By  Fusion  of  Nitrates  with  Silica  or  Potassium  Chromate 107 


CHAPTER  IX. 
DETERMINATION  OF  CARBONIC,  BORIC,  AND  PHOSPHORIC  ACIDS. 

Determination  of  Carbon  Dioxide 

(a)  By  Loss  after  Decomposition  of  Carbonate  with  Acid 1 09 

(6)  By  Loss  after  Fusion  with  Borax  or  Microcosrnic  Salt Ill 

(c)  By  Direct  Weighing 112 

Determination  of  Boric  Acid  by  the  Method  of  Gooch 115 

Determination  of  Phosphoric  Acid 

(a)  By  Precipitation  as  Magnesium  Ammonium  Phosphate 117 

(6)  By  Precipitation  as  Phosphomolybdate 118 

1.  Direct  Weighing  of  the  Phosphomolybdate 119 

2.  Reprecipitation  as  Ammonium  Magnesium  Phosphate 120 


ANALYSIS   OF  ALLOYS. 

CHAPTER  X. 
ALLOYS  OF  SILVER,  COPPER,  LEAD,  BISMUTH,  CADMIUM,  AND  TIN. 

Errors  in  Complete  Analyses 121 

Separation  of  Silver  from  Other  Metals 123 

Analysis  of  a  Silver  Coin  (Ex.  25) 123 

Separation  of  Tin  as  Stannic  Oxide 125 

Separation  of  Lead  from  Other  Metals 120 

Analysis  of  Soft  Solder  (Ex.  26) 128 

Analysis  of  Rose's  Metals  (Ex.  27) 129 

Separation  of  Copper  and  Cadmium 130 

Analysis  of  Wood's  Metal  (Ex.  28) 131 


CONTENTS.  xiii 

CHAPTER  XI. 
ANALYSIS  OF  ALLOYS  CONTAINING  ARSENIC,  ANTIMONY,  AND  TIN. 

PAGE 

Separation  of  Arsenic,  Antimony,  and  Tin 134 

F.  W.  Clarke's  Method  of  Separation  of  Arsenic  from  Antimony  and  Tin. . .    134 

Other  Methods  of  Separation  of  Arsenic  from  Antimony  and  Tin 135 

Separation  of  Antimony  and  Tin 136 

H.  Rose's  Method  of  Separation  of  Antimony  from  Arsenic  and  Tin 136 

Classen's  Electrolytic  Separation  of  Antimony  from  Arsenic  and  Tin 137 

Separation  of  Arsenic  as  Trichloride  from  Antimony  and  Tin 138 

Analysis  of  Type-Metal  (Ex.  29) 140 

Analysis  of  Britannia  Metal  (Ex.  30) 142 

CHAPTER  XII. 
ANALYSIS  OF  ALLOYS  CONTAINING  IRON,  NICKEL,  AND  ZINC. 

Separation  of  Zinc  from  Other  Metals 145 

Analysis  of  Brass  or  Bronze  (Ex.  31 ) 146 

Analysis  of  German  Silver  (Ex.  32) 148 

Analysis  of  Manganese-Phosphorus-Bronze  (Ex.  33) 149 


ANALYSIS   OF   MINERALS. 

CHAPTER  XIII. 
MINERALS  CONTAINING  IRON,  ALUMINIUM,  AND  CHROMIUM. 

Selection  and  Preparation  of  Sample,  Pulverizing 152 

Separation  of  Iron,  Aluminium,  and  Chromium  as  Hydroxides 156 

Separation  of  Aluminium  and  Iron  as  Basic  Acetates 157 

Separation  of  Aluminium  and  Iron  as  Basic  Carbonates 159 

Separation  of  Chromium  as  Chromate 159 

Rothe's  Ether  Separation  of  Iron 160 

Volumetric  Separation  of  Iron  from  Aluminium 162 

Separation  of  Iron  from  Aluminium  as  Aluminate 162 

Analysis  of  Chromite  (Ex.  34) 163 

CHAPTER  XIV. 

ANALYSIS  OF  SULPHIDES  CONTAINING  MANGANESE,  NICKEL, 
COBALT,  AND  MERCURY. 

Separation  of  Nickel  and  Cobalt  as  Sulphides 166 

Electrolytic  Separation  of  Manganese  from  Cobalt  and  Nickel 167 

Separation  of  Cobalt  as  Tripotassium  Cobaltic  Nitrite 167 


xiv  CONTENTS. 


Separation  of  Cobalt  and  Nickel  by  Means  of  Nitroso-/?-Naphthol 168 

Analysis  of  Smaltite  (Ex.  35) 169 

Analysis  of  Pyrite,  Arsenopyrite,  or  Chalcopyrite  (Ex.  36) 172 

Analysis  of  Cinnabar  (Ex.  37) 174 

CHAPTER  XV. 

ANALYSIS  OF  CARBONATES  CONTAINING  CALCIUM,   BARIUM, 
STRONTIUM,  AND  MANGANESE. 

Separation  of  Calcium,  Barium,  and  Strontium  from  Magnesium  and  the 

Alkali  Metals 170 

Separation  of  Calcium  as  Oxalate 176 

Separation  of  Calcium,  Barium,  and  Strontium  as  Sulphates 177 

Separation  of  Barium  as  Chromate  from  Strontium  and  Barium 178 

Separation  of  Calcium  and  Strontium  as  Nitrates 179 

Analysis  of  Dolomite  (Ex.  38) 180 

CHAPTER  XVI. 

ANALYSIS  OF  SILICATES  AND  SEPARATION  OF  SODIUM  AND 

POTASSIUM. 

Separation  of  Magnesium  from  the  Alkalies 186 

Separation  of  the  Alkalies 187 

Decomposition  of  Silicates 188 

(a)  By  Fusion  with  Alkali  Carbonates 188 

(6)  By  the  Method  of  J.  Lawrence  Smith 190 

(c)  By  Fusion  with  Boric  Oxide  According  to  Jannasch 190 

Analysis  of  Feldspar  (Ex.  39) 192 


ELECTROLYTIC   METHODS. 

CHAPTER  XVII. 

THE    IONIC    THEORY;     ELECTROLYTIC    APPARATUS    AND 
MANIPULATION. 

The  Ionic  Theory  of  Electrolysis 197 

Significance  of  Current  Density 200 

Secondary  Electrolytic  Reactions 200 

Production  and  Control  of  the  Electric  Current 202 

Electrolytic  Apparatus 205 

Electrolysis  of  Warm  Solutions 208 

Washing  and  Drying  of  Deposited  Metals 208 

Rotation  of  Electrodes .  209 


CONTENTS.  XV 

CHAPTER  XVIII. 
ELECTROLYTIC  DETERMINATION  OF  METALS. 

PAGE 

Determination  of  Copper  by  Deposition  from  a  Nitric  or  Sulphuric  Acid 

Solution 213 

Separation  of  Copper  from  Other  Metals 214 

Decomposition  of  Copper  Ores 215 

Determination  of  Copper  by  Deposition  from  an  Ammonium-Oxalate  Solu- 
tion   215 

Determination  of  Copper  in  Copper  Sulphate  by  the  Ammonium-Oxalate 

Method  (Ex.  40) 215 

Determination  of  Copper  in  the  Refined  Metal  by  Nitric-Acid  Method 

(Ex.  41) 217 

Determination  and  Separation  of  Silver  from  Other  Metals 217 

Determination  of  Silver  in  Silver  Nitrate  (Ex.  42) 218 

Analysis  of  a  Silver  Coin  (Ex.  43) 218 

Determination  of  Nickel  and  Cobalt 219 

(a)  By  Ammonium-Oxalate  Method 219 

(6)  By  Ammonia  Method 220 

Analysis  of  a  Nickel  Coin  (Ex.  44) 220 

Determination  of  Tin  by  the  Ammonium-Oxalate  Method 221 

Determination  of  Lead  as  the  Peroxide 222 

Decomposition  of  Lead  Ores 223 

Determination  of  Zinc 224 

(a)  By  Ammonium-Oxalate  Method 224 

(6)  By  Acetic- Acid  Method 224 

Determination  of  Zinc  in  Ores 225 


VOLUMETRIC  ANALYSIS. 

CHAPTER  XIX. 
CALIBRATION  OF  APPARATUS. 

Volumetric  Apparatus 227 

Errors  in  Reading  Burettes  and  Pipettes 228 

Standard  Temperatures 231 

Standards  of  Volume 232 

Calibration  of  Flasks  and  Burettes 233 

(a)  By  Weighing  Water 233 

(6)  By  Morse  and  Blalock  Bulbs 235 

Calibration  of  Morse  and  Blalock  Bulbs  (Ex.  45) 242 

Calibration  of  Flasks  by  Means  of  Morse  and  Blalock  Bulbs  (Ex.  46) 244 

Calibration  of  Burettes  by  Means  of  Morse  and  Blalock  Bulbs  (Ex.  47).  . . .  244 

Problems.  .  245 


xvi  CONTENTS. 

CHAPTER  XX. 

ACIDIMETRY. 

PAGE 

Standard  and  Normal  Solutions 247 

Percentage  Given  by  Number  of  Cubic  Centimeters  of  Acid  Used 249 

Effect  of  Carbon  Dioxide  on  Indicators 250 

Basicity  of  Acids  with  Various  Indicators 251 

Properties  of  Litmus,  Cochineal,  Methyl  Orange,  and  Phenolphthalein 253 

Strength  and  Nature  of  Standard  Acid  Most  Desirable  for  General  Use.  .  . .  255 

Problem 257 

CHAPTER  XXI. 
STANDARD  ACIDS. 

Methods  of  Standardizing  Acids 258 

Preparation  and  Standardization  of  Fifth-Normal  Hydrochloric  Acid  (Ex. 

48) 263 

Determination  of  Sodium  Hydroxide,  Carbonate,  and  Bicarbonate 265 

Analysis  of  Caustic  Soda  (Ex.  49) 267 

Determination  of  Temporary  and  Permanent  Hardness  of  Water  (Ex.  50).  .  269 
Preparation  and  Standardization  of  Fifth-Normal  Sulphuric  Acid  (Ex.  51).  271 

Determination  of  Specific  Gravity 272 

Problems 277 

CHAPTER  XXII. 
STANDARD  ALKALIES. 

Standard  Alkaline  Solutions 279 

(a)  Caustic  Soda 279 

(6)  Alcoholic  Solutions  of  Caustic  Potash 281 

(c)  Barium  Hydroxide 281 

(d)  Ammonia 282 

Preparation  of  n/5  Caustic  Soda  and  Determination  of  Ammonia  in  Ammo- 
nium Chloride  (Ex.  52) 282 

The  Kjeldahl  Method  of  Determining  Nitrogen 283 

(a)  Digestion 284 

(6)  Distillation 285 

(c)  Wilfarth's  Modification 287 

(d)  Gunning's  Modification 287 

(e)  Gunning-Jodlbauer  Modification 288 

(/)  Forster's  Modification 288 

Determination  of  Nitrogen  in  Milk  by  the  Kjeldahl-Gunning  Method 

(Ex.  53) 289 

Determination  of  Nitrogen  in  Potassium  Nitrate  by  the  Kjeldahl-Forster 

Method  (Ex.  54) 291 


CONTENTS. 

CHAPTER  XXIII. 
TITRATION  OF  BORIC  AND  CARBONIC  ACIDS. 

PAGE 

Titration  of  Boric  Acid 293 

(a)  By  Thompson's  Glycerine  Method 293 

\b)  By  Jones'  Mannitol  Method 294 

Determination  of  Carbon  Dioxide  in  Water 296 

(a)  Free 296 

(6)  Semi-Combined 297 

(c)  Total 298 

Determination  of  Carbon  Dioxide  in  Gases  by  Pettenkofer's  Method 300 

Determination  of  Carbon  Dioxide  in  the  Air  (Ex.  55) 304 


OXIDATION  AND  REDUCTION  METHODS. 

CHAPTER  XXIV. 
POTASSIUM   PERMANGANATE   AND   DICHROMATE   SOLUTIONS. 

Normal  Oxidizing  Solutions 306 

Indicate  rs 307 

Potassium- Permanganate  Solution 308 

Standardization 309 

(a)  By  Iron.  .  , 309 

(6)  By  Oxalic  Acid  and  the  Oxalates 313 

(c)  By  Direct  Measurement  of  Oxygen 313 

Preparation  and  Standardization  of  n/5  Potassium-Permanganate  Solution 

(Ex.  56) 315 

Determination  of  Iron  in  Iron  Ores 318 

Determination  of  Ferrous  and  Total  Iron  in  an  Iron  Ore  (Ex.  57) 320 

Analysis  of  Manganese  Ores 322 

Determination  of  Manganese  by  Volhard's  Method 323 

Determination  of  Available  Oxygen  in  Pyrolusite  (Ex.  58) 325 

Determination  of  Manganese  in  Pyrolusite  by  Volhard's  Method  (Ex.  59).  .   326 

Determination  of  Tin ; 327 

Analysis  of  Stannous  Chloride  (Ex.  60) 328 

Determination  of  Calcium 328 

Potassium-Dichromate  Solution 330 

Standardization  by  Means  of  Iron 330 

Preparation  of  n/5  Potassium-Dichromate  Solution  (Ex.  61) 332 

Determination  of  Iron  in  an  Iron  Ore  (Ex.  62) 333 

Determination  of  Chromium  hi  Chrome  Iron  Ore  (Ex.  63) 333 

Problems...  .  334 


xviii  CONTENTS. 

CHAPTER  XXV. 
IODOMETRIC  METHODS. 

PAGE 

Preparation  and  Standardization  of  Iodine  and  Sodium  Thiosulphate  Solu- 
tions    335 

Preparation  and  Standardization  of  n/10  Iodine  and  Sodium  Thiosulphate 

Solutions  (Ex.  64) 339 

Determination  of  Oxidizing  and  Reducing  Substances 342 

Titration'  of  Acids  by  lodometric  Methods , 343 

Determination  of  Available  Oxygen  in  Pyrolusite  (Ex.  65) 345 

Determination  of  Sulphur  Dioxide  hi  Sodium  Sulphite  (Ex.  66) 346 

Analysis  of  Bleaching  Powder 346 

Determination  of  Available  Chlorine  in  Bleaching  Powder  (Ex.  67) 347 

Problems.  .                                                                                                         .  348 


PRECIPITATION   METHODS. 

CHAPTER  XXVI. 
DETERMINATION  OF  CHLORIDES,  CYANIDES,  AND  SILVER. 

Theory  of  Indicators 349 

(a)  In  Acidimetry  and  Alkalimetry 349 

(6)  In  Precipitation  Methods 350 

Titration  of  Chlorides 350 

Titration  of  Cyanides 351 

Standard  Silver-Nitrate  Solution 352 

Determinations  by  Means  of  Silver  Nitrate 353 

Determination  of  Cyanogen  in  Potassium  Cyanide  (Ex.  68) 354 

Determination  of  Silver  in  Alloys 355 

Determination  of  Silver  in  a  Coin  (Ex.  69) 355 

Problems 356 

CHAPTER  XXVII. 
DETERMINATION  OF  PHOSPHORIC  ACID. 

Determination  of  Phosphoric  Acid 357 

(a)  By  the  Uranium  Method 357 

(6)  By  Pemberton's  Alkalimetric  Titration  of  the  Phosphomolybdate .  .   360 
(c)  By  the  Reduction  of  the  Molybdate  Precipitate  and   Titration 

with  Permanganate  Solution .   362 


CONTENTS.  xix 


TECHNICAL  ANALYSIS. 

CHAPTER  XXVIII. 
ANALYSIS  OF  IRON,   STEEL,  AND  COAL. 

PAGE 

Determination  of  Silicon 365 

Determination  of  Sulphur 367 

(a)  By  Oxidation  and  Precipitation  as  Barium  Sulphate 367 

(6)  By  Evolution  Methods 369 

Determination  of  Phosphorus 372 

Determination  of  Total  Carbon 374 

(a)  By  Oxidation  with  Chromic  Acid 374 

(6)  By  Oxidation  after  Dissolving  Iron  in  Copper  Solution 376 

1 .  Combustion  in  Oxygen 377 

2.  Combustion  with  Chromic  Acid 380 

(c)  Determination  of  Carbon  after  Volatilization  of  Iron  in  a  Stream 

of  Chlorine 381 

Determination  of  Graphitic  Carbon 382 

Determination  of  Total  Carbon  by  Eggertz's  Method 383 

Determination  of  Manganese 385 

(a)  By  Ford's  Method 385 

(6)  By  Volhard's  Method 387 

(c)  By  Deshay's  Method 388 

Proximate  Analysis  of  Coal 390 

Determination  of  Moisture 390 

Determination  of  Volatile  Combustible  Matter 391 

Determination  of  Ash 391 

Determination  of  Sulphur 392 

Proximate  Analysis  of  Coal  (Ex.  70) 392 

Determination  of  the  Calorific  value  of  Fuels 395 

The  Parr  Calorimeter 397 

Determination  of  the  Calorific  Value  of  Coal  by  Means  of  the  Parr  Calorim- 
eter (Ex.  71) 403 

CHAPTER  XXIX. 
WATER  ANALYSIS. 

Sanitary  Analysis 406 

Physical  Examination 408 

Chemical  Examination 410 

Determination  of  Free  and  Albuminoid  Ammonia 41 1 

Determination  of  Nitrites 416 

Determination  of  Nitrates. 418 

Determination  of  Oxygen  Required 419 

Determination  of  Chlorides 421 

Determination  of  Total  Solids .  422 


xx  CONTENTS. 

PAQB 

Bacteriological  Examination  of  Water 423 

Bacterial  Count  (Ex.  72) 425 

Test  for  B.  Coli  (Ex.  73) 428 

Interpretation  of  the  Results  of  a  Sanitary  Water  Analysis 428 

Analysis  of  Water  for  Use  in  Boilers 430 

Calculation  of  Results .  433 


CHAPTER  XXX. 
ANALYSIS  OF  OILS  AND  FATS. 

Chemical  Composition  of  Oils  and  Fats 436 

Acidity .- 437 

Kottstorfer  Value 439 

Reichert  and  Reichert-Meissl  Value 440 

The  Polenske  Value 441 

Hehner  Value 443 

Iodine  Value 443 

Determination  of  Rosin  in  Shellac 446 

Acetyl  Value 447 

Specific  Gravity 448 

Index  of  Refraction 449 

Maumene"  Number  and  Specific  Temperature  Reaction 449 

Melting-Point  of  Fatty  Acids 451 

Detection  of  Phytosterol  and  Cholesterol 451 

Flash  Point 452 

Oil  Analysis 454 

Table  of  Iodine  Values 457 

Table  of  Reichert-Meissl  Values 459 

Table  of  Polenske  Values 459 

Table  of  Kottstorfer  Saponification  Values 459 

Table  of  Hehner  Values 459 

Table  of  MaumenS  Number 460 

Table  of  Specific  Temperature  Number 461 

Table  of  Acetyl  Values 461 

CHAPTER  XXXI. 
GAS   ANALYSIS. 

Absorbents  for  Oxygen 462 

Absorbents  for  Carbon  Monoxide 464 

Absorbents  for  Illuminants 466 

Determination  of  Hydrogen 467 

Determination  of  Methane 470 

Hempel's  Gas  Apparatus 472 

Analysis  of  Illuminating-Gas  (Ex.  74) 478 


CONTENTS.  xxi 

PAGE 

The  Orsat  Apparatus 4{  1 

Analysis  of  Flue  Gases 4c4 

Problems 485 

CHAPTER  XXXII. 
STOICHIOMETRY. 

Atomic  Weights 486 

Factors 487 

Use  of  Logarithms 487 

Indirect  Gravimetric  Analyses 491 

Factor  Weights 492 

Calculation  of  Formula?  of  Salts  and  Minerals 493 

Balancing  Equations 496 

Calculation  of  Volumetric  Determinations 499 

APPENDIX. 

Reagents 505 

Tables: 

International  Atomic  Weights 509 

Chemical  Factors  and  Their  Logarithms 510 

Logarithms 514 

Antilogarithms 516 

Specific  Gravity  of  Ammonia  Solutions 518 

"  "      "  Hydrochloric  Acid 519 

"  "      "  Nitric  Acid 520 

"  "      "  Sulphuric  Acid 523 

Vapor  Tension  of  Water 526 

Density  of  Water 527 


QUANTITATIVE    ANALYSIS. 


INTRODUCTION. 

1.  Object  of  Quantitative  Analysis. — A    quantitative    analysis 
has  for  its  purpose  the  determination  of  the  proportion  in  which 
one  or  more  constituents  exist  in  a  compound  substance.    While 
in  a  qualitative  analysis  the  object  is  to  prove  the  presence  or 
absence  of  a  given  constituent,  in  the  quantitative  analysis  the 
amount    of   the    constituent   must   be    determined.     Unless   the 
constituents  of  the  substance  examined  are  already  known,  the 
qualitative  must  precede  the  quantitative  analysis.     The  student 
of  the  latter  art  must  therefore  be  familiar  with  the  former.     In 
quantitative  analysis  most   of  the  methods  employed  may  be 
grouped  in  two  classes  designated  as  GRAVIMETRIC  AND  VOLU- 
METRIC methods. 

2.  Gravimetric  Methods. — In  a  gravimetric  determination  the 
substance  to  be  determined  is  separated  from  the  other  material 
present  and  weighed.     It  may  be  separated  either  as  an  element 
or  as  part  of  a  compound  which  is  of  definite  composition,  and 
which  can  be  obtained  pure  and  be  weighed  accurately.     Copper, 
for  instance,  by  means  of  an  electric  current,  may  be  deposited 
from  a  solution  of  a  copper  salt  in  a  bright  coherent  form  so  that 
it  may  be  readily  washed,  dried,  and  weighed.     It  may  also  be 
precipitated  as  sulphide,  which  can  also  be  washed,  dried,  and 
weighed.     As   the   percentage   of   copper   in   copper   sulphide   is 
known,  the  amount  of  copper  present  may  be  readily  calculated. 

3.  Volumetric  Methods. — In  a  volumetric  determination  a  solu- 
tion is  used  which  contains  a  known   amount  of  a    substance 


2  QUANTITATIVE  ANALYSIS. 

which  can  react  in  a  definite  manner  with  the  constituent  to  be 
determined.  The  volume  of  this  so-called  standard  solution 
which  is  just  sufficient  to  complete  the  reaction  is  measured  and 
the  weight  of  the  cdnstituent  to  be  determined  is  calculated. 
The  volumetric  determination  of  silver  may  be  carried  out  as 
follows- 

A  pure  chloride,  such  as  sodium  chloride,  may  be  weighed  out 
and  dissolved  in  a  measured  amount  of  water.  This  solution  is 
added  in  small  quantities  to  the  silver  solution,  until  the  metal 
is  completely  precipitated.  The  volume  added  is  measured  by 
means  of  appropriate  instruments,  and  the  amount  of  sodium 
chloride  necessary  to  precipitate  the  silver  is  computed.  Since 
one  molecule  of  sodium  chloride,  or  58.5  parts,  is  necessary  to  pre- 
cipitate one  atom  of  silver,  or  107.9  parts,  the  weight  of  the  silver 
may  be  readily  calculated. 

4.  Relative  Advantage  of  the  Two  Methods. — Equally  accurate 
results  in  general  may  be  obtained  by  either  gravimetric  or  volu- 
metric methods,  though  in  the  case  of  a  given  substance  the  best 
method  known  may  belong  to  one  or  the  other  class.     Gravi- 
metric processes  and  manipulations  are  generally  simpler,  while 
volumetric  determinations  usually  require  less  time,  provided  the 
standard  solutions  are  once  prepared.     Where  many  determina- 
tions must  be  made  volumetric   methods   are  preferable,  while 
single  determinations  may  usually  be  conducted  more  rapidly  by 
gravimetric  methods. 

5.  Skill  and  Knowledge  Required. — The  successful  carrying  out 
of  a  quantitative  determination  by  either  of  these  general  methods 
requires  a  large  amount  of  skill  in  manipulation  as  well  as  chem- 
ical knowledge.     This  is  evident  from  the  consideration  that  a 
correct   quantitative  analysis  requires  that  both  solutions  and 
solids  be  carried  through  numerous  operations  without  loss.     The 
operator  must  also  have  a  very  complete  knowledge  of  the  chem- 
ical reactions  taking  place,  so  that  he  can  be  certain  that  the 
products  finally  weighed  are  absolutely  pure  and  contain  all  of  a 
given  constituent.     This  is  rendered   the  more  difficult  as  the 
complexity  of  the   substance   analyzed   increases.     The   precipi- 
tates obtained  from  complex  substances  are  very  apt  to  be  con- 
taminated with  some  of  the  elements  present  which  alone  would 


INTRODUCTION.  3 

not  be  precipitated.     Only  careful  testing  and  laborious  purifica- 
tion of  such  precipitates  will  insure  correct  results. 

6.  Patience  and  Honesty  Necessary. — The  ways  in  which  errors 
may  be  made,   especially  by  the  inexperienced  worker,  are  so 
numerous  that  unless  the  greatest  care  is  taken  in  carrying  out 
the  work  the  results  will  certainly  be  worthless.     Persons  who 
are  not  conscientious  enough  to  reject  determinations  in  which 
avoidable  errors  are  known  to  be  present,  such  as  spilling  a  part 
of  a  solution,  should  not  undertake  quantitative  analysis.     To 
the  beginner  the  numerous  precautions  seem  to  require  almost 
endless  patience  and  expenditure  of  time.     If  a  sufficient  amount 
of  laboratory  practice  is  taken,  the  worker  will  ultimately  acquire 
the  skill  which  enables  him  to  carry  on  rapidly  and  without  error 
the  most  difficult  operations.     A  skilful  worker  may  also  operate 
on  many  determinations  at  once,  having  learned  to  economize 
his   time.     A   reputation   for   producing   absolutely   trustworthy 
results  fully  repays  the  analyst  for  the  time  and  labor  expended. 
Absolute  truthfulness  and  patience  are  therefore  personal  quali- 
ties indispensable  to  the  successful  analyst. 

7.  Neatness. — In  order  to  be  absolutely  certain  of  one's  analyti- 
cal results  a  number  of  habits  must  be  acquired  by  rigidly  adher- 
ing to  fixed  rules  of  procedure.     All  apparatus  must  be  kept  per- 
fectly clean,  and  the  working-table  and  surroundings  in  a  neat 
and  orderly  condition.     No  assurance  can  be  given  of  the  accuracy 
of  an  analysis  which  has  been  carried  out  in  vessels  from  which 
material  remaining  from  former  work  may  not  have  been  re- 
moved.    It  does  not  help  matters  to  say  that  one  does  not  think 
there  could  have  been  enough  impurity  to  affect  the  result.     Abso- 
lute certainty  on  this  point  is  required  of  the  chemist. 

8.  Records.— One  of  the  most  common  sources  of  error  is  found 
in   the    careless   recording    of   results.    Weights   are    frequently 
recorded  on  loose  pieces  of  paper,  backs  of  envelopes,  etc.,  which 
may  obviously  be  easily  lost.     Frequently  the  figures  are  accom- 
panied with  no  indication  of  the  nature  of  the  substance  weighed 
or  the  analysis  carried  out.     The  only  safe  method  consists  in 
always  recording  the  weights  or  other  observations  in  a  laboratory 
note-book  kept  for  the  purpose,  and  keeping  this  original  record 
for  future  reference.    The  page  should  be  distinctly  and  fully 


4  QUANTITATIVE  ANALYSIS. 

headed  with  the  date,  the  nature  of  the  determination,  and  num- 
ber of  the  analysis  or  other  mark  of  identification  if  several  similar 
determinations  are  being  carried  out.  The  weights  or  other  observa- 
tions should  be  carefully  verified  when  the  record  has  been  made. 
All  subsequent  records  or  observations  on  this  determination 
should  be  made  on  this  page.  A  brief  outline  of  the  method  of 
analysis  should  be  written  on  the  page  opposite  that  on  which 
the  weights  and  other  figures  are  recorded.  This  outline  should 
be  written  before  the  analysis  is  begun.  The  student  should 
study  the  process  until  the  reason  for  each  step  is  clear.  The 
mechanical  operation  of  carrying  out  an  analysis  according  to 
directions  without  understanding  the  process  is  of  very  little  value. 

9.  Calculation  of  Results. — When  the  accuracy  of  the  record  of 
the  work  has  been  assured,  the  most  common  general  source  of 
error  is  found  in  the  calculation  of  the  result.    Many  chemists 
who   are   otherwise  very  capable  find   the   calculation   of   their 
results  very  difficult.    The  student  is  therefore  advised  to  pay 
special  attention  to  the  method  of  calculating  the  results  of  his 
determinations  in  the  exercises  given  in  this  book  as  well  as 
carefully  solving  the  problems  given  for  practice.    Unless  these 
rather  simple  calculations  are   thoroughly  understood,   he   will 
find  himself  utterly  unable  to  interpret  the  results  in  the  many 
complicated  analyses  the  chemist  is  called  upon  to  perform.     It 
need  hardly  be  said  that  mere  multiplication  and  division  should 
be  carried  out  correctly,  and  yet  as  a  matter  of  fact  many  re- 
ported results  are  erroneous  for  this  reason  alone. 

10.  Limit  of  Accuracy.  —While  the  analyst  must  be  certain  of 
the  accuracy  of  his  records  and  calculations,  and  of  the  cleanliness 
of  his  apparatus  and  surroundings,  he  must  not  make  the  mis- 
take of  doing  part  of  his  work  with  a  far  greater  degree  of  accu- 
racy than  the  remainder.     If  he  weighs  a  precipitate  to  the  fourth 
place  of  decimals  and  multiplies  this  weight  by  the  percentage  to 
which  the  element  sought  is  present,  the  result  need  not  be  carried 
out  to  the  sixth  or  eighth  decimal  place.     If,  for  instance,  a  pre- 
cipitate of  barium  sulphate  weighed  .4325  gram,  on  multiplying 
this  number  by  .5886,  which  represents  the  percentage  of  barium  in 
barium  sulphate,  the  figure  .25456950  is  obtained.     The  last  four 
figures  of  this  number  are  absolutely  without  significance  and  must 


INTRODUCTION.  5 

be  discarded.  As  the  weight  of  the  barium  sulphate  was  taken  to 
but  four  places  after  the  decimal  point,  the  amount  of  barium 
present  cannot  possibly  be  calculated  to  more  than  four  places 
and  should  be  given  as  .2545  gram.  Similarly,  if  an  analysis  is 
carried  out  by  a  process  or  for  a  purpose  in  which  an  error  of 
one  per  cent  may  be  present,  no  pains  need  be  taken  to  secure 
much  greater  accuracy  than  this  in  any  part  of  the  process. 

That  unavoidable  errors  exist  in  all  analytical  methods,  no 
matter  how  carefully  carried  out,  is  beyond  dispute.  The  expe- 
rience of  all  workers  proves  that  it  is  impossible  to  carry  through 
an  analysis  twice,  and  obtain  exactly  identical  results.  With 
the  greatest  possible  care,  results  will  differ  by  an  amount  which 
for  a  given  method  can  be  made*  to  approach  a  certain  minimum 
value.  For  a  considerable  number  of  quantitative  methods  this 
f3rror  is  .1%.  Some  duplicates  may  agree  more  closely  than 
this,  while  others  will  differ  somewhat  more  widely.  The  aver- 
age error  in  a  series  of  results  will  be  found  to  be-  at  least  .1% . 
Most  quantitative  methods  have  a  tendency  to  give  a  high  or  a 
low  result  because  of  impurities  in  the  substance  weighed  or 
measured,  or  failure  to  obtain  all  of  it.  The  quantitative  result 
is  therefore  always  a  more  or  less  close  approximation  to  the 
truth,  with  a  general  minimum  difference  from  the  true  result  of 
.1%.  Results  are  therefore  generally  computed  to  hundredths 
of  per  cent,  though  in  most  cases  the  number  in  the  second  place 
after  the  decimal  point  is  of  very  little  significance.  It  simply 
helps  to  establish  the  correct  number  in  the  tenths  place. 

There  is  therefore  very  little  need  of  making  measurements 
or  weighings  closer  than  1  part  in  1000.  The  ordinary  analytical 
balance  gives  the  true  weight  of  a  substance  within  one  or  two 
tenths  of  a  milligram.  If  the  amount  weighed,  therefore,  is  200 
milligrams,  the  error  of  weighing  is  about  the  same  as  in  the 
remainder  of  the  work.  If  a  gram  is  weighed  out,  the  error  in 
weighing  is  1  or  2  parts  hi  10,000,  which  is  usually  a  much  smaller 
amount  than  can  be  estimated  in  the  remainder  of  the  work. 
The  analytical  chemist,  therefore,  need  generally  spend  no  time 
or  effort  in  overcoming  an  error  which  is  less  than  .1%  or  which 
in  the  ordinary  analysis  amounts  to  less  than  a  few  tenths  of  a 
milligram.  The  larger  the  amount  of  the  substance  taken  for 


6  QUANTITATIVE  ANALYSIS. 

analysis,  the  more  accurate  will  be  the  result  if  due  care  is  taken. 
The  time  consumed  in  handling  increasing  amounts  of  chemicals 
soon  becomes  excessive,  so  that  a  limit  of  from  J  to  1  gram  in 
the  weight  of  the  precipitate  is  found  advisable  in  practice.  If 
a  constituent  is  present  in  very  small  amount,  a  very  large 
amount  of  the  substance  to  be  analyzed  must  be  taken  to  obtain 
enough  of  the  constituent  to  work  with. 

ii.  Utility  and  Importance  of  Quantitative  Analysis. — The  art 
and  science  of  quantitative  analysis  is  of  fundamental  impor- 
tance to  most  branches  of  chemistry.  Historically,  rapid  strides 
in  the  theory  of  the  subject  were  made  only  after  the  necessity 
was  recognized  of  taking  into  account  the  proportions  in  which 
elements  combine  by  weight.  The  most  fundamental  concep- 
tion of  the  atom  is  that  of  a  portion  of  matter  possessing  a  defi- 
nite weight.  The  chemical  formulas  which  are  so  generally  used 
and  seem  to  express  so  much  are  primarily  expressions  of  the 
results  of  a  quantitative  analysis  of  the  compounds  concerned. 
In  the  industrial  world  we  find  one  manufacturing  process  after 
another  coming  under  the  control  of  the  analytical  chemist,  who 
tests  the  purity  of  the  raw  materials  and  the  finished  products 
and  reduces  the  loss  of  valuable  material  to  a  minimum.  Andrew 
Carnegie,  the  famous  iron  and  steel  master,  tells  us  that  he  was 
the  first  man  in  America  to  employ  a  chemist  for  the  iron  and 
steel  industry.  To-day  every  part  of  the  process  is  guided  by 
the  chemist's  analysis  of  the  material,  and  every  plant  is  equipped 
with  a  chemical  laboratory.  Many  other  industries  are  similarly 
controlled  by  the  chemist.  In  the  commercial  world  an  increas- 
ing number  of  substances  are  daily  bought  and  sold  on  the  value 
assigned  by  the  chemical  analysis. 

In  view  of  the  fundamental  importance  of  the  subject  to  the 
science  and  art  of  chemistry  and  to  its  industrial  and  commercial 
applications,  the  student  is  urged  to  spare  no  pains  to  acquire 
the  most  thorough  knowledge  of  the  subject  and  the  greatest 
possible  skill  in  its  application. 


CHAPTER  I. 
THE  BALANCE. 

12  Construction. — The  chemical  balance  consists  essentially  of 
three  parts,  namely,  the  beam,  post,  and  pans.  An  agate  or 
steel  triangular  prism,  called  a  knife-edge,  is  securely  fastened  to 
the  centre  of  the  beam,  and  a  similar  one  to  each  end.  An  agate 
or  steel  plane  is  fastened  to  the  post,  and  on  this  plane  the  beam 
rests,  the  contact  being  formed  by  the  central  knife-edge.  The 
pans  are  suspended  from  the  ends  of  the  beam  by  means  of 
agate  or  steel  planes  set  in  pieces  of  metal  called  stirrups.  These 
planes  rest  on  the  terminal  knife-edges  of  the  beam.  The  object 
of  this  method  of  supporting  the  beam  and  pans  is  to  enable  the 
system  to  oscillate  with  the  least  possible  friction.  A  long,  slen- 
der pointer  is  attached  to  the  centre  of  the  beam.  The  oscilla- 
tions of  the  beam  can  be  observed  by  means  of  the  movements  of 
the  end  of  this  pointer  across  the  front  of  a  scale  attached  to 
the  bottom  of  the  post.  In  this  manner  very  slight  movements 
may  be  observed.  By  means  of  a  thumb-screw  a  second  beam 
can  be  raised  or  lowered,  so  as  to  lift  the  real  beam  of  the  balance 
off  its  knife-edges  when  the  balance  is  not  in  use,  or  lower  it  into 
place  when  it  is  desired  to  make  a  weighing.  Adjustable  sup- 
ports are  also  provided  for  the  pans.  A  levelling  device  is  attached 
to  the  post,  so  that  by  means  of  thumb-screws  under  the  balance 
its  position  may  be  adjusted  so  as  to  place  the  post  in  an  absolutely 
vertical  position.  The  instrument  is  enclosed  in  a  glass  case  in 
order  to  protect  the  parts  from  dust  and  mechanical  injury,  as 
well  as  to  prevent  air-currents  and  sudden  heating  and  cooling 
of  the  parts  during  the  weighing. 

13.  Weighing. — The  weight  of  a  given  substance  is  obtained  by 
placing  it  on  one  of  the  pans  and  placing  weights  on  the  other 
pan  until  the  pointer  indicates  the  same  position  of  the  beam  as 


8 


QUANTITATIVE  ANALYSIS. 


it  occupied  when  the  pans  were  empty.  The  smallest  fractional  * 
weights  are  obtained  by  moving  a  bent  wire  called  a  rider  along 
the  beam  and  placing  it  on  the  various  divisions  of  the  beam 
until  an  exact  counterbalance  is  obtained.  The  rider  must 


FIG.  1. 

weigh  as  many  milligrams  as  the  total  number  of  divisions  of 
the  beam,  the  last  division  being  directly  over  the  terminal  knife- 
edge,  and  the  zero-mark  being  directly  over  the  central  knife- 
edge.  Balances  are  usually  constructed  for  riders  of  6  or  12 
milligrams.  As  the  6  or  12  divisions  of  the  beam  are  each  divided 
into  10  parts,  the  smallest  weight  indicated  by  the  rider  will  be 
the  tenth  of  a  milligram. 

*  All  weights  of  a  denomination  less  than  one  gram  are  called  fractional 
weights. 


THE  BALANCE. 


9 


It  will  be  observed  that  the  chemical  balance  gives  the  equal- 
ity in  weight  between  the  substance  to  be  weighed  and  the  weights 
used.  We  assume  that  if  the  earth  attracts  two  substances 
equally,  they  are  equal  in  mass.  When  a  substance  has  been 
weighed  in  a  balance,  it  can  be  stated  that  its  mass  is  equal  to  the 


FIG.  2. 

mass  of  the  weights  used  to  counterbalance  it.  If  the  force  of  the 
earth's  attraction  should  vary,  it  would  act  upon  the  masses  on 
the  pans  equally;  therefore  the  weights  found  are  independent 
of  the  force  of  gravity  at  the  time  and  place  of  weighing. 

14.  Sensibility. — The  beam,  pans,  weights,  etc.,  constitute  a  sys- 
tem which  operates  in  many  respects  like  a  pendulum,  of  which 
the  central  knife-edge  is  the  point  of  support  about  which  the 
system  oscillates.  The  centre  of  gravity  of  the  system  must  be 
below  the  central  knife-edge,  for  if  it  were  at  the  knife-edge  the 
system  would  remain  at  rest  wherever  placed;  while  if  it  were 
above  the  knife-edge,  the  beam  would  be  in  unstable  equilibrium. 
The  nearer  the  centre  of  gravity  is  to  the  central  knife-edge,  the  more 
sensitive  the  balance  becomes.  A  small  weight  is  fastened  to  the 
pointer  by  means  of  a  screw.  By  raising  this  weight  the  centre 
of  gravity  may  be  brought  nearer  and  nearer  to  the  central  knife- 
edge.  As  this  is  done,  a  smaller  and  smaller  weight  will  be  found 
necessary  to  displace  the  beam  a  given  distance  and  move  the 


10  QUANTITATIVE  ANALYSIS. 

pointer  through  a  given  angle  or  over  a  given  number  of  divisions 
on  the  scale;   that  is,  the  balance  becomes  more  and  more  sensi- 
tive.   By  inspection  of  Fig.  3  it  is  evident  that  if  the  centre  of 
D    gravity  were  at  A,  a  displacement  of  the  oscillating 
body  through  a  given  angle,  (7,  would  raise  the  centre 
of  gravity  a  greater  distance  than  if  it  were  located 
at  some  higher  point,  as  at  B.     The  oscillations  be- 
come  slower  and   slower,  however,  so  that  a  balance 
should  not  be  made  more  sensitive  than  is  required  for 
the  work  at  hand. 

When  the  weights  are  placed  on  the  pans  of  a 
balance,  the  position  of  the  centre  of  gravity  is  not  changed  if 
the  terminal  knife-edges  of  the  beam  are  in  the  same  straight 
line  with  the  central  knife-edge,  because  the  weights  act  as  if 
placed  on  the  terminal  knife-edges.  However,  the  rate  of  oscilla- 
tion decreases  with  increasing  load.  If  the  beam  bends,  the 
terminal  knife-edges  are  lowered,  and  consequently  the  centre  of 
gravity  of  the  system  is  lowered  and  the  sensibility  of  the  balance 
decreased.  Even  if  the  beam  does  not  bend,  the  increased  fric- 
tion with  increased  load  decreases  the  sensibility.  Some  balance- 
makers  place  the  terminal  knife-edges  slightly  above  the  one  in 
the  centre,  so  that  when  the  balance  is  loaded  the  sensibility 
increases  until  the  beam  bends  sufficiently  to  bring  the  terminal 
into  the  same  plane  with  the  central  knife-edge,  after  which  the 
sensibility  decreases  with  increasing  load. 

The  sensibility  of  a  balance  for  a  given  load  is  obtained  by 
loading  the  pans  equally  and  noting  the  number  of  divisions  of 
the  scale  the  pointer  is  moved  by  adding  a  one-milligram  weight 
to  one  of  the  pans.  Sensibility  is  defined  therefore  as  the  number 
of  divisions  of  the  scale  through  which  the  pointer  is  displaced  by  a 
load  of  one  milligram. 

15.  Zero-point. — When  a  weighing  is  to  be  made  it  is  found 
more  convenient  and  more  accurate  if,  instead  of  waiting  until  the 
beam  and  pointer  come  to  rest,  one  ascertains  where  they  would 
come  to  rest.  This  is  done  by  noting  the  points  on  the  scale  to 
which  the  pointer  swings,  and  then  calculating  the  point  of  rest. 
When  the  beam  with  empty  pans  is  set  swinging,  it  will  seldom 
be  found  that  the  pointer  tends  to  come  to  rest  with  the  point  at 


THE  BALANCE.  11 

the  zero-point  of  the  scale.  An  adjustable  weight  is  attached 
to  the  end  of  the  beam,  so  that  by  moving  it  from  or  toward  the 
central  knife-edge  the  pointer  may  come  to  rest  very  near  the 
zero-point  of  the  scale.  In  practice  it  will  be  found  impossible  to 
keep  the  balance  adjusted  so  that  the  pointer  shall  indicate  exactly 
zero.  Unequal  changes  in  temperature  and  other  causes  occasion 
variations  in  the  point  of  rest  from  day  to  day,  and  also  some- 
what regular  changes  during  each  day.  The  point  of  rest  is 
brought  to  the  centre  of  the  scale  by  adjusting  the  weight  on 
the  beam  only  when  the  pointer  comes  to  rest  more  than  one 
division  from  the  zero-point  of  the  scale.  When  a  weighing  is 
to  be  taken  the  balance  is  set  swinging  with  empty  pans,  and 
.the  point  of  rest  calculated  from  the  movements  of  the  pointer. 
This  point  of  rest  is  called  the  zero-point.  The  readings  are 
recorded  in  the  following  manner  and  the  calculation  made  as 
indicated,  the  central  division  of  the  scale  being  designated  10 : 

5.3  14.0 

5.6  13.6 

6.0 


Average        5.6  13.8 

5.6+13.8 
Zero-point  = ~ =9.7 

16.  Reducing  Weights  to  Vacuo. — While  the  balance  gives  the 
true  weight  of  a  substance  independent  of  variations  in  the  force 
of  gravity,  the  weight  of  the  atmosphere  introduces  an  error 
which  is  in  some  cases  appreciable.  A  given  substance  weighs 
less  in  the  air  than  in  a  vacuum  because  it  displaces  a  volume 
of  air  equal  to  its  own  volume.  The  weights  also  are  affected  by 
the  buoyant  force  of  the  air,  but  if  the  specific  gravity  of  the 
weights  and  of  the  substance  to  be  weighed  is  the  same  the  two 
errors  counterbalance  each  other.  If  the  specific  gravity  of  the 
weights  be  denoted  by  w,  and  that  of  the  substance  by  w',  and 
the  weight  of  the  substance  in  grams  by  a,  then 

—  =  number  of  c.c.  of  air  displaced  by  the  weights,  and 
7= number  of  c.c.  of  air  displaced  by  the    substance  weighed; 


12  QUANTITATIVE  ANALYSIS. 

therefore 
—,——  multiplied  by  the  weight  of  a  c.c.  of  air  at  the  time  of 

weighing  gives  the  amount  to  be  added  to  obtain  the  weight  in 
vacuo. 

17.  Methods  of  Weighing. — The  simplest  method  of  weighirg 
consists  in  adding  or  removing  weights  until  the  pointer  does 
not  move  when  the  pans  are  released  by  a  steady  pressure  on 
the  button  connected  with  the  spring  supporting  the  pans.  This 
method  will  give  the  true  weight  within  three-tenths  of  a  milligram 
on  a  good  balance.  When  the  weight  must  be  obtained  more 
accurately,  the  balance  is  set  swinging  and  the  point  of  rest  found 
by  the  method  used  for  finding  the  zero-point.  By  adjusting  the 
weights  the  point  of  rest  may  be  made  to  coincide  with  the  zero- 
point  of  the  empty  pans.  When  the  highest  possible  accuracy 
is  desired,  the  weights  are  adjusted  until  the  point  of  rest  nearly 
coincides  with  the  zero-point  of  the  empty  pans.  The  point  of 
rest  is  accurately  found  by  reading  the  swings.  The  weight 
required  to  make  the  point  of  rest  coincide  with  the  zero-point 
is  then  calculated  by  means  of  the  sensibility  already  found.  If 
this  sensibility  is  2  divisions  of  the  scale  and  the  zero-point  10.1, 
while  the  point  of  rest  with  weights  on  the  pan  is  10.7,  an  addi- 
tional weight  of  .3  milligram  ( — : — ~ — —  )  would  be  required  to  bal- 
ance the  object  being  weighed.  This  method  is  called  weighing 
by  means  of  the  sensibility. 

If  the  distances  between  the  central  and  the  terminal  knife- 
edges  are  not  equal,  the  true  equality  between  the  substances  in 
the  pans  will  not  be  obtained  by  the  methods  of  weighing  already 
given.  The  substance  suspended  from  the  longer  arm  will  have 
the  greater  leverage,  and  must  therefore  be  counterbalanced  by 
a  larger  mass  in  the  other  pan.  In  practice  it  is  found  that  bal- 
ance-makers frequently  fail  to  attain  the  ideal  of  a  balance  with 
arms  equal  within  the  limit  of  error  of  weighing.  In  ordinary 
work  this  source  of  error  may  be  avoided  by  always  placing  the 
substance  to  be  weighed  on  the  same  side.  As  the  same  error  is 
in  this  manner  introduced  into  the  weight  of  the  original  substance 
and  of  the  separated  constituent,  the  percentage  is  not  affected. 


THE  BALANCE.  13 

Even  if  constructed  equal,  unequal  heating  of  the  two  sides  of  the 
balance  may  change  the  relative  lengths  of  the  arms  of  the  beam. 
If  the  true  weight  of  a  substance  is  desired,  it  may  be  found  by 
counterbalancing  it  exactly  with  sand,  shot,  or  other  convenient 
material  and  then  replacing  the  unknown  substance  in  the  pan 
by  weights  until  the  counterbalance  is  exactly  restored.  This 
method  of  weighing  is  called  the  method  of  substitution. 

Another  method  is  to  weigh  a  substance  in  one  pan  and  then 
exchange  weights  and  substance  and  weigh  again.  The  mean 
of  the  weights  obtained  is  taken  as  the  weight  of  the  substance. 
The  true  weight  will  be  equal  to  the  square  root  of  the  product  of 
these  two  weights.  As  in  practice  these  two  weights  are  very 
nearly  equal,  their  mean  differs  from  the  square  root  of  their  prod- 
uct by  an  amount  far  less  than  the  experimental  error. 

Let  R  =  length  of  the  right  arm,  and 
L=     "      "    "    left  arm; 
w  =  true  weight ; 

wf  =  apparent  weight  in  left  pan; 
u/'=       "  "      "  right  pan. 

From  the  principle  of  the  lever, 

w'L=wR    and    wL=w"R, 

whence 

L  __»_«£ 

R  ~~w'~  w' 

Therefore  

w  =\/w'w" . 

If  the  arms  are  very  nearly  equal,  w'  and  w"  will  have  such 

w'  +w" 
nearly  equal  values  that  one  may  place  w  =    —^ • 

18.  Testing  Balance  for  Equality  of  Arms. — By  this  method  a 
balance  can  be  tested  for  equality  of  arms;  for  if  they  are  equal,  a 
substance  should  weigh  the  same  in  either  pan.  The  relative 
lengths  can  be  ascertained  by  weighing  a  ten-gram  weight  first 
on  one  pan  and  then  on  the  other.  A  small  weight  will  have  to  be 


14  QUANTITATIVE  ANALYSIS. 

added   or  subtracted   each   time   to   produce  equilibrium.    Let 
these  small  weights  be  designated  as  m  and  n. 

Let  A  =one  of  the  10-gram  weights; 

B=  the  second      "       weight; 

A  be  the  weight  which  is  being  weighed,  and  when  it  is 
placed  in  the  right-hand  pan  its  weight  is  found  to  be  B+m, 
while  when  it  is  placed  in  the  left-hand  pan  its  weight  is  equal 
to  B  +n.  If  the  weights  as  well  as  the  arms  of  the  balance  are 
equal,  m  and  n  will  each  equal  zero.  If  A  and  B  are  unequal  or 
equal,  and  the  arms  of  the  balance  unequal,  m  and  n  will  be 
unequal.  By  the  principle  of  the  lever  as  above, 

AR  =L(B  +ra), 
AL=R(B+n). 

Solving  these  equations,  we  obtain 

?          m—n 
L~     ~~2B~' 

19.  Errors  in  Weighing. — The  weighing  of  large  objects  is 
attended  by  considerable  error,  because  the  surfaces  condense  air 
and  water  vapor,  forming  a  layer  whose  thickness  varies  with  the 
temperature  and  the  degree  of  humidity  of  the  atmosphere.  This 
error  may  be  avoided  by  using  a  counterpoise,  which  is  identical 
hi  shape  and  size  with  the  substance  weighed.  Another  source 
of  error  arises  from  efforts  which  are  made  to  clean  objects  very 
thoroughly,  especially  glass.  For  this  purpose  a  vigorous  rub- 
bing is  frequently  given.  This  may  give  rise  to  a  static  electrifi- 
cation of  the  surface,  which  produces  attraction  or  repulsion 
between  the  object  to  be  weighed  and  neighboring  bodies. 
Changes  in  weight  to  the  extent  of  6  milligrams  have  been  pro- 
duced in  this  manner.  Under  favorable  conditions  such  a  charge 
may  remain  from  20  minutes  to  half  an  hour  before  being  dissi- 
pated. It  is  therefore  advisable  to  allow  glass  vessels  to  stand 
in  the  balance-case  at  least  20  minutes  after  a  thorough  cleaning 
before  weighing. 


THE  BALANCE.  15 

EXERCISE  i. 
Determination  of  the  Sensibility  of  a  Balance. 

Ascertain  the  sensibility  of  a  balance  by  first  finding  the  zero-point  with 
empty  pans.  Then  place  a  one-milligram  weight  on  the  right-hand  pan  or 
place  the  rider  on  the  one-milligram  division  and  again  find  the  zero-point. 
The  number  of  divisions  on  the  scale  between  these  two  points  is  the 
sensibility  for  zero-load.  In  taking  the  zero-point  the  balance  is  brought 
into  vibration  by  opening  the  door  and  directing  a  current  of  air  against 
one  of  the  pans  by  a  quick  downward  movement  of  the  hand.  Aftei 
closing  the  door,  note  the  distance  the  pointer  moves  to  the  right  and  to 
the  left  across  the  scale.  Read  to  one-tenth  of  the  scale-divisions,  taking 
two  readings  on  one  side  and  three  on  the  other.  If  the  readings  are  irregu- 
lar, more  must  be  taken  until  the  difference  between  successive  readings 
is  approximately  constant.  Average  the  readings  on  each  side  and  com- 
pute the  zero-point  or  place  where  the  pointer  would  stop.  The  computa- 
tion is  made  as  follows: 

7.7  13.8 

7.9  13.6  7.9  +  13.7 

8.2  Zero-point =  10.8 

Av 7.9  13.7 

After  adding  a  one-milligram  weight  to  the  right-hand  pan  the  zero-point 
is  determined  again  in  the  same  manner.  If,  for  instance,  7.9  was  found 
as  the  zero-point  with  one  milligram  on  the  right-hand  pan,  then  the  sensi- 
bility will  be  10.8  —  7.9  =  2.9  divisions.  The  sensibility  of  the  balance  with 
loads  of  5,  10,  20,  30,  40,  50,  70,  and  90  grams  is  now  obtained.  For  this 
purpose  a  5-gram  weight  is  placed  in  each  pan  and  the  zero-point  taken. 
A  one-milligram  weight  is  added  to  the  right-hand  pan  and  the  zero  point 
again  taken.  The  number  of  divisions  of  the  scale  between  these  two  points 
is  the  sensibility  of  the  balance  with  a  load  of  five  grams.  If,  when  the 
weights  of  equal  denomination  are  placed  on  the  opposite  pans,  the  swings 
of  the  pointer  indicate  that  the  zero-point  is  off  the  scale  or  very  far  from 
its  centre,  a  small  fractional  weight  of  suitable  size  must  be  added  to  one 
pan  so  as  to  bring  the  zero-point  near  the  centre  of  the  scale.  This  is  most 
conveniently  done  by  placing  the  lighter  weight  on  the  right-hand  pan  and 
moving  the  rider  until  equality  is  nearly  reached.  The  zero-point  is  then 
carefully  taken  and  the  rider  moved  one  division  further  to  the  right  or  to 
the  left  and  the  zero-point  taken  again.  The  number  of  divisions  between 
these  two  points  gives  the  sensibility  with  the  load  used. 

20.  Maximum  Load  of  Balance. — If  the  balance  shows  a  marked  decrease 
in  sensibility  at  any  given  load,  the  limit  of  its  safe  use  has  been  reached 
and  no  increase  of  the  load  should  be  made.  A  decrease  of  50  per  cent  of 
the  maximum  sensibility  indicates  overloading  of  the  balance.  The  sensi- 
bilities obtained  together  with  the  corresponding  loads  should  be  recorded 
in  the  laboratory  note-book  in  tabular  form  for  future  use. 


16  QUANTITATIVE  ANALYSIS. 

EXERCISE  2. 
Determination  of  the  Relative  Length  of  the  Arms  of  a  Balance. 

Determine  the  zero-point  of  the  balance  with  empty  pans.  Ha\ing 
marked  one  of  the  10-gram  weights  so  as  to  distinguish  it  from  the  other,  and 
calling  this  weight  A ,  place  it  on  the  right-hand  pan.  Place  the  other  10-gram 
weight,  which  is  det>nated  B,  on  the  left-hand  pan.  If  the  zero-point 
has  not  changed  materially,  locate  it  accurately  by  taking  a  set  of  readings. 
The  weight  in  milligrams  corresponding  to  a  change  in  the  position  of  the 
zero-point  is  found  by  dividing  the  number  of  divisions  the  zero-point  has 
been  displaced  by  the  sensibility  at  the  given  load.  For  example,  if  the 
zero-point  with  empty  pans  was  11.1,  and  after  A  and  B  had  been  placed 
on  the  pans  it  was  7.9  and  the  sensibility  with  10  grams  load  is  2.9  divi- 
sions, then  I  —  29  )  =  1  *  milligrams  must  be  added  to  B  to  make  it 

weigh  as  much  as  A.  This  small  weight  is  m.  Interchanging  the  weights  and 
again  taking  the  zero-point  and  calculating,  we  obtain  another  small  weight 
to  be  added  or  subtracted  from  B,  which  is  designated  n.  For  example, 
let  it  be  —2.1  nig.  The  relative  length  of  the  arm  is  obtained  from  the 
formula 


In  the  illustration  given, 


In  this  case  the  right  arm  is  ^nger  than  the  left,  nd  the  true  weight  of  a 
substance  would  be  obtained  by  multiplying  its  apparent  weight  by  the 
factor  1  00016. 

EXERCISE  3. 
Calibration  of  Weights. 

For  identification  after  calibration,  one  of  the  duplicate  weights  is  in 
each  case  marked  by  a  sharp  steel  point.  In  the  case  of  the  two  100 
milligram  weights,  the  unmarked  one  is  recorded  as  100  mg.,  while  the 
marked  one  is  indicated  by  100  mg.  If  three  weights  of  equal  denomi- 
nation are  found  in  the  same  set,  the  third  one  is  marked  twice  and  two 
horizontal  bars  over  its  denomination  serve  to  distinguish  it  from  the 
remainder.  If  the  arms  of  the  balance  have  been  found  to  be  equal, 
the  weights  may  be  compared  by  the  ordinary  process  of  weighing.  If,  as 
is  frequently  the  case,  the  arms  are  unequal,  one  of  the  methods  already 
given  for  obtaining  the  true  weight  of  a  substance  must  be  used.  The 
method  of  substitution  is  very  convenient  for  this  purpose.  A  second  set 
of  weights  is  necessary  in  using  this  method.  The  weights  of  this  set  are 
placed  on  the  left-hand  pan  only  and  are  used  merely  as  counterpoise. 
For  comparison  of  the  10-milligram  weights  the  10-milligram  counterpoise 


THE  BALANCE.  1? 

is  placed  on  the  left-hand  pan  and  one  of  the  two  10-milligram  weights 
placed  on  the  right-hand  pan  and  the  zero-point  taken.  The  other  10- 
milligram  weight  is  then  substituted  for  the  one  on  the  right-hand  pan 
and  the  zero-point  again  taken.  If  these  two  zero-points  are  identical, 
the  two  weights  are  equal.  If  they  are  not  identical,  the  number  of  divisions 
of  the  scale  between  them  is  divided  by  the  sensibility.  This  gives  the 
difference  in  weight  of  the  weights  compared,  the  heavier  one  being  the 
one  which  gave  the  zero-point  farthest  to  the  left.  This  relation  is  recorded 
as  in  the  following  instance:  10  mg.  =  10  mg.+  .1  mg. 

In  comparing  the  gram  weights  the  differences  may  be  greater  than 
one  milligram.  The  rider  or  other  fractional  weights  must  then  be  used  to 
obtain  equality  between  the  counterpoise  and  the  weights  compared. 

If  the  rider  is  10  or  12  milligrams  it  is  also  compared  with  one  of  the 
10-milligram  weights.  If  the  rider  is  5  or  6  milligrams,  it  must  be  com- 
pared with  the  5-milligram  weight  and  these  two  compared  with  one  of  the 
10-milligram  weights.  The  following  comparisons  are  also  made: 


10  mg.+ 1 J  mg with    20  mg. 

20    "  +  10mg.+  10mg.  +  rider  (orrider+5  mg.) "  50     " 

50    "  +  20mg.+  10mg.+  10mg.+rider "  100    " 

100.   "    .  .^ "  100    " 

100    "  +100mg.  ..J_1_. "  200    " 

200    "   +100  mg.+  lOO  mg.+  50  mg.+20  mg.+  lO  mg.+ 

lOmg.  +  rider ^ "  500     " 

500    "  +200  mg.+  lOO  mg.+  lOO  mg.+50  mg.+20  mg.+ 

10  mg.,  etc "  1  gram 

1  gram+ fractionals "  2  gram 

2  grams with     2  grams 

2      "     +  2~  grams  +J  gram "  5     " 

5      '*     +2  grams +2  grams +1  gram "  10     " 

10      "      ._ "  10     " 

10      u      +10  grams.  ..._._ ^_    "  20     " 

20      "     +10   grams+10   grams+5   grams+2   grams+2 

grams+ 1  gram "  50     " 

21.  Calculation  of  the  Results  of  the  Calibration. — The  method  of  calcu- 
lating the  results  is  shown  by  the  following  example  of  the  calibration 
of  a  set  of  weights.  The  following  comparisons  were  made  with  the  results 
indicated : 

5  mg.  =  rider +  .  1  mg. 

10    "    -5mg.+rider -.1    " 

10    "    -lOmg.  .„_ -.1    " 

20    "    =10mg.+  10mg.  . ._._ +.1    " 

50    "    =20mg.+  10mg.+  10mg.+^_mg.+rider +.1    " 

100    "    =50mg.  +  20mg.  +  10mg.+  10mg.  +  5mg.+rider +.1    " 


18  QUANTITATIVE  ANALYSIS. 

100  mg.  =  100  mg.  • -  .1  mg. 

200    "    =  100mg.  +  iOOmg +  .2    " 

500    "    = fractional  weights +  .  2    " 

1  gram  =  fractional  weights —  .  2    " 

2  grams  =  fractional  weights+ 1  gram +  .  2    " 

2      "      =  2"  grams.  .^ 

5      "      =2  grams +2  grams +1  gram +.5  " 

10      "      —  5  grams  +2  grams +2  grams  +1  gram +  1.0  " 

10      "      =  10  grams.  .^ 

20      "      =  10  grams+  To  grams.  .  ._._ +  .4  " 

50      "      =  20  grams+10  grams+10  grams+5  grams+2  grams 

+  2  grams+ 1  gram +  2.0  " 

Using  the  5-milligram  weight  as  the  basis  of  comparison  and  assuming 
for  this  comparison  that  it  weighs  exactly  5  milligrams,  we  would  obtain 
the  following  values  for  the  fractional  weights,  where  the  values  to  the  left 
of  the  equality  sign  merely  designate  the  weights  being  calibrated,  while 
the  figures  to  the  right  give  the  comparative  values  of  these  weights  in 
terms  of  the  5-milligram  weight: 

5  mg.  =  .005    gram  50  mg.  =  .0491  gram 

Rider    =.0049     "  100    "    =.0982     " 

10mg.  =  .0098     "  100    "    =.0982      " 

10    "    =.0097     "  200    "    =.1963      " 

20    "    =.0196     "  500    "    =.4910     " 

Sum  of  fractional  weights  =.  9818      " 

The  one-gram  weight  is  taken  as  the  standard  for  the  entire  set  of  weights 
and  is  assumed  to  be  correct  or  equal  to  1.0000  gram.  As  the  sum  of  the 
fractional  weights  have  been  found  to  be  .0002  gram  heavier  than  the  1-gram 
weight,  the  sum  of  these  weights  should  be  1.0002  grams.  As  by  assuming 
the  5-mg.  weight  to  be  correct,  we  obtained  a  total  of  .9818  gram,  we  must 
increase  the  values  assigned  by  .0184  gram  (1.0002 -.9818  gram).  Half 
of  this  increase  must  be  added  to  the  500-mg.  weight,  one-fifth  to  the  value 
given  to  the  200-mg.  weight,  etc.  The  following  values  will  result  from 
this  calculation : 

500  mg.  =  .5002  gram (.4910+  .0092) 

200    "  =.2000  "  (.1963+. 0037) 

100    "  =.1000  ''  (.0982+. 0018) 

100    "  =.1000  "  (.0982+. 0018) 

50    "  =.0500  "  (.0491+. 0009) 

20    "  =.0200  "  (.0196+. 0004) 

10    "  =.0099  "  (.0097+. 0002) 

10    "  =.0100  "  (.0098+. 0002) 

5    "  =.0051  "  (.0050+. 0001) 

Rider  =.0050  "  (.0049+ .0001) 


THE  BALANCE.  19 

Assuming  the  1-gram  weight  to  be  correct,  the  following  values  are 
obtained  for  the  larger  weights: 

1  gram  =  1 . 0000  gram  10  grams  =  10 . 0031  grams 

2  grams  =2. 0004  grams  10      "      =10.0031      " 
2      "      =2.0004      "  20      "      =20.0066      " 
5     "      =5.0013      "  50     "      =50.0169     " 

When  careful  work  is  being  done  the  weight  of  a  substance  is  obtained 
by  using  the  values  found  by  the  calibration  instead  of  those  marked  on 
the  weights. 

22.  Weighing  Substances  Containing  Water. — Most  substances 
submitted  to  analysis  may  be  weighed  on  watch-glasses  without 
any  appreciable  change  in  weight  during  weighing  due  to  loss  or 
gain  of  water.     If  any  doubt  exists  on  this  point,  the  substance 
after  being  exactly  counterbalanced  may  be  left  on  the  scale-pan 
for  about  the  same  length  rf  time  as  was  required  for  weighing 
and  its  weight  again  taken.     If  any  change  has  occurred,  the 
substance  must   be  weighed  in  a  weighing-bottle.    These  bottles 
are  made  of  thin  glass  with  ground-glass  stoppers,  and  may  be 
obtained  of  any  suitable  size.     The  substance  to  be  analyzed  is 
quickly  transferred  to  the  bottle  after  having  been  brought  to 
the  condition,  so  far  as  moisture  is  concerned,  in  which  it  is  desired 
to  analyze  it.     The  bottle  is  then  weighed  and  a  sufficient  quan- 
tity of  the  contents  carefully  tipped  out  into  a  beaker  or  other 
vessel  in  which  to  operate  on  the  compound.     The  stopper  is 
quickly  replaced  and  the  bottle  weighed.     The  difference  is  the 
amount  taken  out  for  analysis. 

23.  Use  of  Desiccator. — After  a  substance  has  been  heated,  and 
while  it  is  cooling,  preparatory  to  being  weighed,  it  should  be 
placed  in  a  desiccator,  which  is  a  glass  vessel  containing  a  dehy- 
drating agent,  such  as  concentrated  sulphuric  acid  or  fused  cal- 
cium chloride.     A  cover  is  ground  to  fit  the  desiccator,  and  the 
joint  is  made  air-tight  by  greasing  the  ground  surface  with  vasel- 
ine.    In  the  dry  air  of  the  desiccator  most  substances  cool  without 
absorbing  moisture.     It  is  best  to  weigh  a  substance  as  soon  as 
cool,  usually  in  ten  to  twenty  minutes.     Some  substances  begin 
to  absorb  moisture  as  soon  as  placed  on  the  scale-pan.     Such 
substances  should  be  weighed  as  quickly  as  possible,  reheated, 
cooled,   and   weighed.     In   taking   the   second   weight,   however, 


20  QUANTITATIVE  ANALYSIS. 

the  weights  should  be  placed  on  the  scale-pan  before  the  sub- 
stance to  be  weighed  is  taken  out  of  the  desiccator,  so  that  as 
soon  as  it  is  placed  on  the  scale-pan  the  weight  may  be  ascer- 
tained by  taking  the  swings  of  the  pointer  or  by  moving  the  rider. 

24.  Rules  to  be  Observed  in  Using  the  Balance. — The  pans 
should  be  brushed  off  with  a  earner s-hair  brush  before  weighing. 

Care  should  be  taken  not  to  spill  any  substance  whatever  on 
the  floor  of  the  balance-case.  It  should  always  be  kept  clean. 

Students  should  not  adjust  or  attempt  to  repair  a  balance. 
Report  any  irregularities  to  the  instructor. 

All  volatile  liquids  and  acids  must  be  weighed  in  closed  weigh- 
ing-bottles. 

Only  metals,  alloys,  and  pieces  of  minerals  should  be  placed 
directly  on  the  pans  of  the  balance.  All  other  substances  should 
be  weighed  on  watch-crystals  or  in  a  weighing-bottle.  The 
beam  and  pans  must  always  be  supported  when  substances  which 
weigh  more  than  a  gram  are  taken  off  or  placed  on  the  pans. 

The  substance  to  be  weighed  must  always  be  placed  on  the 
left-hand  pan,  and  the  weights  on  the  right-hand  pan. 

Always  count  the  weights  twice — first,  by  noting  the  ones 
missing  from  the  box;  second,  by  noting  the  weights  when  taking 
them  off  the  pan.  An  error  in  noting  the  weight  of  the  sub- 
stance to  be  analyzed  frequently  involves  serious  loss  of  time. 

Crucibles  and  substances  which  are  to  be  weighed  must  be 
allowed  to  become  perfectly  cold  before  being  placed  on  the  scale- 
pan.  A  warm  substance  produces  a  current  of  air  which  by.  its 
buoyant  effect  apparently  lessens  the  weight.  The  amount  of 
air  and  moisture  condensed  on  the  surface  is  always  less  in  the 
case  of  a  warm  body.  Allow  at  least  fifteen  minutes  for  a  platinum 
crucible  and  precipitate  to  cool  in  a  desiccator,  while  twenty 
minutes  should  be  allowed  for  the  cooling  of  a  porcelain  crucible 
under  the  same  conditions. 

Lower  the  support  of  the  beam  to  start  the  balance  swinging 
with  a  slow  even  motion  so  as  not  to  damage  the  delicate  knife- 
edges  by  suddenly  bringing  them  against  the  planes.  When 
through  weighing,  be  sure  not  to  leave  the  balance  swinging,  and 
remove  the  rider  from  the  beam.  Note  all  weights  in  a  book. 
Do  not  use  scraps  of  paper. 


CHAPTER  II. 
GENERAL  OPERATIONS. 

A  NUMBER  of  the  operations  of  gravimetric  analysis  are  com- 
mon to  most  determinations,  no  matter  what  element  is  concerned 
or  employed.  The  element  to  be  determined  must  first  be  PRE- 
CIPITATED by  the  addition  of  the  reagent  necessary  to  form  an 
insoluble  compound.  The  precipitate  must  be  separated  from 
the  solution  by  FILTRATION  and  WASHING,  and  finally  the  precipi- 
tate must  be  DRIED  and  WEIGHED. 

25.  Precipitation. — A  careful  study  of  the  solubility  of  the  various 
compounds  of  an  element  must  be  made  before  the  most  suitable 
form  for  precipitation  can  be  selected.  The  most  insoluble  pre- 
cipitate is  generally  the  best  form.  The  solubility  in  both  hot 
and  cold  water  as  well  as  in  a  solution  of  the  salts  present  under 
the  conditions  of  precipitation  must  be  studied.  In  many  cases 
the  solubility  of  a  precipitate  is  very  much  reduced  by  the  pres- 
ence of  an  excess  of  the  precipitating  reagent  in  the  solution. 
This  follows  from  the  law  of  ionization  of  a  saturated  solution. 
All  inorganic  salts,  acids,  and  bases  dissociate  in  water  solution 
into  parts  called  ions,  which  in  the  case  of  salts  are,  on  the  one 
hand,  the  atoms  of  the  metal,  and  on  the  other  hand  the  acid 
radical.  Barium  sulphate,  for  instance,  breaks  up  into  Ba  and 
S04.  Acids  dissociate,  giving  the  acid  hydrogen  atom  H  as  one 
ion,  and  the  remainder  of  the  acid  as  the  other  ion.  Bases  dis- 
sociate, giving  OH  as  one  ion  and  the  metallic  atom  as  the  other 
ion.  If  a  and  b  represent  the  number  of  ions  in  unit  volume  of  a 
solution  to  be  precipitated,  and  ab  the  number  of  undissociated 

molecules,  then  the  equation  • — r-=  c,  where  c  is  a  constant,  ex- 
presses the  relation  between  these  quantities.  Taking  barium  sul- 
phate as  an  example,  in  a  saturated  solution  of  this  salt  there  will 
be  present  some  undissociated  molecules,  some  Ba  ions  and  some  S04 

21 


22  QUANTITATIVE  ANALYSIS. 

ions.  If,  now,  some  barium  chloride  molecules  are  added  to  the 
solution,  this  salt  will  dissociate  into  Ba  and  Cl  ions.  The  relation 
of  the  number  of  ions  and  molecules  expressed  by  the  equation 

— T~=C  no  longer  holds  for  the  solution,  since  a  or  the  number  of 

barium  ions  has  been  increased  by  the  barium  ions  produced  by 
the  dissociation  of  the  barium  chloride.  As  c  is  found  by  experi- 
ment to  remain  constant,  a  change  in  the  values  of  b  and  ab  must 
take  place.  If  some  of  the  sulphate  ions  (b)  unite  with  an  equal 
number  of  barium  ions  (a),  there  will  be  produced  some  mole- 
cules of  barium  sulphate  (ab),  the  numerator  of  the  fraction  will 
have  been  decreased,  and  the  denominator  increased.  This  proc- 
ess will  continue  until  the  former  value  of  c  will  have  been  reached 
and  equilibrium  restored.  As  the  solution  was  already  saturated 
with  barium  sulphate,  the  added  number  of  molecules  of  this  salt 
must  be  precipitated.  If,  therefore,  the  salt  used  in  precipitation  is 
quite  insoluble,  an  excess  of  the  reagent  containing  one  of  the  ions 
of  this  salt  will  dimmish  the  solubility  to  a  negligible  quantity,  by 

increasing  the  values  of  a  or  b  in  the  general  equation,  — jp=c- 

In  some  cases  an  excess  of  the  reagent  used  in  precipitation  is 
to  be  avoided,  because  the  precipitate  redissolves  in  the  excess  of 
the  reagent.  Aluminium  hydroxide,  for  instance,  redissolves  to 
an  appreciable  extent  in  an  excess  of  ammonia,  the  aluminium 
probably  acting  as  an  acid,  and  forming  an  aluminate  with  the 
base.  Silver  chloride  dissolves  in  an  excess  of  sodium  chloride 
or  hydrochloric  acid,  forming  a  double  chloride.  In  such  cases  as 
these,  care  must  betaken  to  add  only  a  very  slight  excess. 

26.  Contamination  of  the  Precipitate  with  Soluble  Salts. — 
Another  difficulty  met  with  in  precipitation  is  the  tendency  of 
many  salts  to  carry  down  other  salts  which  are  themselves  very 
soluble.  It  is  frequently  impossible  to  wash  such  a  precipitate 
free  from  the  contamination  In  many  cases  the  precipitate 
must  be  redissolved  and  reprecipitated.  As  the  solution  contain- 
ing the  bulk  of  the  soluble  salt  is  separated  from  the  insoluble 
salt  by  the  first  precipitation,  during  the  second  precipitation  only 
a  small  amount  of  the  soluble  salt  is  present,  and  the  rather  small 
fraction  of  this  amount  which  is  carried  down  may  be  neglected. 


GENERAL  OPERATIONS.  23 

Various  theories  have  been  given  to  account  for  this  phenomenon. 
If  the  precipitant  is  added  rapidly,  particles  of  the  solid  will,  no 
doubt,  surround  and  enclose  portions  of  the  solution.  For  this 
reason  it  is  advisable  to  add  the  precipitating  reagent  in  a  fine 
stream,  with  vigorous  stirring  of  the  solution.  Heating  the  solu- 
tion also  increases  the  solubility  of  the  soluble  salts. 

Besides  the  mechanical  action  of  a  precipitate,  it  is  also 
undoubtedly  true  that  more  or  less  insoluble  chemical  compounds 
are  formed  by  the  union  of  soluble  and  insoluble  salts.  Barium 
sulphate  has  been  shown  to  unite  with  ferric  sulphate,  forming  a 
double  sulphate  of  barium  and  iron  which  loses  S03  on  ignition, 
leaving  the  sulphate  of  barium  colored  red  with  ferric  oxide. 

27.  Digestion  of  Precipitates. — Many  very  insoluble  precipi- 
tates, such  as  barium  sulphate  and  calcium  oxalate,  come  down 
in  such  a  fine  state  of  subdivision,  that  in  attempting  to  filter 
them,  they  pass  through  the  pores  of  the  paper.  Such  precipitates 
should  be  allowed  to  stand  for  a  considerable  time  in  contact  with 
the  mother-liquor,  as  the  size  of  the  particles  will  then  increase. 
The  same  object  is  more  quickly  accomplished  if  the  solution  is 
boiling  during  the  addition  of  the  precipitant,  and  is  then  allowed 
to  digest  at  a  temperature  near  the  boiling-point.  This  change 
in  the  size  of  the  particles  is  due  to  the  well-known  fact  that  the 
solubility  of  a  finely  divided  substance  is  greater  than  that  of 
large  particles  of  the  same  substance.  If  both  large  and  small 
particles  are  present  in  contact  with  the  solution,  the  smaller 
ones  will  tend  to  dissolve  and  produce  a  solution  which  is  super- 
saturated with  respect  to  the  large  particles.  This  process  is 
quite  rapid  in  the  case  of  moderately  soluble  salts,  but  with  the 
insoluble  compounds  used  in  quantitative  analysis,  the  action  is 
very  slow.  Heating  the  solution  increases  the  solubility,*  and 
therefore  accelerates  the  solution  of  the  fine  powder.  The  final 
result  of  the  process  is  that  the  small  particles  dissolve  and  the 
large  particles  grow  in  size  by  the  addition  of  the  small  ones. 

As  the  solubility  of  the  small  crystals  is  greater  than  that  of 
the  large  ones,  complete  precipitation  is  not  obtained  unless  the 

*  Though  this  is  generally  true,  there  are  a  few  substances  which  are  more 
soluble  in  cold  than  in  hot  water.  Among  these  are  lithium  and  strontium  sul- 
phates and  calcium  hydroxide. 


24  QUANTITATIVE  ANALYSIS. 

precipitate  is  allowed  to  stand  for  some  time  in  contact  with  the 
solution.  Digesting  the  precipitate  hot  hastens  this  process,  as 
it  removes  the  small  crystals.  Shaking  the  precipitate  thor- 
oughly accomplishes  the  same  object,  as  it  hastens  the  solution 
of  the  small  particles  by  bringing  them  into  contact  with  the 
solvent  and  prevents  supersaturation  by  bringing  the  large  par- 
ticles into  contact  with  the  solution. 

28.  Filtration. — The  separation  of  the  precipitate  from  the 
•solution  is  accomplished  by  nitration.  The  difficulties  encoun- 
tered at  this  point  are  of  two  kinds:  Finely  divided  precipitates 
pass  through  the  pores  of  the  paper  or  other  filtering  medium, 
while  gelatinous  solids. prevent  the  solution  from  passing  through. 
The  former  difficulty  is  overcome  by  digesting  the  precipitate 
with  the  solution  in  a  warm  place.  The  filter-paper  should  first 
be  moistened  with  water,  and  if  the  precipitate  comes  through, 
the  first  portions  of  the  turbid  filtrate  should  be  repeatedly  passed 
through  the  filter-paper  until  it  becomes  clear.  The  pores  of  the 
paper  will  then  be  filled  with  the  precipitate.  The  filtration  is 
most  rapidly  accomplished  if  the  solution  is  hot,  boiling  water 
passing  through  the  pores  of  paper  about  six  times  as  fast  as  cold 
water.  If  the  precipitate  is  gelatinous,  so  that  filtration  is  slow 
and  the  precipitate  does  not  pass  through,  recourse  may  be  had 
to  filtration  by  suction.  The  most  convenient  way  of  producing 
a  vacuum  is  by  the  Bunsen  filter-pump.  A  great  many  forms  of 
such  pumps  are  on  the  market,  varying  in  efficiency  and  durabil- 
ity. The  paper  must  be  fitted  perfectly  to  the  funnel,  and  the 
point  supported  by  means  of  a  perforated  platinum  cone  or  a 
small  square  of  linen  or  some  similar  device.  The  piece  of  linen, 
which  should  be  about  3  cm.  square,  is  neatly  and  evenly  folded 
on  the  centre  of  the  filter-paper,  which  is  then  folded  again  in 
the  usual  manner.  The  paper  must  be  carefully  fitted  to  the 
funnel,  and  pressed  firmly  down  into  the  apex  and  then  held  there 
by  the  index  finger  of  the  left  hand  while  it  is  moistened  with  a  little 
distilled  water  from  the  wash-bottle.  All  parts  of  the  paper  must 
be  in  contact  with  the  glass.  If  the  sides  of  the  funnel  are  uneven, 
this  is  impossible  and  another  funnel  must  be  selected.  A  funnel 
whose  sides  curve  down  to  the  stem  must  always  be  rejected. 

Very  few  funnels  are  made  with  the  correct  angle,  so  that  the 


GENERAL  OPERATIONS. 


25 


filter-paper  must  generally  be  folded  with  the  edges  uneven,  as 

shown  at  c  in  Fig.  4.     If  the  paper  as 

folded  does  not  fit  the  funnel,  it  must 

be  folded  again,   allowing  a  larger  or 

smaller  lap.     When  the  paper  has  been 

placed  in  the  funnel,  the  latter  should 

be  inserted  into  the  rubber  stopper  of 

the  filter-flask,  and    after   filling  with 

water  the  suction  applied.     If  the  paper 

does  not  break  while  the  water  is  being 

sucked   through,  it  has  been  properly 

fitted  to  the  funnel  and  the  solution  to  be  filtered  may  be  poured 

on  it. 


FIG.  5. 

A  much  simpler  method  of  filtering  by  suction  is  by  the  use 
of  the  GOOCH  CRUCIBLE,  as  shown  in  Fig.  5.  The  bottom  of  this 
crucible  consists  of  a  plate  which  is  perforated  with  a  large  num- 


26  QUANTITATIVE  ANALYSIS. 

ber  of  small  holes.  A  layer  of  asbestos  in  water  is  floated  over 
the  bottom  and  sucked  down  to  a  compact  mass,  the  thickness  of 
which  is  varied  according  to  the  fineness  of  the  precipitate.  The 
crucible  is  then  connected,  by  means  of  a  rubber  band,  with  a 
funnel  which  is  connected  as  usual  with  the  filter-flask.  Solu- 
tions which  attack  paper  may  be  filtered  in  this  manner.  The 
Gooch  crucible  is  especially  well  adapted  for  precipitates  which 
must  be  dried  at  temperatures  below  ignition. 

The  asbestos  must  be  purified  by  digestion  with  strong  aqua 
regia  for  several  hours  and  washing  with  water  until  free  from 
chlorides.  The  asbestos  is  then  washed  into  a  glass-stoppered 
stock  bottle.  After  the  layer  of  asbestos  has  been  formed  in  the 
crucible,  it  must  be  washed  with  water  until  fine  particles  of 
asbestos  are  no  longer  washed  out. 

29.  Washing  Precipitates. — When  a  filtering  medium  has  been 
secured  by  means  of  which  the  precipitate  can  be  rapidly  and 
completely  separated  from  the  solution,  the  next  object  of  the 
analyst  is  to  secure  thorough  washing  of  the  precipitate.  Water 
is  the  most  common  reagent  used  for  this  purpose,  and  when  pos- 
sible, it  should  be  hot.  In  some  cases  precipitates  are  insoluble 
in  water  only  when  other  salts  are  present.  Volatile  salts,  such 
as  ammonium  chloride,  are  used  for  this  purpose,  as  the  subse- 
quent ignition  expels  them.  When  non-volatile  salts  must  be 
used,  they  must  finally  be  washed  out  with  the  least  amount  of 
pure  water.  In  some  cases  alcohol  or  ammonia  serves  as  the 
washing  liquid.  The  most  rapid  and  thorough  way  of  washing 
is  to  leave  the  precipitate  in  the  beaker  and  simply  decant  the 
wash-water  through  the  paper.  The  precipitate  may  then  be 
thoroughly  mixed  with  the  successive  portions  of  wash- water 
and  finally  transferred  to  the  paper  and  the  washing  completed. 
This  process  is  called  washing  by  decantation.  When  the  precipi- 
tate is  immediately  transferred  to  the  funnel  the  amount  of  wash- 
water  used  is  less,  but  the  time  required  is  generally  greater  and 
the  washing  is  less  thorough.  Stirring  up  the  precipitate  for 
this  reason  with  the  stream  of  water  from  the  wash- bottle  is 
advantageous  and  should  be  done  whenever  possible,  and  more 
especially  when  the  precipitate  is  large.  To  avoid  spattering, 
the  stream  of  water  is  first  directed  against  a  portion  of  the  paper 
not  covered  with  the  precipitate.  In  all  cases  the  filtrate  must 


GENERAL  OPERATIONS.  27 

be  tested  for  the  constituent  to  be  washed  out,  and  for  the  final 
test  several  cubic  centimeters  should  be  taken. 

The  filtration  and  washing  of  gelatinous  precipitates  is  very 
much  facilitated  by  adding  the  paper  pulp  from  2  or  3  quanti- 
tative filter-papers  before  precipitation.  As  the  precipitate  is 
intimately  mixed  with  the  paper  pulp  the  wash-water  comes  into 
intimate  contact  with  the  precipitate.  On  igniting  such  a  pre- 
cipitate it  is  left  in  a  finely  powdered  state  and  its  re-solution  is 
comparatively  easy.  The  filter-paper  may  be  pulped  by  mois- 
tening with  a  little  concentrated  hydrochloric  acid. 

30.  Preparation  of  Pure  Salts. — Before  applying  a  method  of 
analysis  to  an  unknown  substance  it  is  necessary  to  analyze  some 
substance  of  known  composition  in  order  to  test  the  method  and 
the  individual's  skill  in  using  it.     Owirg  to  personal  peculiarities, 
individual   chemists   sometimes   utterly  fail   to   get   satisfactory 
results  by  using  methods  which  in  the  hands  of  others  give  uni- 
formly satisfactory  results.     If,  therefore,  a  method  is  first  applied 
to  material  of  known  composition,  the  chance  of  failure  on  an 
unknown  substance  is  avoided.     For  this  purpose,  pure  recrys- 
tallized  and  carefully  dried  salts  are  most  frequently  used.     The 
method  of  preparing  a  number  of  these  salts  will  therefore  be 
given. 

31.  Recrystallization   is  the  method  most  commonly  used  for 
purification.     By  this  process  the  impurities  are  gotten  rid  of  in 
one  of  two  ways.     If  insoluble,  they  are  filtered  off;    if  soluble, 
they  remain  in  the  solution  when  the  salt  which  composes  the 
bulk   of   the   substance   crystallizes   out.     Recrystallization   also 
serves  the  important  purpose  of  securing  a  product  containing 
the  theoretical  amount  of  water  of  crystallization,  since  even  a 
salt  which  is  correctly  called  chemically  pure,  inasmuch  as  it  con- 
tains no  foreign  material,  may  contain  varying  amounts  of  water. 

In  most  cases  the  so-called  commercial  salts,  instead  of  the 
more  expensive  chemically  pure  quality,  may  be  used  in  the 
preparation  of  the  pure  recrystallized  salt.  If  the  salt  is  in  large 
crystals,  it  should  be  coarsely  powdered  in  a  porcelain  mortar. 
It  is  then  dissolved  in  hot  water,  so  as  to  obtain  a  nearly  satu- 
rated solution.  Any  insoluble  material  must  be  filtered  off. 
During  this  process,  the  salt  will  crystallize  out  in  the  filter-paper 
unless  the  funnel  is  kept  warm.  This  is  best  accomplished  by 


28 


QUANTITATIVE  ANALYSIS. 


means  of  some  form  of  the  hot-water  funnel,  which  is  a  hollow 
metallic  funnel  which  can  be  filled  with  wp.ter  and  heat  applied 
from  a  Bunsen  burner,  as  shown  in  Fig.  6.  A  folded  filter-paper 
should  be  employed.  If  in  spite  of  the  hot-water  funnel  the  salt 
crystallizes  out  on  the  paper,  the  solution  must  be  diluted.  The 
filtered  solution  must  be  stirred  vigorously  while  the  salt  is  crys- 
tallizing out.  This  prevents  the  formation  of  large  crystals  or 
aggregations  of  small  crystals,  which  are  very  apt  to  enclose  por- 
tions of  the  mother-liquor  in  the  interstices.  As  soon  as  the  solu- 
tion is  cold  the  crystals  should  be  filtered  off  and  as  much  of 
the  mother-liquor  as  possible  sucked  out,  while  with  a  porcelain 
spatula  or  spoon  the  same  object  is  accomplished  by  pressing 
the  crystals  into  a  compact  mass.  The  remainder  of  the  mother- 
liquor  may  be  washed  out  by  ice-cold  distilled  water,  unless  the 

salt  is  very  soluble.  The  salt 
should  be  redissolved  in  distilled 
water  and  recrystallized.  Its 
purity  is  increased  by  each  re- 
crystallization,  but  ordinarily 
two  crystallizations  are  suffi- 
cient; and  if  a  C.  P.  salt  is  used 
in  the  first  instance  one  crystal- 
lization is  frequently  sufficient. 
After  being  freed  from  the 
mother-liquor  by  suction,  press- 
ing, and  washing,  the  salt  is 
removed  from  the  funnel  and 
spread  out  on  large  sheets  of 
absorbent  or  filter-paper,  or,  still 
better,  on  a  porous  unglazed 
porcelain  plate.  The  crystals  are 
pressed  between  the  paper  or 
rubbed  on  the  plate  with  a  porce- 
lain spatula  until  no  more  liquor  is  absorbed.  It  is  then  allowed 
to  stand  in  the  air  with  occasional  stirrirg  to  break  up  large 
lumps  and  to  expose  moist  crystals  to  the  air  until  it  is  dry, 
which  can  be  determined  by  ascertaining  if  the  crystals  adhere  to 
a  clean  and  dry  glass  or  porcelain  surlace.  This  test  must  be 
made  from  time  to  time  so  that  the  salt  shall  not  be  exposed 


FIG.  6. 


GENERAL  OPERATIONS.  29 

for  too  long  a  time  to  the  air,  as  in  that  case  water  of  crystalliza- 
tion may  be  lost.  When  dry,  the  crystals  must  be  transferred 
to  a  clean  dry  glass-stoppered  bottle. 

32.  Preparation  of  Double  Salts. — The  process  of  recrystalliza- 
tion  will  by  no  means  secure  in  the  case  of  every  salt  a  degree 
of  purity  sufficient  to  permit  its  use  as  a  standard.  Many  salts 
are  not  easily  obtained  of  constant  composition  so  far  as  the  per- 
centage of  acid  and  base  are  concerned,  while  others  cannot  be 
dried  so  as  to  retain  a  definite  percentage  of  water.  Many  salts 
which  alone  cannot  be  prepared  pure  can  be  purified  by  the  for- 
mation of  double  salts,  especially  of  ammonium.  The  alums 
belong  to  this  class  as  well  as  many  double  sulphates  which  are 
not  alums,  such  as  the  double  ammonium  sulphates  of  zinc,  nickel, 
cobalt,  ferrous  iron,  as  well  as  the  double  chlorides  of  copper, 
platinum,  etc.  Some  of  these  double  salts  are  so  stable  that 
they  are  not  decomposed  by  solution  in  water,  so  that  they  can 
be  recrystallized  in  the  same  manner  as  the  simple  salts.  The 
alums  can  be  purified  in  this  manner.  The  less  stable  double 
salts  decompose  in  water  solution,  so  that  part  of  the  less  soluble 
constituent  crystallizes  out  first,  then  crystals  of  the  double  salt 
appear.  The  double  sulphate  of  magnesium  and  potassium 
behaves  in  this  manner.  This  difficulty  may  be  overcome  by 
having  an  excess  of  the  more  soluble  constituent  present.  For 
this  purpose  and  in  the  preparation  of  any  of  the  double  salts, 
the  constituents  of  the  double  salt  may  be  dissolved  separately  in 
hot  distilled  water  and  then  the  solutions  mixed  and  vigorously 
stirred  while  the  salt  crystallizes  out.  Ordinarily  the  single  salts 
may  be  taken  in  the  proportion  of  their  molecular  weights.  Fer- 
rous ammonium  sulphate  [FeS04.(NH4)2S04.6H2O]  may  be  made 
by  dissolving  the  constituents  separately  in  the  proportions  278  to 
132  [FeS04.7H.O=278  and  (NH4)2S04  =  132]  and  then  pouring 
the  solutions  together.  As  the  iron  oxidizes  easily,  the  tempera- 
ture of  the  water  must  not  be  allowed  to  exceed  40°,  and  the 
process  must  be  carried  out  rapidly.  If  an  unstable  double  salt, 
like  potassium  magnesium  sulphate  (MgS04.K,S04.6H20),  must  be 
prepared,  the  magnesium  sulphate  must  be  present  in  excess  to 
the  extent  of  J  to  J  of  its  molecular  weight.  MgS04.7H20  having 
a  molecular  weight  of  246,  we  must  use  (246  +80)  326  parts  of 
this  salt  to  174  parts  of  the  potassium  sulphate  (K2S04  =174). 


30  QUANTITATIVE  ANALYSIS. 

33.  Precipitation  by  Change  of  Solvent. — For  various  reasons 
jecrystallization  is  not  always  applicable,  and  recourse  to  other 
means  must  be  made.  Common  salt  or  sodium  chloride  differs 
so  little  in  its  solubility  in  hot  and  cold  water  that  it  cannot  con- 
veniently be  recrystallized.  Advantage  is  here  taken  of  the  fact 
that  this  salt  is  much  less  soluble  in  concentrated  hydrochloric 
acid  than  in  water.  This  property  is  based  on  the  fact  already 
explained  on  p.  21,  that  an  increase  in  the  number  of  one  of  the 
ions  of  a  salt  diminishes  the  solubility  of  that  salt.  As  chlorine 
is  one  of  the  ions  of  sodium  chloride  and  also  of  hydrochloric 
acid,  the  addition  of  the  latter  to  a  concentrated  solution  of  the 
former  precipitates  it  in  large  quantities.  As  the  impurities  are 
present  in  only  very  small  quantities,  the  solution  is  very  far 
from  being  saturated  for  these  salts,  even  when  they  are  chlorides 
of  other  metals  than  sodium.  In  order  not  to  dilute  the  solu- 
tion the  hydrochloric  acid  is  conducted  into  it  in  the  gaseous  form. 

Sodium  carbonate  may  be  purified  in  a  similar  manner  by 
conducting  carbon  dioxide  into  a  saturated  solution  of  the  sodium 
salt.  In  this  case,  however,  the  precipitate  is  the  more  insoluble 
sodium  bicarbonate  which  is  reconverted  into  the  carbonate  by 
gentle  heat.  Another  illustration  of  this  method  of  purification  is 
found  in  the  preparation  of  pure  ferrous  sulphate.  Recrystalli- 
zation  is  inadvisable  in  this  case,  because  ferrous  salts  oxidize 
very  rapidly  when  their  solutions  are  heated  in  contact  with  the 
air.  A  cold  saturated  solution  of  this  salt  is  made  and  crystallized 
out  by  the  addition  of  an  equal  volume  of  alcohol.  As  ferrous 
sulphate  is  quite  insoluble  in  50%  alcohol,  a  good  crop  of  crystals 
is  obtained  by  stirring  this  mixture.  The  crystals  are  washed 
with  alcohol  and  dried  in  the  usual  manner.  As  the  moist  crys- 
tals oxidize  quite  readily,  the  presence  of  alcohol  is  here  advan- 
tageous, since  it  facilitates  drying  by  its  volatility  and  by  dis- 
placing the  water. 

34.  Precipitation  by  Double  Decomposition.  —  Insoluble  salts 
must  be  prepared  by  precipitation  under  conditions  in  which  no 
other  substance  comes  down.  The  carbonates  of  the  alkaline 
earths  are  prepared  in  this  manner.  The  chlorides  and  nitrates 
of  these  metals  can  be  easily  obtained  in  the  necessary  state  of 
purity.  If  other  metals  are  present  besides  the  alkali  metals, 
they  may  be  removed  from  the  solution  of  the  alkaline-earth 


PURE  SALTS. 


"31 


COMPOUNDS  OF  THE  METALS  AND   ACIDS  WHICH  CAN  BE  PRE- 
PARED IN  A  PURE  CONDITION. 


METALS. 

COMPOUNDS. 

Method  of 
Purification. 

Parts  of  the  Salt  Dissolved  by  100  Parts 
of  Water  at 

0° 

20° 

100° 

Aluminium.  . 
Ammonium 

A12(SO4)3.K2S04.24H2O 
NH4C1  

R. 
R 

3.9 
29.7 



15.0 
37.2 



357.5 
77.3 
103.3 

35.7 

11.5 

68.8 

Antimony.  .  . 

Arsenic 

(NH4)2S04  

KSbOC4H4064H2O.  ... 

As2O3 

R. 

R. 

S 

70.6 
37 

7° 

5.26 

75.4 

7.75 



Barium 

BaCl2.2H2O  

R 

36  2 

41  8 

Bismuth. 

Bi20,.  . 

T 

Cadmium 

CdI2  ...                 

R. 
P 

86 

92  6 

133.3 

Calcium 

CaCO3  

Chromium.  .  . 
Cobalt 

Iceland  spar  

K2Cr2O7  

R. 
R 

5 
25  4 

13.1 

102 

CoS04  K2SO4  6H20.  ... 

52  4 

49° 

108  1 

Copper.  .  .  . 

CuSO4.5H2O  

R. 

24.2 

36.5 

204.5 

Iron  

Metal  

FeS04.(NH4)2S04.6H2O 
Metal  .  .          

R. 

18 

29.8 

75° 

78.2 

Lead  . 

Pb(N08)2  

R 

36.5 

52  3 

127 
671.2 

Magnesium.  . 

Manganese.  .  . 
Mercury  

Nickel 

MgSO4  7H2O.  . 

R 

76  9 

119.7 

37.2 
25° 

51.3 

K2S04.MgS04.6H20.  .  . 

MnSO4.(NH4)2S04.6H20 
HgCl2  
Metal  

R. 

R. 
R. 
D 

20.3 



75° 

104.6 

5.73 



7.39 

53.96 

NiSO4.(NH4)2SO4.6H2O 
KNO3  

R. 
R 

35° 

2.5 

8.1 
31.2 

85° 

39.5 

Potassium  .  .  . 
Silver  .    .    . 

13  3 

247 
56.6 
940 

KC1  
AgNO,.  . 

R. 
R. 

28.5 
122 



34.7 
227 



Sodium.  .  .  . 

Metal 

NaCl  

P 

35  7 

36.0 

39.8 

Strontium.  .  . 
Tin 

SrCO3  

P 

SnCl4  2NH4C1 

R 

14-5° 

33 

Zinc 

ZnSO4.(NH4)2S04.6H2O 

R. 

10 

17.3 

80° 

67.8 

In  the  column  headed  "Method  of  Purification,"  the  letters  have  the 
following  signification:  R.,  recrystallization ;  D.,  distillation;  S.,  sublimation; 
I.,  ignition  of  nitrate,  and  P.,  precipitation. 


32  GENERAL  OPERATIONS. 

COMPOUNDS   OF  THE  METALS    AND  ACIDS—  (Continued). 


ACIDS. 

COMPOUNDS. 

Method  of 
Purification. 

Parts  of  the  Salt  Dissolved  by  100  Parts 
of  Water  at 

Q*- 

20° 

100° 

Hydriodic.  .  . 

I.. 

s 

Hydrobromic 
Hydrochloric. 
Nitric  

KI  
KH(IO3\.  . 

11. 

R 

127.9 

15° 

1.53 

144.2 

17° 

5.4 

64.5 
36.0 



209 

KBr  
NaCl  

R. 
P. 

53.3 
35.7 

102 
39.8 

HNO3  

Oxalic  
Phosphoric.  . 

KNO3  
H2C2O4.2H20  

(NH,),C  O.  . 

R. 
R. 

R, 
R 

13.3 
2.2 

15° 

4.2 

31.2 
11.1 



247 
350 

KH3(C,64)2.2H2O  . 

Na2HPO4.12H20.  .  . 

R 

6.3 

23.4 

236.8 
100 

Sulphuric.  .  .  . 

NaNH4HPO4.4H2O. 
KH2PO4  

R. 
R 

16 

H2SO4  

K2S04  

R. 

8.5 



10.9 



26.2 

In  the  column  headed  "Method  of  Purification,"  the  letters  have  the 
following  signification:  R.,  recrystallization;  D.,  distillation;  S.,  sublima- 
tion; I.,  ignition  of  nitrate,  and  P.,  precipitation. 

metal  by  passing  hydrogen  sulphide  through  the  acid  solution, 
filtering  off  any  precipitate,  and  then  adding  ammonia  and  again 
filtering.  To  the  solution  thus  purified,  ammonium  carbonate 
solution  is  added  with  vigorous  stirring.  The  precipitated  car- 
bonate is  then  thoroughly  washed  and  dried.  This  method  can- 
not be  used  for  the  preparation  of  the  pure  carbonates  of  the 
other  metals,  because  these  elements  do  not  form  stable  carbonates 
of  definite  composition. 

NOTES. 

Ammonium  Chloride  as  purchased  varies  greatly  in  purity.  Phosphoric 
arsenic,  sulphuric,  and  sulphocyanic  acids  may  be  present  as  well  as 
heavy  metals  and  earths  and  aniline  derivatives.  The  pure  salt  is  best 
prepared  by  neutralizing  C.P.  hydrochloric  acid  by  a  stream  of  ammonia- 
gas  produced  by  heating  pure  concentrated  ammonium  hydrate.  When  the 
acid  is  neutralized  the  solution  is  made  slightly  acid  with  hydrochloric  acid 
and  evaporated  to  dryness  in  a  porcelain  dieh  and  dried  on  the  water-bath. 

Ammon'um  Oxalate  is  examined  in  the  same  manner  as  oxalic  acid. 
It  does  not  lose  water  of  crystallization  as  readily  as  the  acid. 


PURE  SALTS  33 

Ammonium  Sulphate  may  be  made  by  the  method  given  for  ammonium 
chloride,  but  must  be  dried  at  120°  and  the  solution  must  be  neutral  or 
alkaline  before  evaporating  to  dryness. 

Arsenious  Oxide  may  be  tested  for  sulphide  by  heating  1  gram  in  a 
test-tube.  The  first  sublimate  which  forms  must  be  pure  white.  At  the 
end  of  the  operation  no  residue  must  remain  in  the  tube. 

Barium  Carbonate  must  be  prepared  by  precipitation  of  the  C.P.  chloride 
with  ammonia  and  ammonium  carbonate.  The  washed  precipitate  is 
gently  ignited  in  a  platinum  dish. 

Barium  Chloride  may  be  tested  for  metals  by  passing  hydrogen  sulphide 
through  the  acid  solution  and  then  rendering  it  alkaline  with  ammonia. 
Five  grams  of  the  salt  are  dissolved  in  200  c.c.  of  water,  .acidified  with  a 
drop  or  two  of  dilute  hydrochloric  acid,  heated  to  boiling,  and  the  barium 
precipitated  with  sulphuric  acid.  After  digesting  for  some  time,  the 
precipitate  is  filtered  off  and  the  filtrate  evaporated  to  dryness  in  a  platinum 
dish.  The  alkalies,  lime,  etc.,  will  be  found  in  the  residue,  which  should  be 
less  than  .1%  in  the  C.P.  salt. 

Bismuth  Oxide  may  be  prepared  pure  by  dissolving  the  commercial 
article  in  nitric  acid,  diluting  largely  with  water,  filtering  and  washing  the 
precipitate,  which  may  be  dried  and  ignited  in  a  platinum  dish.  A  still 
purer  product  may  be  obtained  by  dissolving  the  basic  nitrate  in  pure 
nitric  acid,  precipitating  with  ammonia  and  ammonium  carbonate,  washing 
and  igniting. 

Cadmium  Iodide  may  be  tested  for  tin,  lead,  copper,  zinc,  and  other 
metals  by  dissolving  2  grams  in  water  and  adding  nitric  acid.  A  white  pre- 
cipitate indicates  tin.  To  one-half  of  the  solution  a  large  excess  of  ammonia 
is  added.  The  solution  must  remain  clear  and  colorless.  The  other  part 
of  the  solution  is  diluted  with  water  and  filtered  after  adding  excess  of 
potash  solution.  Pass  hydrogen  sulphide  through  the  filtrate  and  acidify. 
No  precipitate  must  be  produced  either  in  the  alkaline  or  acid  solution. 

Calcium  and  Strontium  Carbonates  are  prepared  by  the  method  given 
for  barium  carbonate.  During  the  drying,  especially  of  the  calcium  car- 
bonate, the  platinum  dish  must  not  be  allowed  to  become  even  faintly  red. 
It  is  advisable  to  support  the  dish  about  an  inch  above  a  wire  gauze,  which 
is  heated  by  the  Bunsen  burner.  A  very  pure  form  of  calcium  carbonate 
may  be  purchased  in  the  form  of  Iceland  spar. 

Copper  prepared  electrolytically  and  of  a  high  percentage  of  purity 
may  be  readily  obtained. 

Copper  Sulphate. — This  salt  must  almost  invariably  be  recrystallized 
and  carefully  dried  in  order  to  insure  the  presence  of  the  theoretical  amount 
of  water.  The  dry  crystals  on  exposure  to  the  air  effloresce.  The  loss 
of  water  is  indicated  by  the  appearance  of  white  spots  in  place  of  the  clear 
blue  of  the  hydrated  salt. 

Ferrous  Ammonium  Sulphate,  frequently  called  MOHR'S  SALT,  may  be 
purchased  remarkably  pure.  The  crystals  should  be  small  and  free  from 


34  GENERAL  OPERATIONS. 

ferric  iron.  A  gram  of  the  salt  dissolved  in  boiled  water  which  has  been 
allowed  to  cool  out  of  contact  with  the  air  must  show  only  a  faint  red  with 
thiocyanate. 

Hydrochloric,  Sulphuric,  and  Nitric  Acids  may  be  easily  obtained  almost 
absolutely  free  from  impurities.  The  percentage  of  acid  present  may  be 
ascertained  from  the  tables  pp.  479-486  after  carefully  taking  the  specific 
gravity. 

Iodine  must  always  be  purified  by  sublimation.  For  this  purpose  the 
commercial  article  is  mixed  with  one-fourth  or  one-fifth  of  its  weight  of 
potassium  iodide  by  grinding  them  together  in  a  porcelain  mortar.  The 
material  is  placed  in  a  watch-crystal  or  crystallizing-dish,  which  is  placed 
on  a  piece  of  asbestos  board  having  a  hole  cut  in  the  centre.  The  iodine 
is  sublimed  by  gentle  heat  from  a  Bunsen  burner,  the  first  portion  of  the 
vapor  being  allowed  to  escape  as  the  material  is  frequently  quite  moist. 
Another  watch-crystal  or  crystallizing-dish  is  now  placed  over  the  first  one, 
the  upper  one  being  kept  cold  by  means  of  moist  filter-paper  or  cloth.  The 
upper  dish  is  removed  before  all  of  the  iodine  is  sublimed.  It  is  best  to 
sublime  the  iodine  immediately  before  use  as  it  is  very  hygroscopic. 

Iron. — The  soft  iron  wire,  which  is  sold  for  standardizing  purposes,  con- 
tains from  .2  to  .4%  of  carbon.  For  careful  work  the  percentage  of  iron 
or  carbon  must  be  determined  gravimetrically. 

The  DOUBLE  AMMONIUM  SALTS  of  MANGANESE,  COBALT,  NICKEL,  and  ZINC 

must  be  prepared  by  the  general  methods  given. 

Mercury. — On  being  shaken  in  a  clean  dry  bottle  the  surface  must 
remain  perfectly  bright  and  the  metal  must  not  adhere  to  the  surface  of 
the  glass. 

Mercuric  Chloride  may  be  tested  for  mercurous  chloride  by  treating  a 
gram  of  the  finely  ground  sample  with  15  c.c.  of  pure  ether.  It  must  dis- 
solve completely.  The  filtrate  from  the  hydrogen-sulphide  precipitate 
must  leave  no  weighable  residue.  On  testing  this  precipitate  with  dilute 
ammonia,  filtering  and  acidifying  the  filtrate  with  hydrochloric  acid,  no 
yellow  precipitate  or  color  must  be  produced  indicating  absence  of  arsenic 

Oxalic  Acid  must  leave  no  residue  on  ignition.  An  alkaline  residue 
indicates  the  presence  of  sodium  or  potassium.  Sulphuric  acid  is  tested 
for  by  dissolving  5  grams  in  100  c.c.  of  water,  adding  a  few  drops  of  dilute 
hydrochloric  acid  and  barium  chloride.  No  precipitate  must  form  after 
standing  several  hours  in  a  hot  place.  Ammonia  and  ammonium  sulphide 
must  produce  no  coloration.  On  heating  2  grams  in  a  test-tube  with  caustic- 
soda  solution  no  ammonia  must  be  given  off.  A  very  pure  acid  may  be 
made  by  dissolving  the  impure  material  in  a  mixture  of  equal  parts  of 
alcohol  and  ether,  filtering,  evaporating  off  the  alcohol  and  ether  after  the 
addition  of  water,  and  recrystallizing  the  product. 

Potash  Alum. — Recrystallization  and  carefully  drying  may  be  necessary 
to  insure  the  correct  percentage  of  water. 


PURE  SALTS.  35 

Potassium-acid  lodate  may  be  purchased  remarkably  pure.  Meineke 
found  two  samples  100.001  and  100.010%  pure. 

Potassium  Bromide  and  Iodide  can  usually  be  obtained  quite  pure. 
The  presence  of  bromates  or  iodates  is  shown  by  the  blue  color  produced 
by  added  starch  solution  and  a  little  dilute  hydrochloric  acid  to  the  dilute 
solution  and  in  the  case  of  the  bromide  a  little  potassium  iodide.  The 
heavy  metals  and  sulphuric  acid  are  tested  for  in  the  usual  manner.  The 
salts  must  be  dried  at  100°. 

Potassium  Bichromate  is  generally  obtained  very  pure.  The  com- 
mercial samples  frequently  contain  sulphates  or  chlorides.  The  C.P.  or 
recrystallized  salt  may  be  dried  by  heating  until  the  salt  is  fused.  It  should 
not  be  heated  higher,  and  all  organic  matter  must  be  carefully  excluded. 

Potassium  Nitrate  is  also  sold  in  a  very  pure  condition.  Manufacturers 
generally  guarantee  a  maximum  impurity  of  sodium  chloride  of  about  1 
part  in  10,000  or  .01%. 

Potassium  Permanganate,  which  is  very  nearly  100%  pure,  may  be 
readily  obtained.  Chlorides,  sulphates,  and  nitrates,  which  are  present  in 
the  commercial  article,  are  absent  from  the  C.P.  salt.  The  latter,  however, 
invariably  contains  small  quantities  of  manganese  dioxide. 

Potassium  Sulphate  must  give  a  clear  neutral  solution  (1  to  20)  and 
must  give  no  reaction  with  hydrogen-sulphide  water,  ammonium  oxalate, 
potassium  carbonate,  or  silver  nitrate.  It  may  be  dried  at  100°. 

Potassium  Tetroxalate  may  be  prepared  by  making  a  hot  saturated 
solution  of  oxalic  acid,  neutralizing  one-fourth  of  it  with  pure  potassium 
carbonate  and  adding  this  with  stirring  to  the  remainder  of  the  solution. 
The  product  is  still  further  purified  by  recrystallization. 

Silver  Nitrate. — Good  commercial  samples  have  been  found  to  contain 
from  .01  to  .03%  of  impurity.  The  crystals  should  be  pure  white. 

Metallic  Silver  may  be  obtained  of  a  high  state  of  purity  guaranteed  by 
government  assay. 

Sodium  Phosphate  and  Sodium-ammonium  Phosphate  must  be  neutral 
to  phenolphthalein  and  free  from  sulphates,  chlorides,  and  nitrates.  If  the 
crystals  are  not  transparent,  but  show  signs  of  efflorescence,  the  salt  must 
be  recrystallized  and  carefully  dried. 

Sodium  and  Potassium  Chlorides  may  be  purchased  pure  enough  for  most 
quantitative  work.  The  method  of  purification  of  sodium  chloride  given  in 
Experiment  6  yields  a  very  pure  product. 

Zinc  and  Magnesium  Sulphates  must  always  be  recrystallized  and  care- 
fully dried.  The  double  sulphate  of  magnesium  and  potassium  is  pre- 
pared as  directed  in  Exercise  V. 

Many  of  the  tests  given  in  this  section  have  been  quoted  from  "Testing 
of  Chemical  Reagents,"  by  Dr.  C.  Krauch.  The  student  is  referred  to  this 
book  or  to  " Chemical  Reagents,  their  Purity  and  Tests,"  by  E.  Merck,  for  the 
testing  of  other  salts. 


36  QUANTITATIVE  ANALYSIS. 

EXERCISE  4. 
Preparation  of  Pure  Copper  Sulphate,  CuS04.sH20. 

Weigh  out  about  150  grams  of  crystallized  copper  sulphate.  Crush  the 
large  crystals  in  a  porcelain  mortar.  Dissolve  in  about  150  c.c.  of  hot  distilled 
water.  If  the  solution  is  not  perfectly  clear,  filter,  using  a  folded  filter-paper 
and  a  hot-water  funnel.  The  clear  filtrate  must  be  vigorously  stirred  as  soon 
as  crystals  begin  to  separate.  Cool  the  solution  by  placing  the  beaker  in  cold 
tap-water  or  in  ice-water.  Continue  the  stirring  until  no  more  crystals  sepa- 
rate out.  Filter  off  the  crystals  immediately,  using  a  funnel  of  suitable  size 
with  a  platinum  cone  or  a  perforated  porcelain  dish  in  the  bottom  of  the 
funnel.  Connect  the  funnel  by  means  of  a  rubber  stopper  to  a  filter-flask 
and  remove  as  much  of  the  mother-liquor  as  possible  by  means  of  strong 
suction  from  the  filter-pump.  Press  the  crystals  into  a  compact  mass  by 
means  of  a  spatula.  Pour  about  50  c.c.  of  ice-water  over  the  crystals  and 
suck  it  through  as  rapidly  as  possible  with  the  suction.  When  no  more 
liquid  comes  through,  spread  out  the  crystals  by  means  of  a  spatula  on  an 
unglazed  porcelain  plate  or  on  several  folds  of  filter-paper.  When  no 
more  liquid  seems  to  be  absorbed ,  spread  the  crystals  out  in  an  even  layer. 
Cover  them  with  a  piece  of  filter-paper  and  allow  them  to  dry  in  the  air  for 
several  hours  with  occasional  stirring.  Test  for  moisture  from  time  to  time 
by  placing  a  few  crystals  on  a  clean  and  dry  porcelain  or  glass  surface.  Dry 
crystals  will  not  adhere  to  such  a  surface.  As  soon  as  dry,  transfer  to  a 
clean  dry  bottle  with  ground-glass  stopper.  Care  must  be  taken  in  removing 
from  the  filter-paper  not  to  detach  shreds  of  filter-paper  with  the  crystals. 
If  the  crystals  have  been  exposed  too  long  to  the  air,  portions  will  appear 
white  from  loss  of  water  of  crystallization.  If  the  work  has  been  carried  on 
in  a  room  containing  hydrogen  sulphide  or  ammonium  sulphide  in  the  atmos- 
phere, black  spots  will  appear  on  the  crystals.  If  such  crystals  cannot  be 
removed  the  work  must  be  repeated. 

EXERCISE  5. 
Preparation  of  Potassium-magnesium  Sulphate,  K2S04.MgS04.6H20. 

Weigh  out  about  163  grams  of  magnesium  sulphate  and  dissolve  in  115  c.c. 
of  water.  Dissolve  87  grams  of  potassium  sulphate  in  350  c.c.  of  water. 
If  necessary,  filter  the  solutions  separately,  using  the  hot-water  funnel  and 
folded  filter-papers.  Pour  the  hot  solution  of  potassium  sulphate  rapidly 
and  with  vigorous  stirring  into  the  hot  magnesium  solution.  Stir  con- 
stantly while  the  double  salt  crystallizes  out,  and  finally  cool  the  solution  by 
surrounding  the  beaker  with  cracked  ice.  When  no  more  salt  crystallizes  out, 
filter  the  crystals  off  in  a  funnel  with  a  platinum  cone  or  perforated  porce- 
lain plate  in  the  bottom.  Suck  the  mother-liquor  off  by  means  of  the 
filter-pump,  pressing  the  crystals  together  with  a  porcelain  spatula.  Wash 
the  crystals  with  50  c.c.  of  ice-cold  distilled  water,  sucking  them  ory  with  the 


GENERAL  OPERATIONS. 


37 


pump.  Transfer  the  crystals  to  an  unglazed  porcelain  plate  or  several  folds 
of  filter-paper.  Remove  the  mother-liquor  by  pressing  the  crystals  on  the 
porcelain  plate  with  a  porcelain  spatula  or  by  pressing  them  with  the  hand 
between  the  folds  of  the  filter-paper.  Finally,  spread  them  out  to  dry  in 
the  air,  testing  them  for  moisture  from  time  to  time  by  placing  them  on  a 
clean  and  dry  porcelain  or  glass  surface.  The  dry  crystals  will  not  adhere 
to  such  a  surface.  When  dry,  transfer  to  a  clean  and  dry  bottle. 

EXERCISE  6. 
Preparation  of  Pure  Sodium  Chloride. 

About  70  grams  of  common  salt  are  weighed  out  and  dissolved  in  200  c.c. 
of  distilled  water.  If  necessary,  the  solution  is  filtered.  Gaseous  hydrochloric 
acid  is  now  passed  into  the  solution.  It  is  generated  in  the  flask  A,  which 


FIG.  7. 

contains  about  150  grams  of  common  salt.  300  grams  of  concentrated  sul- 
phuric acid  are  poured  into  220  c.c.  of  distilled  water  and  allowed  to  cool 
somewhat.  This  acid  is  transferred  to  the  dropping-funnel  and  allowed  to 
drop  slowly  on  the  salt.  B  is  a  wash-bottle  containing  concentrated  hydro- 
chloric acid  to  cat2h  any  sodium  sulphate  which  might  spatter  over.  The 


38  QUANTITATIVE  ANALYSIS. 

thistle-tube  C  touches  the  salt  solution  in  the  beaker.  Toward  the  end  of 
the  operation  the  flask  may  be  heated  so  as  to  expel  the  hydrochloric  acid. 
The  stream  of  hydrochloric  acid  is  allowed  to  pass  until  no  more  precipita- 
tion is  noticed  in  the  salt  solution.  The  crystals  are  filtered  off  and  washed 
with  concentrated  c.p.  hydrochloric  acid.  When  no  more  liquid  can  be 
sucked  out  by  the  pump,  the  sodium  chloride  is  transferred  to  a  porcelain 
dish  and  heated  with  the  Bunsen  burner  until  dry  and  free  from  hydro- 
chloric acid.  The  pure  salt  is  then  transferred  to  a  clean  and  dry  glass- 
stoppered  bottle. 

RULES  FOR  USING  AND  CLEANING  PLATINUM  UTENSILS. 

Platinum  when  hot  easily  forms  alloys  with  other  metals. 
For  this  reason  metals  must  never  be  heated  in  platinum.  The 
same  rule  applies  to  all  compounds  of  easily  reducible  metals, 
such  as  silver,  copper,  lead,  and  bismuth.  Even  stannic  oxide  is 
sometimes  reduced  to  metallic  tin,  which  alloys  with  the  platinum. 

Heat  platinum  only  with  a  clear  blue  flame.  A  deposit  of 
carbon  is  injurious.  The  vessel  should  also  be  placed  beyond  the 
inner  cone  of  the  flame,  as  reducing  gases  may  pass  through  the 
hot  platinum. 

As  phosphorus  and  sulphur  combine  with  platinum,  making 
it  brittle,  compounds  of  these  elements  should  not  be  heated  in 
platinum  under  reducing  conditions. 

Caustic  alkalies  should  not  be  fused  in  platinum  as  it  is  oxi- 
dized and  dissolved.  Nickel,  iron,  silver,  or  copper  dishes  may 
be  used  for  this  purpose. 

The  carbonates  of  the  alkalies  may  be  fused  in  platinum, 
although  a  little  platinum  is  dissolved,  especially  when  potassium 
nitrate  is  present. 

Platinum  vessels  may  be  cleaned  by  digestion  with  hydro- 
chloric or  nitric  acids,  but  not  with  a  mixture  of  these  acids. 
Basic  impurities  not  removed  in  this  manner  maybe  dissolved  by 
fused  acid  potassium  sulphate,  while  acid  impurities  such  as  silica 
may  be  removed  by  fused  sodium  carbonate.  The  surface  of 
platinum  should  be  kept  smooth  and  bright  by  polishing  with 
sea  sand  which  is  free  from  sharp  particles. 

Platinum  vessels  change  weight  very  slowly,  losing  a  few  tenths 
of  a  milligram  after  strong  ignition,  a  little  more  after  a  fusion, 
especially  with  an  alkaline  oxidizing  mixture,  and  still  more  on 
being  polished. 


CHAPTER  III. 

DETERMINATION  OF  WATER. 

35.  Conditions  in  which  Water  is  Held. — Water  is  present  in 
the  great  majority  of  substances  submitted  to  chemical  analysis, 
and  the  amount  present  must  frequently  be  determined.     It  may 
be  held  MECHANICALLY  or  combined   CHEMICALLY   as  water  of 
crystallization,  or  it  may  be  an  inherent  part  of  the  molecule.     In 
considering  its  determination,  the  manner  of  separating  it  from 
the  other  constituents  present  must  be  studied  as  well  as  the 
method  of  weighing  it.     The  simplest  method  of  determining  it 
involves  heating  the  compound  to  the  temperature  at  which  the 
water  volatilizes.     This  temperature  differs  greatly,  ranging  from 
the  ordinary  temperature  of  the  air  to  that  of  the  blast-lamp. 
Where  the  water  is  held  mechanically,  as  in  most  minerals,  as 
well  as  metals  and  non-crystalline  substances,  a  temperature  of 
100°  or  a  few  degrees  above  that  point  is  sufficient  to  dry  the 
substance   completely.     In   the    case   of   salts   which   crystallize 
with  water,  the  temperature  at  which  it  is  completely  given  off 
varies  from  100°  centigrade  to  a  red  heat.      In  some  cases  a  definite 
number  of  molecules  of  water  of  crystallization  will  be  driven 
off  at  one  temperature,  while  a  higher  temperature  is  required 
to  completely  dehydrate  the  substance.     The  silicates,  as  a  class, 
hold  water  with  great  persistence.     Glass  ma}^  be  heated  to  a 
red  heat  for  months,  and  still  give  off  a  trace  of  water.     In  this 
case  the  water  is  probably  combined  with  the  silica,  forming  a 
more  or  less  complicated  silicic  acid. 

36.  Methods  of   Determining  Water.— Whenever  a  substance 
can  be  dried  completely  by  simply  heating  it  to  a  temperature  at 
which  no  other  constituent  is  volatilized,  the  amount  of  water 
may  be  determined  by  weighing  the  substance  before  and  after 
drying  it.    In  this  case  the  amount  of  water  present  may  also 


40  QUANTITATIVE  ANALYSIS. 

be  determined  by  allowing  the  water  to  be  absorbed  by  a  weighed 
amount  of  some  hygroscopic  substance,  like  calcium  chloride  or 
concentrated  sulphuric  acid.  Such  a  determination  would  give 
the  total  amount  of  water  present.  This  water  may  be  present 
either  as  HYGROSCOPIC  water  or  as  water  of  CRYSTALLIZATION  or 

Of  CONSTITUTION. 

If  it  is  necessary  to  determine  the  amount  of  each,  the  sub- 
stance must  first  be  heated  to  a  temperature  at  which  only  the 
hygroscopic  water  is  given  off.  At  a  temperature  of  105°  the 
hygroscopic  water  will  certainly  be  expelled  in  every  case,  but 
as  at  that  temperature  many  crystals  lose  a  part  or  all  of  their 
water  of  crystallization,  a  lower  temperature  must  sometimes  be 
used.  In  such  cases,  the  substance  is  placed  in  a  desiccator  in 
which  the  air  is  dried  by  sulphuric  acid  or  calcium  chloride.  Even 
under  these  circumstances  many  crystals  lose  water.  By  diluting 
the  sulphuric  acid,  however,  an  atmosphere  may  be  obtained 
having  a  tension  of  water  vapor  equal  to  or  greater  than  that 
produced  by  the  water  of  crystallization.  Long  standing  in  such 
an  atmosphere  will  free  the  crystal  of  adherent  or  hygroscopic 
water. 

37.  Sources  of  Error. — When  substances  lose  other  constitu- 
ents than  water  on  heating,  the  error  may  frequently  be  avoided 
by  adding  an  anhydrous  substance  which  is  capable  of  retaining 
the  volatile  substance.  The  oxides  of  calcium,  lead,  and  bis- 
muth have  been  used  as  retainers  for  fluorine,  chlorine,  sulphur, 
etc. 

Not  only  must  loss  of  volatile  constituents  on  heating  be 
guarded  against,  but  a  possible  increase  in  weight  from  the  ab- 
sorption of  oxygen,  carbon  dioxide  or  air  must  be  kept  in  mind. 
Metals  in  their  lower  state  of  oxidation  may  absorb  oxygen,  and 
pass  to  the  higher  state  of  oxidation.  A  familiar  example  of  this 
is  ferrous  sulphate,  which  will  absorb  oxygen  even  at  the  ordinary 
temperature.  Sulphides  of  some  metals  easily  take  up  oxygen, 
becoming  sulphates.  Some  basic  oxides  of  the  metals  absorb 
carbon  dioxide,  forming  carbonates.  Some  minerals,  such  as 
the  zeolites,  have  been  found  to  absorb  air  after  the  water  has 
been  expelled. 


DETERMINATION  OF  WATER.  41 


EXERCISE  7. 

Determination  of  Water  of  Crystallization  in  Copper  Sulphate, 
CuS04.5H2O. 

38.  Weighing  the  Salt. — A  watch  crystal  of  convenient  size  is  weighed, 
a  one-gram  weight  is  added  to  the  right-hand  pan  and  recrystallized  copper 
sulphate  is  placed  on  the  watch-crystal  by  means  of  a  spatula  until  equi- 
librium is  restored.     Exactly  one  gram  of  copper  sulphate  will  then  have 
been  weighed  out.     In  placing  the  salt  on  the  watch-crystal  by  means  of 
the  spatula,  the  latter  should  not  be  allowed  to  touch  the  watch-crystal 
nor  the  scale-pan.     The  spatula  with  a  moderate  amount  of  the  salt  in  it 
is  held  in  the  right  hand  so  that  by  tapping  it  gently  with  the  left  hand  the 
crystals  will  fall  on  the  watch-glass.     The  arms  supporting  the  beam  are 
slightly  lowered  by  the  central  thumb-screw,  so  that,  by  releasing  by  means 
of  the  left  hand  the  spring  supporting  the  scale-pans,  the  pointer  will  indi- 
cate when  enough  salt  has  been  added  by  remaining  at  zero  or  moving  to 
the  right.     After  a  little  experience  the  quickness  with  which  the  pointer 
moves  will  indicate  the  amount  of  salt  to  be  added  or  taken  away.     When 
the  pointer  moves  slowly  to  the  left,  only  a  few  small  crystals  at  a  time 
are  allowed  to  fall  on  the  watch-crystal  until  the  pointer  ceases  to  move. 
The  swings  may  then  be  taken  if  necessary  and  the  final  adjustment  made. 
If  too  much  has  been  added,  a  considerable  amount  should  be  very  care- 
fully taken  off,  and  added  in  small  amounts  as  before.     When  exactly  a 
gram  is  weighed  out,  the  calculation  of  percentage  is  very  simple,  as  the  number 
of  milligrams  of  water  lost  is  equal  to  the  tenths  of  per  cent.     At  first  it  may 
seem  easier  to  calculate  from  an  amount  weighed  out  which  is  only  approxi- 
mately one  gram  than  to  endeavor  to  obtain  exactly  one  gram,  but  it  is 
found  that  an  experienced  worker  can  weigh  more  readily  than  make  the 
required  calculations. 

39.  Determination  of    Four   Molecules  of   Water. — The  watch-glass  with 
copper  sulphate  is  now  transferred  to  an  air-oven,  heated  to  115°.     The 
temperature  is  regulated  by  adjusting  the  flame  of  the  Bunsen  burner 
from  time  to  time  until  the  thermometer  indicates  a  constant  temperature. 
After  about  one  hour  the  copper  sulphate  is  transferred  to  a  desiccator, 
and  after  ten  minutes  weighed.     It  is  again  heated  in  the  air-bath  one- half 
hour  and  cooled  and  weighed.     This  is  repeated  until  the  weight  is  con- 
stant.    In  most  analytical  work  a  weight  is  considered  constant  if  the  change 
is  not  greater  than  0.3  of  a  milligram,  since  errors  of  weighing  may  be  as 
great  as  this.     Frequently  a  change  of  weight  of  1  milligram  warrants  a 
discontinuation  of  the  heating,   since  the  next  heating  would   probably 
produce  a  much  smaller  change  in  weight.     As  the  watch-crystal  may 
have  lost  weight  on  heating,  the  copper  sulphate  is  brushed  off  and  the 
watch-crystal  weighed.     The  difference  between  this  weight  and  the  last 
weight  of  the  dried  copper  sulphate  and  the  watch-crystal  gives  the  weight 


42 


QUANTITATIVE  ANALYSIS. 


of  the  dried  copper  sulphate.  The  difference  between  the  weight  of  the 
dried  copper  sulphate  and  of  the  crystals  taken  is  the  weight  of  water 
expelled.  The  percentage  of  water  lost  in  this  manner  will  be  found  to  be 
very  nearly  that  calculated  for  4  molecules  of  water  or  28.85%.  The 
correct  weight  of  the  watch-crystal  may  also  be  obtained  by  drying  it  in 
the  air-bath  at  115°  until  its  weight  is  constant  before  placing  the  copper 
sulphate  on  it. 

3oa.  Determination  of  the  Fifth  Molecule  of  Water.— If  the  air-bath  can 
be  heated  to  210°-215°,  the  fifth  molecule  of  water  may  be  determined 
in  the  same  manner  as  the  first  four.  For  this  purpose  either  another  gram 
of  the  crystals  may  be  weighed  out  and  the  total  amount  of  water  deter- 
mined or  the  portion  from  which  the  4  molecules  have  been  expelled  may 
be  heated  to  constant  weight  at  210°  to  215°  and  the  percentage  correspond- 
ing to  the  fifth  molecule  determined.  If  the  air-bath  cannot  be  heated 
to  215°,  the  watch-crystal  containing  the  copper  sulphate  may  be  placed 
on  a  sand-bath  which  consists  of  an  iron  plate  filled  with  sand  and  placed 
on  a  tripod  so  as  to  be  heated  by  means  of  a  Bunsen  burner.  Until  the 
bulk  of  the  water  has  been  expelled,  the  thermometer-bulb  should  be 
immersed  in  the  sand  so  as  to  touch  the  watch-crystal,  which  should  be 


FIG.  8. 

pressed  down  firmly  in  the  sand.  When  the  copper  sulphate  has  been 
heated  about  one  hour  in  this  manner,  the  bulb  of  the  thermometer,  after 
being  freed  from  sand,  should  be  placed  on  the  copper  sulphate  in  a  slanting 
position.  The  watch-crystal  should  be  covered  with  another  and  the 
Bunsen  burner  regulated  until  the  thermometer  indicates  210°  to  215° 
and  the  heating  continued  for  one-half  hour,  when  the  copper  sulphate  is 
carefully  brushed  off  the  thermometer  and  the  former  cooled  in  the  desicca- 
tor and  weighed.  The  heating  with  the  thermometer  on  the  copper  sulphate 
is  repeated  until  the  weight  is  constant.  The  watch-crystal  is  now  weighed 


DETERMINATION  OF  WATER.  43 

after  removing  the  copper  sulphate  and  the  percentage  of  water  calculated. 
The  theoretical  percentage  for  5  molecules  is  36.07. 

EXERCISE  8. 
Determination  of  Water  of  Crystallization  in  Barium  Chloride,  BaCl2.2H2O. 

A  crucible,  preferably  of  platinum,  with  its  lid,  is  placed  on  a  pipe-stem 
triangle  and  heated  with  the  Bunsen  burner  for  a  few  minutes,  cooled  in  a 
desiccator,  and  weighed.  Two  grams  of  pure  crystallized  barium  chloride 
are  weighed  out  and  transferred  to  the  crucible.  The  crucible  is  placed 
on  the  pipe-stem  triangle  and  heated  gently  by  a  small  Bunsen  burner 
flame,  the  lid  being  on  the  crucible.  The  temperature  is  gradually  raised 
until  the  crucible  attains  a  low  red  heat,  at  which  it  is  maintained  for  about 
ten  minutes.  It  is  then  cooled  in  a  desiccator  and  weighed.  The  heating 
and  weighing  is  repeated  until  the  weight  is  constant.  Theoretical  per- 
centage of  water  of  crystallization  in  crystallized  barium  chloride  is  14.74. 

40.  Penfield's  Method  of  Determining  Water. — A  very  simple 
and  accurate  method  of  determining  water,  especially  when  it  is 
present  in  small  amount,  is  that  of  Pen  field.*  The  apparatus 
used  consists  simply  of  a  glass  tube  closed  at  one  end  and  having 
one  or  two  bulbs  blown  on  the  middle  of  the  tube  If  a  high  tem- 
perature is  required  to  expel  the  water,  a  hard  glass  tube  must  be 
used,  otherwise  ordinary  soft  glass  tubing  may  be  used.  Tubes 
20  cm.  long  and  6  mm.  internal  diameter  will  be  found  a  convenient 
size.  When  the  amount  of  water  given  off  is  large,  a  bulb  should 
be  blown  on  the  middle  of  the  tube.  The  tube  is  first  dried  by 
placing  in  an  air-bath  heated  to  a  little  above  100°  and  sucking 
out  the  moist  air  by  means  of  a  smaller  tube.  The  tube  is  then 
cooled  and  weighed  and  the  substance  to  be  analyzed  is  carefully 
transferred  to  the  bulb  at  the  closed  end  of  the  tube,  which  is  again 
weighed,  the  difference  between  this  weight  and  the  weight  of  the 
empty  tube  giving  the  weight  of  the  substance  analyzed. 

A  short  piece  of  glass  tubing  drawn  out  to  a  capillary  is  attached 
to  the  Penfield  tube  by  means  of  a  short  piece  of  rubber  tubing. 
This  simple  device  prevents  the  loss  of  water  by  circulating  air 
currents.  Moist  filter-paper  or  cloth  is  wrapped  around  the  bulb 
in  the  middle  of  the  tube,  and  the  bulb  at  the  end  containing  the 

*  Am.  Jour.  Sci.,  3d  series,  Vol.  XLVIII,  p.  31.  1894. 


44  QUANTITATIVE  ANALYSIS. 

substance  being  analyzed  is  heated  high  enough  to  expel  the 
moisture.  This  may  generally  be  accomplished  with  the  Bunsen 
burner,  although  some  substances  require  the  blast-lamp.  The 
water  condenses  on  the  cold  middle  portion  of  the  tube  and  col- 
lects in  the  bulb,  which  prevents  it  from  running  back  to  the 
heated  portion  and  cracking  the  glass.  When  all  of  the  water 
has  been  expelled  the  flame  is  moved  so  as  to  melt  off  the  bulb 
containing  the  substance  being  analyzed;  when  this  has  been 
accomplished  the  tube  is  cooled  and  weighed  after  removing  the 
capillary  tube  at  the  end.  The  tube  is  now  placed  in  the  air- 
bath  and  dried  as  in  the  beginning  of  the  experiment.  It  is  then 
cooled  and  again  weighed,  the  loss  of  weight  being  equal  to  the, 
amount  of  water  obtained. 

If  other  volatile  substances  besides  water  are  given  off,  a 
retainer  must  be  used.  The  oxides  of  calcium,  lead,  or  bismuth 
have  been  used  for  this  purpose.  The  retainer  need  not  be 
weighed,  but  it  must  be  thoroughly  dried  before  being  placed  in  the 
tube,  and  enough  should  be  added  after  the  weighed  substance 
to  be  analyzed  has  been  introduced  to  fill  the  bulb  and  form  a 
layer  about  two  centimeters  deep.  The  retainer  must  be  heated 
first  and  then  the  flame  is  increased  so  as  to  also  heat  the  bulb. 
When  all  of  the  water  has  been  expelled  the  part  of  the  tube  con- 
taining the  retainer  is  melted  off  and  the  determination  com- 
pleted as  already  described,  when  no  retainer  is  required. 

The  Penfield  method  is  well  adapted  to  the  determination  of 
water  in  minerals  and  substances  which  require  high  heat  in  order 
to  expel  the  moisture.  The  results  are  accurate  if  the  operation 
is  carefully  carried  out,  although  only  moderate  amounts  of  water 
can  be  weighed  in  this  manner. 

41.  Efficiency  of  Various  Drying-agents  for  Gases. — When  the 
water  is  to  be  weighed  directly,  it  must  be  absorbed  in  a  weighed 
amount  of  some  dehydrating  agent.  The  substances  commonly 
used  for  this  purpose  are  PHOSPHORUS  PENTOXIDE,  CONCEN- 
TRATED SULPHURIC  ACID,  and  FUSED  CALCIUM  CHLORIDE.  PhoS- 

phorus  pentoxide  is  the  most  efficient  drying-agent  for  gases 
known.  It  is  doubtful  if  any  water  at  all  remains  in  a  gas  which 
has  been  dried  over  this  reagent.  On  the  contrary,  gases  dried 
over  concentrated  sulphuric  acid  give  up  moisture  to  phosphorus 


DETERMINATION  OF  WATER.  45 

pentoxide,  while  gases  dried  with  fused  calcium  chloride  give  up 
moisture  both  to  concentrated  sulphuric  acid  and  to  phosphorus 
pentoxide.  One  litre  of  air  dried  with  calcium  chloride  at  15°  C. 
gives  up  1  milligram  of  water  to  concentrated  sulphuric  acid, 
while  phosphorus  pentoxide  is  capable  of  absorbing  .002  milli- 
gram of  water  per  litre  after  the  air  has  been  dried  over  con- 
centrated sulphuric  acid. 

Equally  correct  results,  however,  may  be  obtained  by  the  use  of 
the  less  efficient  dehydrating  agents  if  care  is  taken  to  dry  the  air 
when  it  enters  the  apparatus  to  the  same  extent  as  when  it  leaves. 
This  object  is  best  secured  by  using  the  same  dehydrating  agent 
for  both  purposes.  Some  moisture  will  be  present  in  the  air 
which  passes  over  the  hydrated  substance,  but  as  exactly  the 
same  amount  of  water  is  present  in  the  air  on  leaving  the  second 
set  of  drying-tubes  the  water  actually  absorbed  by  these  tubes  will 
be  that  taken  up  within  the  apparatus  only. 

42.  Drying  Properties  of  Fused  Calcium  Chloride. — When  cal- 
cium chloride  is  used  care  must  be  taken  to  keep  all  of  the 
tubes  in  a  series  of  the  same  temperature,  as  this  salt  differs  in  its 
efficiency  at  different  temperatures.  At  0°  centigrade,  1  litre 
of  air  after  being  dried  over  calcium  chloride  contains  .3  milli- 
gram of  water;  if  dried  at  15°  it  contains  1  milligram  of  water, 
while  if  dried  at  30°,  3.3  milligrams  of  water  remain.  In  one  ex- 
periment where  air  was  passed  through  a  series  of  U-tubes 
(A,  B,  C,  D,  E)  filled  with  calcium  chloride,  which  were  main- 
tained at  different  temperatures,  the  gains  and  losses  in  the 
weights  noted  below  were  observed.  Before  entering  the  first 
tube,  A,  the  air  was  dried  by  passing  through  a  tube  of  calcium 
chloride  maintained  at  the  temperature  of  A. 

Tubes.  Temparatures.  Changes  in  Weight. 

A 16.9°  .0000  gram 

B 0.0°  .0452      " 

C 30.2°  .1565      " 

D 0.0°  .1569      " 

E 16.9°  .0442      " 

Total  change  in  weight  . 0014  gram 

It  is  extremely  difficult  to  obtain  fused  calcium  chloride  which 
is  entirely  free  from  calcium  oxide.  It  will,  therefore,  be  found 
to  absorb  a  small  amount  of  carbon  dioxide.  To  neutralize  the 


46  QUANTITATIVE  ANALYSIS. 

calcium  oxide  the  calcium  chloride  should  be  kept  in  an  atmo- 
sphere of  carbon  dioxide  or  hydrochloric  acid  for  some  time  and 
then  the  acid  gas  displaced  by  air.  This  is  best  accomplished  by 
filling  tubes  with  it  and  passing  a  slow  stream  of  the  acid  gas 
through  them,  and  finally  drawing  air  through.  When  moist  air 
passes  over  calcium  chloride  some  of  it  dissolves  in  the  water 
absorbed,  and  this  liquid  mass  on  the  surface  of  the  lumps  after- 
ward solidifies.  The  so-called  Marshand  tube  is,  therefore,  best 
adapted  for  use  with  calcium  chloride.  This  U-tube  has  a  bulb 
blown  in  the  exit  tube  on  one  side.  The  moist  air  should  be 
led  in  at  this  point,  so  that  the  water  may  condense  in  the  bulb, 
which  may  be  kept  cold  by  moist  filter-paper  wrapped  around  it. 
The  calcium  chloride  in  the  half  of  the  tube  nearest  the  bulb 
should  be  in  large  lumps,  so  that  the  tube  shall  not  be  clogged 
by  the  dissolved  calcium  chloride.  The  other  half  of  the  U-tube 
should  be  filled  with  finer  material,  so  as  to  dry  the  gas  thoroughly. 
A  plug  of  glass  wool  should  be  inserted  above  the  fine  calcium 
chloride,  so  as  to  prevent  small  particles  from  being  carried  out 
by  the  current  of  air. 

43.  Drying  Properties  of  Concentrated  Sulphuric  Acid. — Con- 
centrated sulphuric  acid  is  about  equally  efficient  at  0°  and  30° 
centigrade.     It  should  be  free  from  sulphur  dioxide.     If  pres- 
ent, this  gas  may  be  expelled  by  passing  a  current  of  air  through 
the  acid.     If  the  acid  is  at  all  dark  or  brown  in  color  from  the 
presence  of  organic  matter  it  should  not  be  used,  as  sulphur  diox- 
ide will  result  from  the  slow  oxidation  of  the  organic  material. 
It  absorbs  carbon  dioxide  to  an  appreciable  extent,  but  this  is 
wholly  expelled  by  passing  air  through  it.     U-tubes  should  first 
be  partly  filled  with  glass  beads  and  then  sulphuric  acid  added 
until  the  beads  are  thoroughly  moistened  and  the  liquid  just 
forms  a  seal  at  the  bottom  of  the  tube.     Care  must  be  taken  that 
sulphuric-acid  U-tubes  are  always  held  upright,  so  that  the  acid 
shall  not  come  in  contact  with  the  stoppers  or  enter  the  exit  tubes. 
It  is  therefore  most  convenient  to  attach  a  platinum  wire,  so  that 
they  may  be  suspended  during  the  weighing  and  at  other  times. 
This  wire  should  be  left  on  the  tube. 

44.  Drying  Properties  of  Phosphorus  Pentoxide. — Phosphorus 
pentoxide   is  seldom  used,  because  of  the  difficulty  of  manipu- 


DETERMINATION  OF  WATER.  47 

lating  tubes  containing  it.  When  moist  air  comes  in  contact  with 
it,  a  very  viscuous  liquid  is  produced  which  very  quickly  prevents 
the  further  passage  of  gases  through  it.  U-tubes  are  therefore 
filled  with  alternate  layers  of  glass  wool  and  the  drying  material. 
The  phosphorus  pentoxide  must  not  contain  any  of  the  trioxide. 
If  the  latter  is  present,  the  material  must  be  distilled  in  a  stream 
of  oxygen. 

45.  Making  Impervious  Connections. — Either  cork  or  rubber 
stoppers  may  be  used  in  closing  U-tubes  after  being  filled,  unless 
they  are  provided  with  glass  stoppers.  Both  cork  and  rubber  are 
by  no  means  impervious  to  moisture  and  carbon  dioxide.  The  ex- 
posed surface  must  therefore  be  coated  with  sealing  wax,  paraffine, 
or  shellac  varnish.  If  the  latter  is  used,  a  sufficient  time  should  be 
allowed  for  the  escape  of  the  alcohol  before  weighing  the  tube. 

When  the  tubes  are  connected,  short  pieces  of  rubber  tubing 
should  be  used,  but  the  glass  parts  of  the  apparatus  should  be 
brought  into  contact,  and  the  rubber  tubing  wired  or  tied  to  the 
glass  tube.  The  portion  of  the  rubber  tube  between  the  wire 
or  string  should  be  shellacked.  Only  in  this  manner  can  an  abso- 
lutely tight  and  impervious  joint  be  secured.  When  weighed 
U-tubes  are  disconnected  from  the  other  tubes,  the  ends  should 
be  closed  with  short  pieces  of  rubber  tubing  closed  at  one  end  by 
glass  plugs.  These  stoppers  should  be  removed  while  weighing 
the  tubes. 

EXERCISE  9. 

Determination   of    Water    of    Crystallization    in    Magnesium    Sulphate, 

MgS04.7H20. 

Fill  three  Marshand  tubes  with  calcium  chloride  as  directed  in  the  pre- 
ceding section.  Shellac  the  exposed  parts  of  the  rubber  or  cork  stoppers. 
Connect  these  tubes  with  a  hard-glass  bulb  as  shown  in  Fig.  9,  and  support 
the  series  so  that  the  bulb-tube  may  be  heated  by  means  of  a  Bunsen  burner. 
A  Bunsen  filter-pump  or  a  large  bottle  filled  with  water  having  an  exit  at 
the  bottom  for  the  water,  or  a  siphon  extending  to  the  bottom  of  the  bottle 
and  a  second  tube  passing  through  the  stopper  at  the  top  for  entrance  of 
air,  should  be  provided  to  draw  air  through  the  apparatus.  A  wash-bottle 
containing  concentrated  sulphuric  acid  should  be  inserted  between  the 
last  U-tube  and  the  aspirator.  All  joints  made  by  rubber  tubing  should 
have  the  glass  tubes  in  contact  and  the  rubber  tubes  should  be  wired  or 
tied  to  the  glass  and  the  part  between  should  be  shellacked.  After  the 


48 


QUANTITATIVE  ANALYSIS. 


apparatus  has  been  set  up  the  first  calcium-chloride  tube  should  be  closed 
by  a  rubber  tube  and  glass  plug.     Gentle  suction  should  be  applied  so 


FIG.  9. 

that  on  disconnecting  with  the  aspirator,  the  acid  in  the  wash-bottle  shall 
rise  in  the  long  glass  tube,  showing  a  partial  vacuum  in  the  apparatus. 
The  height  of  this  acid  should  be  carefully  noted,  and  if  an  appreciable 
change  occurs  in  five  minutes,  a  leak  exists  hi  some  of  the  joints,  which 
must  be  found  and  closed  before  proceeding  with  the  analysis.  When  the 
apparatus  has  been  made  air-tight,  light  the  Bunsen  burner  and  heat  the 
bulb-tube  to  redness  and  draw  air  slowly  through  the  apparatus  for  about 
one-half  hour.  The  U-tubes  to  the  ri^ht  of  the  bulb-tube  are  now  discon- 
nected and  weighed  after  being  carefully  cleaned.  In  the  meantime,  the 
bulb-tube  must  be  closed  with  a  rubber  stopper. 

The  magnesium  sulphate  should  be  weighed  out  in  a  glass  tube  of  about 
5  mm.  internal  diameter  and  about  8  cm.  long,  which  has  been  closed  at 
one  end.  This  tube  is  weighed,  about  one-half  gram  of  the  salt  placed  in 
it,  and  again  carefully  weighed.  Another  piece  of  glass  tubing  of  the  same 
diameter  and  of  sufficient  length  is  taken,  and  the  weighing-tube  containing 
the  salt  fastened  to  it  by  means  of  a  short  piece  of  rubber  tubing  of  suitable 
size.  By  means  of  this  handle,  the  weighed  tube  is  introduced  into  the 
bulb,  and  by  tapping  the  glass-tube  the  salt  is  deposited  in  the  bulb,  as 
shown  in  Fig.  10.  The  tube  is  carefully  withdrawn  so  as  not  to  drop  any 


FIG.  10. 

adhering  particles  of  salt,  the  weighed  tube  carefully  detached  from  the 
rubber  connector  and  again  accurately  weighed  with  whatever  magnesium 
sulphate  is  left  adhering  to  it.  The  difference  between  the  last  two  weights 
gives  the  weight  of  magnesium  sulphate  deposited  in  the  bulb-tube.  The 


DETERMINATION  OF  WATER.  49 

weighed  U-tubes  are  connected  in  position  again  and  the  apparatus  once 
more  tested  for  leaks.  If  none  are  found,  the  bulb-tube  is  gently  heated 
by  means  of  the  Bunsen  burner,  while  a  slow  current  of  air  is  drawn  through 
the  tubes.  If  the  U-tubes  become  appreciably  warm  from  the  heat  of  the 
Bunsen  burners,  they  must  be  shielded  from  the  heat  by  means  of  asbestos 
boards  or  other  non-conducting  material.  Finally,  when  the  moisture 
which  condenses  in  the  bulb  has  been  removed  by  the  current  of  ah*,  the 
bulb  is  heated  to  dull  redness  for  about  fifteen  minutes.  The  burner  is 
then  removed,  the  weighed  U-tubes  disconnected  and  weighed  again.  As 
soon  as  the  U-tube  is  removed,  the  bulb-tube  is  closed  with  a  rubber  stopper. 
It  is  then  replaced  and  the  heating  continued  for  about  half  an  hour  and 
the  U-tubes  again  weighed.  This  process  is  repeated  until  the  U-tubes 
no  longer  increase  in  weight.  The  percentage  of  water  is  calculated  from 
the  weight  of  magnesium  sulphate  taken  and  the  increase  in  weight  of  the 
U-tubes. 


DETERMINATION    OF    METALS. 

CHAPTER  IV. 
DETERMINATION   OF  METALS  AS  OXIDE. 

46.  Properties  of  Weighable  Precipitates. — In  Chapter  II  mention 
was  made  of  some  of  the  properties  of  a  salt  or  other  combination 
of  an  element  which  determine  its  availability  for  the  separation 
of  that  element  from  the  solution.     The  discussion  was  limited  to 
the  requirements  of  complete  precipitation  and  thorough  wash- 
ing.    In  other  words,  all  of  the  element  to  be  determined  must 
be  present  in  the  precipitate,  and  all  other  substances  must  be 
removed  from  the  precipitate.     It  is  then  in  condition  to  be 
weighed  as  such.     Generally  the  element  forms  part  of  a  more 
or  less  complex  chemical  compound — oxides,  chlorides,  and  sul- 
phates being  common  forms  in  which  elements  are  weighed.     This 
compound  must  be  of  definite  composition,  so  that  the  percentage 
of  the  element  to  be  determined  is  fixed  and  invariable.    The 
weighed  substance  must  be  non-volatile,  not  strongly  hygroscopic, 
and  in  general  unacted  on  by  the  atmosphere  or  the  fumes  liable 
to  be  present  in  the  laboratory.     Other  things  being  equal,  a 
precipitate  of  simple  composition  is  apt  to  be  of  constant  compo- 
sition, and  therefore  suitable  for  weighing. 

47.  Properties   of  the   Metallic    Oxides.  —  The    oxides    of   the 
metals  have  been  found  in  many  cases  to  possess  all  the  properties 
essential  to  a  weighable  compound.     The  affinity  of  most  ele- 
ments for  oxygen  is  so  strong  that  even  on  ignition  this  element  is 
not  expelled.     The  oxygen  of  the  air  will  even  reoxidize  most 
metals  if  they  have  become  reduced  by  carbon  or  reducing-gases. 
The  almost  universal  solvent  in  which  precipitations  are  made  is 
water  which  is  composed  of  oxygen  to  the  extent  of  about  89%. 
If  a  metal  is  to  be  weighed  as  an  OXIDE,  it  may  be  precipitated  as 

50 


DETERMINATION  OF  METALS  AS  OXIDE.  51 

HYDROXIDE,  CARBONATE,  or  OXALATE.  In  the  case  of  many  metals 
the  salts  of  volatile  acids  may  be  ignited,  leaving  the  metal  as 
oxide.  This  method  is  especially  applicable  to  nitrates  and 
organic  salts.  In  some  cases  the  salts  of  non-volatile  acids  may 
be  converted  into  oxides  by  heating  with  ammonium  carbonate 
or  mercuric  oxide,  the  acid  being  driven  off  hi  combination  as 
the  volatile  mercury  or  ammonium  salt.  These  various  methods 
will  now  be  studied  in  detail,  the  metals  being  considered  in  groups 
in  which  the  method  of  analysis  is  nearly  identical  for  each  group. 


IRON,   ALUMINIUM,   AND   CHROMIUM   GROUP. 

METAL    PRECIPITATED    AS    HYDROXIDE    BY    AMMONIUM    HYDROXIDE 
AND   WEIGHED   AS   OXIDE. 

48.  Precipitation  of  the  Hydroxides  of  Iron,  Aluminium,  and 
Chromium. — The  hydroxides  of  these  three  metals  are  almost 
entirely  insoluble  in  water,  either  hot  or  cold.  They  differ  mark- 
edly in  their  solubilities  in  alkali.  Iron  is  almost  absolutely 
insoluble  in  ammonia  or  the  fixed  alkalies,  while  aluminium  is 
easily  soluble  in  the  fixed  alkalies  and  slightly  so  in  ammonia. 
Chromium  occupies  an  intermediate  position  in  this  respect 
between  iron  and  aluminium,  being  almost  insoluble  in  ammonia 
and  moderately  soluble  in  the  fixed  alkalies. 

Iron  may,  therefore,  be  completely  precipitated  by  sodium, 
potassium,  or  ammonium  hydroxide.  It  is  found,  however,  that 
ferric  hydrate  carries  down  some  of  the  potassium  or  sodium 
which  cannot  be  washed  out.  Ammonium  hydroxide  is,  there- 
fore, always  used  in  precipitating  iron,  which  for  this  purpose 
must  be  in  the  ferric  condition.  Any  ferrous  iron  may  be  converted 
into  ferric  by  boiling  the  solution  after  the  addition  of  a  few  drops 
of  nitric  acid  or  bromine-water. 

As  chromium  and  aluminium  are  soluble  in  sodium  hydroxide, 
these  metals  are  also  precipitated  by  ammonium  hydroxide.  As 
aluminium  hydroxide  is  soluble  in  even  a  slight  excess  of  ammo- 
nia, the  solution  must  be  made  as  nearly  neutral  as  possible. 
Ammonium  chloride  greatly  weakens  the  alkaline  properties  of 
ammonia,  and  for  this  reason  a  considerable  amount  of  this  salt 


52  QUANTITATIVE  ANALYSIS. 

is  added  to  the  solution  of  aluminium.     This  is  unnecessary  in 
•  the  case  of  chromium  and  iron,  because  of  their  insolubility  in 
ammonium  hydroxide. 

Chromium  hydroxide  dissolves  slightly  in  ammonium  hydrox- 
ide in  the  cold,  but  is  completely  precipitated  on  boiling.  Chro- 
mium must  be  in  the  chromic  or  basic  condition.  Chromates  are 
reduced  to  chromic  compounds  by  adding  sodium  sulphite  to 
the  solution  acidified  with  hydrochloric  acid,  or,  still  better,  by 
passing  in  gaseous  sulphur  dioxide. 

49.  Washing. — The  hydroxides  of  these  three  metals  are  gelat- 
inous, and  therefore  there  is  no  danger  of  their  passing  through 
the  paper.     They  clog  the  pores,  however,  so  that  rapid  filtration 
and  washing  can  be  accomplished  only  by  using  suction  or  a  hot- 
water  funnel.     The  washing  must  be  thorough,  as  any  ammo- 
nium chloride  left  in  the  precipitate  reacts  with  the  hydroxides, 
liberating  ammonia  and  forming  a  chloride  of  the  metal  which 
volatilizes  so  that  the  result  obtained  is  low.     When  these  hydrox- 
ides dry  they  shrivel  and  crack  so  that  if  the  washing  is  discon- 
tinued for  a  short  time  fissures  are  produced  in  the  precipitate, 
through  which  the  water  passes  without  taking  the  impurities 
out  of  the  precipitate.     If  the  washing  must  be  discontinued, 
the  stem  of  the  funnel  should  be  closed  with  a  piece  of  rubber 
tubing  and  a  glass  plug  and  the  funnel  filled  with  water.    Many 
of  these  difficulties  may  be  overcome  by  using  paper  pulp,  as 
described  on  page  27. 

50.  Ignition   of   the   Filter-paper.— When   the    filter-paper    is 
burned  neither  the  aluminium  nor  the  chromium  hydroxides  are 
reduced  by  the  hot  carbon.     Ferric  hydroxide,  however,  is  partly 
reduced  to  the  magnetic  oxide  of  iron,  Fe304,  while  the  remainder 
is  converted  into  ferric  oxide,  Fe203.     As  much  of  the  precipitate 
as  possible  should  therefore  be  detached  from  the  paper,  so  that 
the  error  from  this  source  may  be  negligible.     If  the  filter-paper, 
with  whatever  ferric  hydroxide  cannot  be  removed,  is  burned  in 
the  platinum  crucible,  the  iron  which  is  reduced  is  apt  to  alloy 
with  the  platinum  and  be  difficult  of  removal.     It  should,  there- 
fore, be  burned  in  the  flame  of   the  Bunsen  burner  while  held 
by  a  platinum  wire.     If  a  more  open  platinum  dish,  such  as  is 
used  in  milk  analysis,  is  at  hand,  the  moist  precipitate  may  be 


DETERMINATION  OF  METAL  AS  OXIDE.  53 

placed  in  the  dish  with  the  apex  of  the  filter-paper  on  top.  The 
side  of  the  dish  is  now  heated  so  that  the  paper  dries  and  finally 
burns.  The  access  of  the  air  to  the  paper  is  so  good  that  little 
iron  is  reduced. 

51.  Contamination  of  the  Precipitate  with  Silica. — It  is  very 
difficult  to  obtain  precipitates  of  these  three  metals  which  are 
free  from  silica.  The  ammonia  which  is  used  as  a  precipitant 
almost  invariably  contains  silica  which  has  been  derived  from  the 
reagent  bottle,  most  of  which  is  precipitated  and  can  be  filtered 
off.  Appreciable  amounts  of  silica  are  obtained  by  the  disinte- 
gration of  the  glass  of  the  beaker  in  which  the  metal  is  precipi- 
tated. This  silica  is  filtered  off  and  weighed  with  the  precipitate. 
The  digestion  of  the  alkaline  solution  should,  therefore,  be  as  brief  as 
possible,  and  the  excess  of  ammonia  as  small  as  possible.  If  too 
much  ammonia  is  added  it  should  be  neutralized  with  dilute 
hydrochloric  acid.  It  is  also  better  to  conduct  the  precipitation  in 
a  porcelain  dish,  since  porcelain  is  less  easily  attacked  by  alkalies 
than  glass.  If  a  platinum  dish  is  available,  this  source  of  error  is 
entirely  avoided.  If  glass  is  used  the  error  from  this  source  may 
be  reduced,  by  working  rapidly,  to  almost  negligible  quantities  of 
.1  or  .2%.  If  Jena  beakers  are  used  the  error  is  still  less. 

If  the  highest  degree  of  accuracy  is  desired  the  percentage  of 
silica  must  be  determined  and  a  correction  made  on  the  weight  of 
the  original  precipitate.  This  determination  is  carried  out  by 
dissolving  the  precipitate  in  acid,  evaporating  to  dryness  on  the 
water-bath,  redissolving  in  acid  and  water,  and  filtering  off  the 
silica,  which  is  washed,  ignited,  and  weighed.  For  this  purpose 
the  iron  precipitate  may  be  dissolved  by  digesting  the  powdered 
material  with  hydrochloric  acid.  Aluminium  oxide  may  be  dis- 
solved in  the  same  manner,  but  the  action  is  very  slow  unless  paper 
pulp  is  used  during  the  precipitation,  as  directed  on  page  27.  It  dis- 
solves more  easily  if  first  heated  with  eight  parts  of  concentrated 
sulphuric  acid  and  three  parts  of  water,  and  then  water  added. 
It  is  also  rendered  soluble  by  fusion  with  potassium  bisulphate. 
This  method  is  especially  to  be  recommended  for  both  iron  and 
aluminium,  because  the  silica  may  be  rendered  insoluble  by 
dissolving  the  melt  in  dilute  sulphuric  acid  and  evaporating  until 
fumes  of  sulphuric  acid  are  evolved.  Chromium  oxide  is  best 
dissolved  by  fusion  in  the  platinum  crucible  with  sodium  carbo- 


54  QUANTITATIVE  ANALYSIS. 

nate  and  potassium  nitrate,  potassium  chromate  being  formed. 
This  solution  must  be  acidified  with  hydrochloric  acid  and  evap- 
orated to  dryness  and  the  silica  determined  as  in  the  other  oxides. 
52.  Removal  of  Organic  Material. — These  metals  are  not 
completely  precipitated  if  organic  matter  is  present  in  the  solu- 
tion. It  may  be  detected  and  also  removed  by  evaporating  the 
solution  to  dryness  and  igniting.  This  is  not  applicable  if  chlo- 
rides are  present.  In  that  case  boiling  with  concentrated  nitric 
acid  or  potassium  chlorate  and  hydrochloric  acid  must  be  resorted 
to.  The  solution  may  also  be  treated  with  excess  of  sulphuric  acid 
and  evaporated  to  dryness  and  ignited.  If  the  amount  of  organic 
matter  is  small,  the  ignition  will  frequently  be  unnecessary,  as 
the  charred  material  will  be  entirely  destroyed  by  the  concentrated 
sulphuric  acid  produced  by  the  evaporation.  A  few  drops  of 
nitric  acid  followed  by  heating  the  solution  will  render  it  clear 
more  quickly. 

EXERCISE  10. 

Determination  of  Aluminium  in  Potash  Alum, 
K2S04.A12(S04)3.24H20. 

Weigh  carefully  2  grams  of  pure  recrystallized  potash  alum.  If  bal- 
anced watch-crystals  are  used,  place  them  on  the  scale-pans  and  ascertain 
if  the  pointer  indicates  zero  when  the  beam  and  pans  are  released.  If  it 
does  not,  balance  the  watch-crystals  by  means  of  a  small  weight  or  the 
rider.  Place  a  2-gram  weight  on  the  right-hand  watch-crystal  and  weigh 
out  2  grams  of  the  alum  as  directed  in  Exercise  4.  Transfer  to  a  500-c  c. 
beaker,  brushing  off  the  watch-crystal  with  a  clean  camel's-hair  brush,  and 
dissolve  in  200  c.c.  of  water.  Add  about  50  c.c.  of  ammonium  chloride 
solution,  which  should  be  filtered  if  not  perfectly  clear.  The  ammonium 
chloride  may  also  be  introduced  together  with  paper  pulp  by  adding  15  c.c. 
of  concentrated  hydrochloric  acid  to  two  or  three  ashless  filter-papers  *  and 
25  c.c.  of  dilute  ammonia.  Warm  on  a  hot  plate  or  an  asbestos  board  or 
wire  gauze  over  a  Bunsen  burner.  Add  filtered  dilute  ammonia  with  con- 
stant and  vigorous  stirring  of  the  solution  until  the  odor  of  ammonia  can 
be  detected  after  vigorous  stirring.  Red  litmus-paper  should  be  only 
slowly  turned  blue  when  held  over  the  solution.  If  too  much  ammonia  has 
been  added  it  should  be  nearly  neutralized  with  dilute  hydrochloric  acid. 

*  The  weight  of  the  ash  of  this  paper  is  stated  on  the  package  obtained  from 
the  dealers  in  chemical  apparatus  and  is  usually  small  enough  to  be  neglected. 
The  paper  should  be  kept  in  a  box  or  other  receptacle  where  it  can  be  kept  per- 
fectly clean. 


DETERMINATION  OF  METAL  AS  OXIDE. 


55 


53.  Washing  and  Transferring  the  Precipitate  to  the  Filter-paper. — 
After  the  solution  has  thus  been  made  faintly  alkaline,  heat  it  nearly  to 
the  boiling-point  for  a  few  minutes.  Fit  a  funnel  with  an  ashless  filter-paper 
preparatory  to  filtering  by  suction  as  directed  in  Chapter  II.  An  11  or 
I2£  cm.  filter-paper  should  be  used  and  the  funnel  should  be  of  such  a  size 
that  a  space  at  least  a  quarter  of  an  inch  is  left  free  above  the  paper.  Allow 
the  precipitate  to  settle  a  few  minutes  and  then  decant  the  clear  liquid 
through  the  paper,  using  suction.  Guide  the  stream  of  water  by  holding 
the  stirring-rod  against  the  lip  of  the  beaker.  Add  about  75  c.c.  of  hot 
water  from  the  wash-bottle,*  which  for  this  purpose  is  kept  on  a  steam-bath 
or  hot  plate.  Stir  the  precipitate  well  with  the  stream  of  water.  For 
this  purpose  the  jet  of  water  must  be  quite  large  and  care  must  be  taken 
not  to  spatter  the  precipitate  out  of  the  beaker.  Allow  the  precipitate  to 
settle  and  decant  through  the  funnel  as  before.  Repeat  the  washing  by 
decantation  twice  aud  then  transfer  the  main  precipitate  to  the  funnel. 
Holding  the  stirring-rod  against  the  beaker  with  the  left  hand,  sweep  the 
precipitate  from  the  beaker  by  means  of  a  stream  of  water  from  the  wash- 
bottle,  which  is  held  in  the  right  hand  as  shown  in  Fig.  11.  When  the 


FIG.  11. 

bulk  of  the  precipitate  has  been  transferred  in  this  manner,  the  particles 
adhering  firmly  to  the  glass  must  be  removed  by  means  of  a  so-called  police- 
man, which  is  made  by  inserting  the  end  of  a  rather  large-sized  stirring-rod 

*  A  so-called  hot-water  wash-bottle  is  prepared  by  fastening  asbestos  paper, 
a  sheet  of  cork,  or  other  insulating  material  around  the  neck  of  an  ordinary  wash- 
bottle  by  means  of  pieces  of  wire  or  string.  A  thick  cord  closely  wound  around 
the  neck  of  the  flask  will  serve  the  same  purpose.  The  water  should  not  be 
allowed  to  boil  or  steam  will  enter  the  mouth  when  blowing  out  the  water.  To 
overcome  this  difficulty,  a  piece  of  rubber  tubing  should  be  placed  on  the  glass 
tube  which  enters  the  mouth.  By  closing  this  tube  with  the  teeth  when  not 
blowing,  steam  is  kept  out  of  the  mouth. 


56  QUANTITATIVE  ANALYSIS. 

into  a  short  piece  of  rubber  tubing.  The  rubber  tube  should  be  left  pro- 
jecting slightly  beyond  the  end  of  the  glass  tube  and  sealed  together  with 
a  little  bicycle  cement. 

The  bottom  as  well  as  the  sides  of  the  beaker  must  be  rubbed  clean 
with  this  policeman.  Both  the  stirring-rod  and  the  policeman  must  finally 
be  freed  from  particles  of  the  precipitate.  If  portions  of  the  precipitate 
cannot  be  rubbed  off  the  glass  they  should  be  dissolved  in  a  few  drops  of 
dilute  hydrochloric  acid  and  reprecipitated  by  neutralizing  with  ammonia 
and  warming  the  solution.  When  all  of  the  precipitate  has  been  transferred 
to  the  funnel,  it  is  washed  with  hot  distilled  water  until  free  from  chlorides. 
The  exposed  portion  of  the  filter-paper  must  also  be  well  washed.  Test 
the  wash-water  from  time  to  time  with  nitric  acid  and  silver  nitrate,  finally 
using  several  cubic  centimeters.  In  washing  this  precipitate,  care  must 
be  taken  that  all  of  the  water  is  not  sucked  off  the  precipitate  so  that  it 
cracks,  producing  fissures  through  which  the  wash-water  passes  without 
dissolving  the  ammonium  chloride.  If  for  any  reason  the  washing  must 
be  discontinued,  the  stem  of  the  funnel  should  be  closed  with  a  short  piece 
of  rubber  tubing  and  a  glass  plug  and  the  funnel  filled  with  water. 

54.  Ignition  and  Weighing  of  the  Precipitate. — During  the  washing  of  the 
precipitate  a  platinum  crucible  with  the  lid  is  supported  on  a  clean  pipe- 
stem  or  platinum  triangle  resting  on  a  tripod  and  heated  to  redness  with  a 
Bunsen  burner.  The  air-vents  of  the  Bunsen  burner  are  adjusted  so  that 
the  flame  is  entirely  colorless.  The  crucible  must  be  kept  well  above  the 
inner  cone.  While  still  red-hot  the  crucible  is  seized  with  a  clean  pair  of 
forceps  or  crucible  tongs  and  placed  in  a  desiccator,  which,  for  this  purpose, 
is  provided  with  a  pipe-stem  or  glass  triangle.  After  fifteen  minutes  it  is 
weighed,  the  weight  being  recorded  on  the  right-hand  side  of  the  NOTE- 
BOOK, the  page  being  fully  labelled  and  dated.  The  left-hand  page  is 
reserved  for  a  brief  description  of  the  process  of  analysis,  and  especially 
any  procedure  which  led  to  disaster  or  failure  and  the  remedy  adopted. 
All  of  this  work  should  be  done  during  the  intervals  when  the  liquid  is 
running  through  the  funnel.  It  will  run  just  as  fast  if  not  watched. 

The  moist  precipitate  with  the  paper  is  taken  out  of  the  funnel  and 
transferred  to  the  weighed  platinum  crucible.  If  there  is  any  delay,  so 
that  the  precipitate  is  partially  dried,  it  must  be  placed  in  the  steam-oven 
and  completely  dried.  It  then  shrinks  to  a  very  small  bulk.  The  lid  of 
the  crucible  must  in  this  case  be  very  carefully  placed  on  the  crucible,  as  a 
little  moisture  left  in  the  hard  lumps  of  the  precipitate  forms  steam  and 
scatters  the  precipitate  with  considerable  force.  When  burning  the  moist 
precipitate,  the  paper  is  placed  in  the  crucible  with  the  apex  up,  so  that 
spattering  during  drying  will  not  throw  the  precipitate  out  of  the  crucible. 
Any  portions  of  the  precipitate  left  on  the  funnel  are  wiped  off  with  small 
pieces  of  quantitative  filter-paper  and  placed  in  the  crucible.  The  covered 
crucible  is  then  placed  on  the  triangle  and  heated  with  a  very  small  flame 


DETERMINATION  OF  METAL  AS  OXIDE.  57 

until  steam  and  volatile  matter  cease  to  escape.  The  heat  is  gradually 
increased  until  the  water  and  most  of  the  volatile  matter  are  expelled  from 
the  precipitate  and  the  paper.  The  lid  is  now  taken  off  the  crucible,  which 
is  placed  on  its  side  and  heated  with  the  full  flame  of  the  Bunsen  burner. 
As  the  carbon  burns,  the  crucible  is  turned  to  expose  fresh  portions  to  the 
action  of  the  air.  The  crucible  is  finally  heated  with  the  blast-lamp  until 
the  aluminium  oxide  is  perfectly  white.  While  still  red-hot,  the  crucible 
is  transferred  to  the  desiccator  and  after  fifteen  minutes,  weighed.  The 
crucible  is  again  heated  in  the  blast-lamp,  cooled  in  the  desiccator,  and 
weighed.  This  is  repeated  until  the  weight  is  constant  within  .2  or  .3 
milligram. 

The  percentage  of  A1203  found  is  calculated  by  dividing  the  weight  of 
the  precipitate  by  the  weight  of  alum  taken  and  multiplying  the  result  by 
100.  The  theoretical  percentage  of  A12O3  in  potash  alum  is  10.76.  If  a 
result  much  higher  than  this  is  obtained,  the  precipitate  may  be  contami- 
nated with  silica,  which  is  determined  as  directed  in  the  next  exercise,  with 
the  exception  that  grinding  the  precipitate  will  be  unnecessary  if  paper 
pulp  has  been  used  during  the  precipitation. 

EXERCISE  ii. 
Determination  of  Iron  in  Soft-iron  Wire.* 

In  this  exercise  the  determination  should  be  carried  out  in  duplicate, 
the  two  analyses  being  carried  on  side  by  side. 

Clean  the  wire  thoroughly  by  rubbing  with  emery-cloth  or  sandpaper 
until  all  rust  is  removed  and  finally  wiping  with  filter-paper.  The  wire 
should  be  wound  in  a  spiral  around  a  lead-pencil  and  should  not  be  touched 
with  the  fingers  after  being  wiped  with  the  filter-paper.  Weigh  out  from 
200  to  300  milligrams  of  the  wire  and  transfer  to  a  200-c.c.  beaker.  Add 
5  c.c.  of  concentrated  nitric  acid  and  a  little  water.  Cover  the  beaker  with 
a  watch-crystal  and  warm  on  the  hot  plate  under  the  hood.  When  the 
iron  is  all  dissolved,  rinse  off  the  watch-crystal  with  a  little  distilled  water 
and  evaporate  to  dryness  on  the  hot  plate.  Add  10  c.c.  of  dilute  hydro- 
chloric acid  and  again  evaporate  to  dryness  on  the  water-bath.  After  the 
ferric  chloride  is  dry, heat  for  one-half  hour  on  the  water-bath,  dissolve  in 
a  few  cubic  centimeters  of  dilute  hydrochloric  acid,  add  a  little  water,  and 
filter  off  the  silica  on  a  small  filter-paper.  Wash  the  paper  free  from  iron 
with  hot  distilled  water  to  which  a  little  hydrochloric  acid  has  been  added. 
The  volume  of  the  filtrate  should  be  about  200  c.c. 

It  is  best  to  precipitate  the  iron  in  a  platinum  dish.  If  such  a  dish  is 
not  available,  an  evaporating-dish  of  Berlin  porcelain  should  be  used  or  a 

*This  wire  is  such  as  is  used  in  standardizing  potassium  permanganate  or 
other  oxidizing  solutions  and  is  sold  for  use  as  piano-strings,  and  to  florists  for 
making  wreaths. 


58  DETERMINATION  OF  METALS. 

Jena  beaker.  The  solution  is  neutralized  with  filtered  ammonia,  and 
brought  to  a  boil.  After  allowing  the  precipitate  to  settle  for  a  few  minutes, 
the  clear  liquid  is  decanted  through  a  12^  or  15-cm.  ashless  filter-paper 
arranged  for  suction  as  described  in  Chapter  II.  The  precipitate  is  washed 
twice  by  decantation  and  then  transferred  to  the  funnel  and  washed  with 
hot  water,  observing  the  precautions  given  in  Exercise  10.  Toward  the 
end  of  the  washing  the  stream  of  water  from  the  wash-bottle  should  be  so 
directed  as  to  loosen  the  precipitate  from  the  sides  of  the  paper  and  wash 
it  down  into  the  point  of  the  funnel.  When  the  precipitate  has  been 
washed  free  from  chlorides  it  is  dried  in  the  steam-  or  air-oven.  The  tem- 
perature of  the  latter  should  not  be  allowed  to  rise  much  above  100°  Centi- 
grade. 

55.  Ignition  of  the  Paper  on  Platinum  Wire. — A  piece  of  glazed  paper, 
preferably  of  a  light  color,  about  8X10  inches  is  spread  out  on  the  desk. 
A  small  camel's-hair  brush,  a  piece  of  platinum  wire  about  4  inches  long 
fastened  to  a  glass  or  other  handle,  a  Bunsen  burner  with  a  long  blue  flame, 
the  ignited  and  weighed  platinum  crucible  and  the  dried  precipitate  in  the 
funnel  are  brought  together.  The  glazed  paper  is  carefully  brushed  clean 
with  the  camel's-hair  brush.  The  crucible  is  placed  in  the  center  of  the 
paper.  The  paper  is  taken  out  of  the  funnel  and  the  precipitate  loosened 
with  the  platinum  wire  and  dropped  into  the  crucible.  Very  little  of  the 
precipitate  should  be  left  on  the  paper.  Rubbing  the  two  inner  surfaces 
of  the  filter-paper  together  will  sometimes  loosen  a  considerable  amount 
of  the  precipitate.  If  any  portion  of  the  precipitate  is  found  adhering  to 
the  funnel  it  should  be  rubbed  off  with  a  small  piece  of  ashless  filter-paper 
which  has  been  moistened  with  a  little  dilute  nitric  acid.  The  filter-paper 
is  now  folded  as  if  to  be  inserted  into  a  funnel.  It  is  rolled  up  tightly  on 
one  of  its  edges  and  the  platinum  wire  is  wrapped  around  the  upper  part 
of  the  paper  which  is  always  freest  from  portions  of  the  precipitate.  It  is 
now  held  by  means  of  the  handle  of  the  platinum  wire  over  the  crucible 
and  the  point  touched  to  the  Bunsen  burner  flame  and  ignited.  The  charred 
paper  is  completely  burned  by  holding  it  in  the  oxidizing  flame  of  the  Bunsen 
burner  and  then  exposing  the  red-hot  carbon  to  the  oxygen  of  the  air. 
The  ash  is  allowed  to  drop  into  the  crucible.  The  wire  is  now  brushed  off, 
the  crucible  is  taken  in  the  left  hand  and  any  particles  which  have  fallen 
on  the  paper  are  brushed  into  it.  The  crucible  is  placed  on  its  side  on  a 
pipe-stem  triangle  and  heated  with  the  Bunsen  burner  until  any  unburned 
carbon  of  the  filter-paper  is  oxidized. 

If  a  platinum  dish  is  available  the  moist  precipitate  with  the  apex  up 
may  be  placed  in  the  dish  which  is  gently  heated  on  one  side  with  the  Bunsen 
burner.  When  the  paper  is  completely  charred  the  dish  may  be  heated 
to  redness  until  the  carbon  is  completely  burned.  The  precipitate  is  then 
heated  with  the  blast  lamp  for  a  few  minutes,  cooled  in  the  desiccator  and 
weighed.  It  is  re-ignited  and  weighed  until  the  weight  is  constant. 


DETERMINATION  OF  METALS  AS  OXIDE.  59 

56.  Calculation. — The  weight  of  Fe2O3  multiplied    by  the  percentage 
of  iron  in   Fe203   (70  per  cent)    gives   the  weight   of   iron  found.     This 
number  multiplied  by  100  and  divided  by  the  weight  of  iron  taken  gives 
the  percentage  of  iron  in  the  wire  which  is  usually  99.6  to  99.8  per  cent. 
The  result  may  also  be  computed  by  logarithms.     The  log.  of  the  weight 
of  the  precipitate  is  added  to  the  log.  of  the  percentage  of  iron  in  Fe203. 
From  this  sum  is  subtracted  the  log.  of  the  weight  of  iron  wire  taken.     This 
log. +2.0  gives  the  log.  of  the  percentage  of  iron  in  the  wire.    The  dupli- 
cates in  this  analysis  should  agree  within  .1  to  .2  per  cent.     If  they  vary 
more  than  this  amount  a  third  or  even  a  fourth  analysis  must  be  made. 

57.  Determination  of  Silica. — If  the  percentage   found  is   greater  than 
99.8  per  cent,  the  error  is  probably  due  to  silica,  especially  if  the  precipita- 
tion was  carried  out  in  a  beaker.     Instead  of  making  another  determination, 
the  percentage  of  silica  should  be  determined  and  a  correction  made  on  the 
percentage  found.     For  this  purpose  the  precipitate  is  ground  quite  fine  in 
an  agate  mortar.     Loss  by  spattering  should  be  avoided  by  covering  the 
mortar  with  a  piece  of  paper  through  which  the  pestle  passes.     When  as 
much  of  the  precipitate  as  can  be  readily  removed  from  the  crucible  has  been 
ground,  the  pulverized  material  is  returned  to  the  crucible  which  is  then 
weighed.     The  difference  between  this  weight  and  the  weight  of  the  crucible 
gives  the  weight  of  the  iron  oxide  taken  for  the  silica  determination.     The 
loose  material  is  transferred  to  a  100-c.c.  beaker,  15  c.c.  concentrated  hydro- 
chloric acid  and  10  c.c.  water  added.     The  beaker  is  digested  on  the  hot 
plate  until  the  iron  is  dissolved.     A  little  acid  is  also  added  to  the  crucible 
which  is  also  heated  until  the  iron  is  dissolved.     This  solution  is  added  to 
that  in  the  beaker.     The  beaker  is  now  placed  on  the  water-bath  and  the 
solution  evaporated  to  dryness  and  heated  dry  for  one-half  hour.     Dilute 
hydrochloric  acid  is  added  and  the  heating  continued  until  the  iron  is  again 
dissolved. 

The  powdered  precipitate,  after  being  weighed  in  the  crucible,  may 
also  be  fused  with  acid  potassium  sulphate.  The  burner  should  be  turned 
down  until  the  heat  is  just  sufficient  to  keep  the  material  fused.  When 
the  iron  is  all  dissolved  the  crucible  is  allowed  to  cool,  transferred  to  a 
beaker  and  the  melt  extracted  with  dilute  sulphuric  acid  and  the  solution 
evaporated  until  fum.es  of  sulphuric  acid  are  evolved. 

The  solution  obtained  by  either  method  is  filtered  through  an  ashless 
filter-paper,  the  silica  in  the  beaker  being  transferred  to  the  paper  by  means 
of  a  policeman  as  described  in  Exercise  10.  The  silica  is  washed  and, 
after  burning  the  paper,  is  ignited,  finally  for  a  few  minutes  with  the 
blast-lamp.  The  crucible  is  cooled  in  the  desiccator  and  weighed.  The 
contents  of  the  crucible  should  be  pure  white,  any  tinge  of  red  indi- 
cating iron  due  to  imperfect  washing  or  undissolved  ferric  oxide.  On 
adding  a  little  pure  hydrofluoric  acid  the  precipitate  should  be  entirely 
volatilized  on  heating.  Any  residue  which,  on  repeated  treatment  with 


60  DETERMINATION  OF  METALS. 

hydrofluoric  acid,  fails  to  volatilize  is  not  silica.  As  only  part  of  the 
original  iron  precipitate  was  taken  for  this  analysis  the  amount  of  silica 
in  the  entire  precipitate  must  be  calculated  and  deducted  to  give  the 
true  weight  of  Fe203  from  which  the  percentage  of  iron  in  the  iron  wire 
must  be  re-calculated,  as  in  the  following  illustration.  0.3350  gram  of  t  he 
iron  wire  was  weighed  out.  0.4820  gram  of  the  ferric  oxide  was  obtained. 
The  percentage  of  iron  calculated  from  this  weight  of  oxide  by  the  method 
already  given  is  100.7.  After  grinding  the  precipitate  for  the  silica  deter- 
mination, 0.4153  gram  remained  in  which  .0043  gram  Si02  was  found. 
The  amount  present  in  the  entire  precipitate  was  obtained  from  the  pro- 
portion 0.4135  :  .0043  ::  0.482  :  x  where  a;  =  .0050.  On  subtracting  the 
silica  from  the  weight  of  the  impure  precipitate  we  obtain  0.4770  as  the 
weight  of  ferric  oxide.  On  recalculating  the  percentage  of  iron  we  obtain 
99.68. 


DETERMINATION  OF  COPPER,  MANGANESE,  NICKEL, 
AND  COBALT  BY  PRECIPITATION  WITH  CAUSTIC 
ALKALI. 

58.  Precipitation  and  Washing. — A  number  of  metals  cannot 
be  precipitated  as  hydroxide  with  ammonia,  because  of  their 
solubility  in  ammonium  salts.  Some  of  these  metals  form  hydrox- 
ides which  are  insoluble  in  sodium  and  potassium  hydroxides, 
and  may  be  precipitated  by  either  of  these  reagents.  Consider- 
able difficulty  is  experienced  in  completely  washing  out  the  excess 
of  alkali  from  the  precipitate  and,  unlike  ammonia,  it  is  not 
expelled  by  ignition.  On  digesting  the  ignited  precipitate  with 
water  the  alkali  can  frequently  be  detected  by  its  alkaline  reac- 
tion. By  filtering,  washing,  and  reigniting,  a  considerable  loss 
in  weight  is  frequently  noted. 

The  solution  should  be  dilute  and  only  a  slight  excess  of  alkali 
added.  The  liability  of  contamination  of  the  precipitate  with 
silica  from  the  action  of  the  alkali  on  the  glass,  which  was  met 
with  in  the  determination  of  iron,  aluminium,  and  chromium  is 
still  more  pronounced  when  the  stronger  alkalies  are  used.  The 
precipitation  should  therefore  never  be  carried  out  in  glass.  Por- 
celain or,  wherever  possible,  platinum  dishes  should  be  used. 
The  precipitate  should  be  tested  for  the  presence  of  silica  by  solu- 
tion in  hydrochloric  acid,  evaporation  to  dry  ness,  again  dissolv- 


DETERMINATION  OF  METALS  AS  OXIDE.  61 

ing  in  acid,  filtering,  washing,  and  weighing  the  residue  as  directed 
under  iron.  These  oxides  dissolve  with  ease  in  acid. 

59.  Composition  of  the  Ignited  Oxides. — Considerable  diffi- 
culty is  experienced  in  obtaining  precipitates  of  constant  compo- 
sition. CUPRIC  HYDROXIDE  is  converted  on  ignition  into  cupric 
oxide,  CuO,  but  if  reduced  by  the  filter-paper  or  reducing  gases 
from  the  flame  to  metallic  copper  it  reoxidizes  to  cuprous  oxide, 
Cu20.  This  is  converted  into  cupric  oxide  by  moistening  with 
nitric  acid,  evaporating  to  dryness  and  igniting. 

MANGANESE  HYDROXIDE  is  oxidized  by  the  oxygen  of  the  air 
even  at  the  ordinary  temperature.  It  is  not  reduced  either  by 
the  carbon  of  the  filter-paper  or  by  the  reducing  gases  of  the 
flame.  On  ignition  in  presence  of  air  it  is  converted  into  man- 
ganoso-manganic  oxide,  Mn304.  It  is  impossible,  however,  to 
obtain  a  precipitate  in  which  the  percentage  of  manganese  cor- 
responds to  the  theoretical  for  Mn304.  Pickering  has  shown  that 
the  amount  of  manganese  in  the  ignited  oxide  varies  from  69.688 
to  74.997%,  according  to  the  temperature  to  which  the  oxide 
has  been  heated  and  other  undetermined  conditions.  For  this 
reason  only  small  amounts  of  manganese  should  be  weighed  as 
Mn304. 

When  COBALTOUS  HYDOXIDE  is  heated  in  contact  with  the 
solution  from  which  it  was  precipitated,  it  is. gradually  converted 
by  the  oxygen  of  the  air  into  cobaltoso-cobaltic  hydroxide.  On 
drying,  it  absorbs  more  oxygen,  but  on  strong  ignition  oxygen  is 
driven  off,  leaving  cobaltous  oxide  CoO.  On  cooling  in  contact 
with  the  air,  oxygen  is  again  absorbed,  the  light  brown  cobaltous 
oxide  changing  more  or  less  completely  to  the  black  cobaltoso- 
cobaltic  oxide  Co304.  If  the  air  is  excluded  by  a  current  of  car- 
bon dioxide  this  action  is  prevented,  and  the  pure  cobaltous 
oxide  is  obtained.  This  oxide  is  readily  reduced  to  metallic 
cobalt  by  ignition  in  a  stream  of  hydrogen  and  the  alkali  which 
could  not  be  completely  removed  by  washing  the  hydroxide  can 
now  be  quite  readily  extracted  with  hot  water. 

NICKELOUS  HYDROXIDE  is  converted  on  ignition  into  nickelous 
oxide  NiO,  which  is  unaltered  on  heating  in  the  air.  It  is  readily 
reduced  to  metallic  nickel  on  ignition  in  a  stream  of  hydrogen  or 
of  carbon  monoxide. 


62  DETERMINATION  OF  METALS. 


EXERCISE  12. 

Determination  of  Copper  in  Crystallized  Copper   Sulphate, 
CuS04.5H2O. 

Weigh  out  1  gram  of  the  pure  salt,  transfer  to  a  platinum  or  porcelain 
dish,  and  dissolve  in  about  200  c.c.  of  distilled  water.  Make  a  dilute  solution 
of  caustic  soda  by  dissolving  about  1  gram  in  25  c.c.  distilled  water.  Heat 
the  copper  solution  nearly  to  boiling  and  add  the  caustic-soda  solution  with 
constant  stirring  until  no  further  precipitation  occurs.  Allow  to  settle 
for  a  few  minutes,  then  decant  through  a  paper  fitted  into  a  funnel  for 
suction.  Wash  the  precipitate  twice  by  decantation  with  hot  water,  then 
transfer  to  the  funnel  and  complete  the  washing  with  hot  water  until  the 
wash-water  is  perfectly  neutral.  Dry  the  precipitate  in  the  funnel  at 
100°  C.  Remove  the  precipitate  from  the  paper  as  directed  in  Exercise  11 
and  place  it  on  a  watch-crystal.  Burn  the  paper  on  the  platinum  wire, 
being  careful  to  wrap  the  wire  around  a  portion  of  the  paper  on  which  there 
is  the  least  amount  of  precipitate.  Let  the  ash  fall  into  a  porcelain  cruci- 
ble. Add  a  few  drops  of  concentrated  nitric  acid,  place  the  lid  on  the 
crucible,  and  evaporate  off  the  excess  of  acid  with  a  very  small  flame 
of  the  Bunsen  burner,  finally  heating  quite  strongly.  If  any  carbon  from 
the  filter-paper  remains  unburned,  repeat  the  treatment  with  nitric  acid. 
When  the  crucible  is  cold  set  it  on  the  glazed  paper  and  transfer  the  main 
portion  of  the  precipitate  to  the  crucible,  brushing  the  watch-crystal  and 
paper  thoroughly.  The  moist  paper  with  the  precipitate  may  also  be  placed 
in  the  crucible  and  the  paper  burned  as  directed  in  Exercise  10.  The 
precipitate  must  be  moistened  with  nitric  acid  as  directed  for  the  ash  of 
the  filter-paper.  Finally  heat  the  crucible  with  the  full  flame  of  the  Bunsen 
burner  for  ten  or  fifteen  minutes,  cool  in  the  desiccator,  and  weigh.  As 
copper  oxide  is  hygroscopic,  the  weight  should  be  taken  rapidly.  The 
crucible  is  reheated  and  when  weighed  the  second  time  the  weights  should 
be  placed  on  the  right-hand  pan  before  the  crucible  is  placed  on  the  left-hand 
pan  so  as  to  make  the  final  adjustment  with  the  rider  or  by  taking  the 
swings  of  the  pointer  before  the  precipitate  gains  appreciably  in  weight. 
The  percentage  of  copper  found  in  copper  sulphate  should  be  within  .1  or 
.2  per  cent  of  the  theoretical  25.46  per  cent.  The  method  gives  high  results. 

EXERCISE  13. 

Determination   of    Nickel  in    Nickel-ammonium  Sulphate, 
(NH4)2S04.NiS04.6H20. 

Weigh  out  2  grams  of  crystallized  nickel-ammonium  sulphate  and  trans- 
fer to  a  porcelain  dish.  Dissolve  in  about  200  c.c.  distilled  water.  Heat 
nearly  to  boiling  and  add,  with  stirring,  a  dilute  but  freshly  made  solution 
of  caustic  soda  until  precipitation  is  complete.  Continue  heating  the  solu- 
tion for  ten  or  fifteen  minutes.  Wash  by  decantation  two  or  three  times, 
then  wash  thorouehlv  with  hot  water  on  the  filter-paper.  Dry  the  pre- 


DETERMINATION  OF  METALS  AS  OXIDE.  63 

cipitate,  detach  from  the  paper,  and  place  it  on  a  watch-crystal.  Burn 
the  filter-paper  over  the  porcelain  crucible.  Treat  the  residue  with  a  drop 
of  concentrated  nitric  acid,  evaporating  off  the  excess.  Add  the  remainder 
of  the  precipitate  and  heat  with  the  Bunsen  burner  to  constant  weight. 
The  precipitate  should  now  be  examined  for  alkali  by  digesting  with  hot 
water  and  testing  with  litmus.  If  alkali  is  present  it  is  removed  by 
repeated  extraction  with  hot  water,  the  precipitate  being  finally  heated  to 
constant  weight  again.  Calculate  the  percentage  of  nickel  found  in  the 
nickel-ammonium  sulphate.  Theoretical  percentage  is  14.86. 

The  nickel  oxide  may  also  be  converted  into  metallic  nickel  by  heating 
in  a  stream  of  hydrogen.  For  this  purpose  the  perforated  lid  and  porce- 
lain stem  cf  the  Rose  crucible  are  most  convenient.  In  place  of  this  an 
ordinary  clay  pipe  may  be  used.  The  hydrogen  from  a  Kipp  generator 
should  be  freed  from  arsine  by  passing  it  through  a  mercuric  chloride  solu- 
tion and  dried  by  passing  through  calcium  chloride  or  concentrated  sul- 
phuric acid.  The  apparatus  is  set  up  as  shown  in  Fig.  12.  The  stream 
of  hydrogen  should  be  passed  for  a  few  minutes  to  displace  the  air  from 
the  wash-bottle  and  the  U-tube.  An  explosion  will  not  pass  through  the 


FIG.  12. 

apparatus  if  a  plug  of  cotton  or  glass  wool  is  piaced  in  the  glass  tube  or 
limb  of  the  U-tube  nearest  the  crucible.  The  crucible  is  then  gently  heated 
with  a  small  flame  from  a  Bunsen  burner,  which  is  turned  on  full  after  a 
few  minutes.  The  crucible  is  finally  allowed  to  become  nearly  cold  in  the 
stream  of  hydrogen.  It  is  then  transferred  to  the  desiccator  for  a  few 
minutes  before  weighing.  The  ignition  in  the  stream  of  hydrogen  is 
repeated  until  the  weight  is  constant.  The  precipitate  is  then  tested  for 
silica  by  dissolving  the  metal  in  a  little  dilute  nitric  acid.  If  any  silica  is 
found,  it  is  filtered  off,  washed,  and  weighed. 


CHAPTER  V. 
DETERMINATION  OF  METALS  AS  OXIDE. 

PRECIPITATION  OF  LEAD,  BISMUTH,  CALCIUM,  BARIUM, 
AND  STRONTIUM  BY  AMMONIUM  CARBONATE. 

60.  Precipitation  by  Ammonium  Carbonate.  —  Some  of  those 
metals  whose  oxides  or  hydroxides  are  soluble  in  water  or  in 
solutions  of  the  alkali  hydroxides  may  be  precipitated  by  ammo- 
nium or  sodium  carbonate.    Ammonium  carbonate   cannot  be 
used  with  some  of  the  metals  because  the  precipitate  dissolves, 
forming  a  double  salt  with  the  ammonium  compounds  produced 
by  the  neutralization  of  the  mineral  acid  with  which  the  base 
was  combined.     Ammonium  carbonate  is  used  in  preference  to 
sodium  or  potassium  carbonate,  because  of  the  greater  ease  of 
freeing  the  precipitate  from  the  ammonium  salts  by  washing  or 
volatilization  during  ignition.     On  the  other  hand,  some  carbon- 
ates of  the  metals  which  can  be  precipitated  by  ammonium  car- 
bonate form  acid  carbonates  which  are  slightly  soluble.     Ammo- 
nium carbonate  as  obtained  either  in  the  solid  form  or  in  solu- 
tion almost  invariably  contains  acid  ammonium  carbonate.     Free 
ammonia  must  therefore  always  be  added  to  the  solution  to  be 
precipitated.      The  precipitation  is  therefore   usually  said  \to  be 
made  by  ammonia  and  ammonium  carbonate,  by  the  latter  term 
being  designated  the  commercial  article.   . 

61.  Conversion  of  the  Carbonate  into  the  Oxide. — The  ease  of 
converting   the   carbonates   of   these   metals   into   oxides   varies 
greatly.     Lead  carbonate  loses  all  of  its  carbon  dioxide  below  red 
heat ;  indeed  at  the  ordinary  temperature  part  of  it  passes  off  so  that 
the  precipitate  formed  by  ammonia  and  ammonium  carbonate  is  a 
basic  lead  carbonate.     In  the  case  of  the  alkaline-earth  metals 
the  precipitate  formed  in  each  case  is  the  normal  carbonate,  and 
it  may  be  dried  and  weighed  as  such.     Calcium  carbonate  may  be 

64 


DETERMINATION  OF  METALS  AS  OXIDE.  65 

completely  converted  by  the  heat  of  the  blast-lamp  into  calcium 
oxide,  so  that  this  metal  may  be  weighed  either  as  the  oxide  or 
as  the  carbonate.  It  is  impossible  to  completely  convert  stron- 
tium or  barium  carbonates  into  oxides.  These  metals  are,  there- 
fore, weighed  as  carbonates.* 

As  LEAD  SALTS  are  very  easily  reduced  to  the  metallic  form, 
the  bulk  of  the  lead  precipitate  must  be  removed  from  the  filter- 
paper  before  the  latter  is  burned.  The  lead  which  remains  on 
the  paper  is  reduced  to  metal,  and  must  be  reconverted  to  oxide 
by  treatment  with  concentrated  nitric  acid  and  ignition  of  the 
lead  nitrate  formed.  The  bulk  of  the  lead  precipitate  is  then 
placed  in  the  crucible  and  by  ignition  converted  into  the  monox- 
ide of  lead. 

If  it  is  desired  to  weigh  the  CALCIUM  as  OXIDE,  the  precipitate 
need  not  be  dried  or  removed  from  the  filter-paper,  but  is  intro- 
duced into  the  crucible,  dried  by  gentle  heat,  ignited  with  access 
of  air  to  burn  the  paper,  and  finally  heated  with  the  blast-lamp 
to  constant  weight.  This  is,  therefore,  the  simplest  and  most 
rapid  method  of  weighing  calcium.  It  has  the  disadvantage, 
however,  that  the  calcium  oxide  is  very  hygroscopic,  and  when 
moist  absorbs  carbon  dioxide  from  the  air.  The  precipitate 
must,  therefore,  be  weighed  as  quickly  as  possible. 

CALCIUM  CARBONATE  is  stable  in  the  air  and  may,  therefore, 
be  weighed  with  the  greatest  accuracy.  If  the  precipitate  is  to 
be  weighed  as  carbonate,  it  must  be  dried  and  removed  from  the 
paper.  This  is  not  difficult,  as  it  forms  a  loose  powder  on  dry- 
ing. It  is  placed  on  a  watch-crystal,  while  the  paper  is  burned  in 
the  crucible  or  on  a  platinum  wire.  By  this  treatment  the  cal- 
cium remaining  on  the  paper  is  converted  into  oxide,  which  must 
be  changed  to  carbonate.  This  is  accomplished  by  sprinkling 
some  dry  powdered  ammonium  carbonate  on  the  calcium  oxide, 
moistening  with  a  drop  or  two  of  water,  and  heating  very  gently 
to  drive  off  the  excess  of  ammonium  carbonate  and  water.  This 
operation  is  repeated,  then  the  bulk  of  the  precipitate  is  added, 
which  is  also  heated  gently  with  the  Bunsen  burner.  After 
weighing  the  precipitate,  it  must  be  treated  with  ammonium 

*  Calcium  carbonate  decomposes  at  825°,  strontium  carbonate  at  1155°, 
and  barium  carbonate  at  1450°. 


66  DETERMINATION  OF  METALS. 

carbonate  and  a  little  water  and  again  heated  and  weighed,  to 
ascertain  if  the  weight  is  constant. 

The  precipitate  of  BARIUM  or  STRONTIUM  may  be  thrown 
moist  into  the  platinum  crucible  and  heated  with  the  Bunsen 
burner  until  dry  and  the  paper  burned.  The  hot  carbon  reacts 
with  the  carbonates  of  these  metals,  forming  carbon  monoxide 
and  the  oxide  of  the  metal.  After  the  complete  combustion  of 
the  filter-paper,  the  precipitate  should  be  treated  with  ammonium 
carbonate  and  water  and  heated  to  expel  the  excess  of  the 
reagent,  finally  to  dull  redness  with  the  Bunsen  burner. 

EXERCISE  14. 
Determination  of  Strontium  in  Strontium  Carbonate,  SrC03. 

Weigh  out  1  gram  of  pure  strontium  carbonate.  Transfer  to  a  400-c.c. 
beaker.  Add  50  c.c.  distilled  water  and  dilute  hydrochloric  acid  drop  by 
drop  with  stirring  or  agitation  of  the  solution  until  the  strontium  is  dissolved. 
Dilute  to  150  c.c.,  add  a  few  cubic  centimeters  dilute  filtered  ammonia,  then 
ammonium  carbonate  solution  with  constant  stirring  until  no  further  pre- 
cipitation occurs.  Allow  the  beaker  to  stand  in  a  warm  place  for  several  hours. 
Filter  and  wash  the  precipitate  with  water  containing  a  little  ammonia. 
Transfer  the  moist  precipitate  to  a  weighed  platinum  crucible.  With 
the  lid  on  the  crucible,  heat  gently  until  the  precipitate  is  dry.  Remove 
the  lid,  place  the  crucible  on  its  side,  and  heat  strongly  until  the  paper  is 
burned.  Cool,  sprinkle  some  powdered  ammonium  carbonate  over  the 
precipitate,  moisten  with  a  little  water,  heat  gently  with  the  lid  on  the 
crucible  until  the  precipitate  is  dry,  then  more  strongly,  but  not  to  red- 
ness. Cool  in  a  desiccator  and  weigh.  Repeat  until  constant  weight  is 
obtained.  The  theoretical  percentage  of  strontium  in  strontium  carbonate 
is  59.37. 

PRECIPITATION  OF  CALCIUM  BY  AMMONIUM  OXALATE. 

62.  Calcium  is  almost  invariably  precipitated  as  oxalate,  since 
this  salt  of  calcium  is  more  insoluble  than  the  carbonate,  while  on 
ignition  it  can  be  converted  into  the  carbonate  or  the  oxide  as 
desired.  It  forms  a  very  finely  divided  powder  when  first  pre- 
cipitated, which  gives  considerable  trouble  by  passing  through 
the  pores  of  the  filter-paper.  By  digesting  the  precipitate  with 
the  hot  solution  from  which  it  has  been  separated,  the  crystals 
increase  in  size  so  that  after  two  or  three  hours  it  can  be  filtered 
without  any  difficulty.  The  same  object  is  more  quickly  accom- 
plished if  the  solution  of  calcium  is  brought  to  a  boil  before  adding 


DETERMINATION  OF  METALS  AS  OXIDE.  67 

the  ammonium  oxalate,  which  should  also  be  hot.  The  solution 
should  also  be  vigorously  stirred  during  the  precipitation.  If 
the  precipitate  is  heated  very  gently,  it  decomposes  almost 
completely,  according  to  the  equation  CaC204  =  CaC03  +  CO. 
Almost  invariably,  however,  the  calcium  carbonate  will  be  dark- 
colored  because  of  the  formation  of  some  free  carbon.  On  heat- 
ing the  precipitate  strongly  enough  to  cause  the  combustion  of 
this  carbon  and  thus  to  give  it  a  pure-white  color,  a  large  portion 
of  the  precipitate  will  be  converted  into  oxide.  That  portion 
which  remains  with  the  filter-paper  will  also  be  converted  into 
oxide.  Instead  of  attempting  to  produce  the  carbonate  in  the 
first  instance,  it  is  simpler  to  heat  the  precipitate  with  the  paper 
from  the  beginning  quite  strongly  with  the  Bunsen  burner  or  the 
blast-lamp  until  it  is  perfectly  white.  It  may  then  be  brought  to 
constant  weight  as  calcium  oxide,  or  it  may  be  converted  into 
carbonate  by  the  addition  of  at  least  an  equal  bulk  of  ammonium 
carbonate,  moistening  with  water  and  heating  gently.  This  is 
repeated  until  constant  weight  is  obtained.  By  weighing  the 
calcium  as  oxide,  the  result  is  undoubtedly  obtained  more  quickly, 
and  if  the  weighing  is  done  rapidly  it  is  quite  as  accurate  as  when 
the  calcium  is  weighed  as  carbonate.  The  weight  of  the  calcium 
oxide  may  also  be  verified  by  converting  it  into  sulphate.  A 
few  drops  of  sulphuric  acid  are  added  cautiously  to  the  oxide. 
The  excess  is  evaporated  off  on  the  hot  plate  and  the  calcium 
sulphate  finally  brought  to  constant  weight  by  heating  to  red- 
ness with  the  Bunsen  burner. 

EXERCISE  15. 
Determination  of  Calcium  in  Calcium  Carbonate. 

Weigh  out  0.5  gram  of  pure  calcium  carbonate.  Transfer  to  a  500-c.c. 
beaker,  add  a  little  water,  and  while  covering  the  beaker  with  a  watch- 
crystal  add  5  c.c.  dilute  hydrochloric  acid.  When  the  calcium  is  dissolved, 
rinse  the  watch-crystal  and  the  sides  of  the  beaker,  bringing  the  bulk  of 
the  solution  to  about  250  c.c.  Neutralize  with  filtered  ammonia,  heat 
nearly  to  boiling  on  the  hot  plate,  and  add  ammonium  oxalate  solution 
slowly  with  vigorous  stirring  of  the  solution  until  no  further  precipitation 
occurs.  Keep  the  solution  on  the  hot  plate,  so  that  it  nearly  boils,  for  several 
hours.  Decant  the  clear  solution  through  the  funnel,  wash  with  hot  water 
by  decantation  several  times,  then  transfer  the  precipitate  to  the  paper 


68  DETERMINATION  OF  METALS. 

and  complete  the  washing  with  hot  water,  testing  the  wash-water  for  chlo- 
rides. Transfer  the  moist  precipitate  to  the  weighed  platinum  crucible,  dry, 
and  burn  the  paper  in  the  usual  manner,  finally  heating  with  the  blast- lamp 
for  about  ten  minutes.  Cool  in  a  desiccator  and  weigh.  Heat  with  the 
blast-lamp  again  for  ten  minutes  and  weigh,  first  placing  the  weight  found 
necessary  by  the  first  weighing  on  the  pan  and  finally  taking  the  crucible 
out  of  the  desiccator  and  making  the  final  adjustment  as  rapidly  as  possible. 
Contjnue  heating  and  weighing  until  constant  weight  is  obtained. 

CONVERT  the  OXIDE  into  the  CARBONATE  by 
adding  at  least  an  equal  bulk  of  powdered  ammo- 
nium carbonate  *  and  moistening  with  water.  Place 
the  cover  on  the  crucible  and  evaporate  off  the 
water  by  gentle  heat,  then  place  the  crucible  on 
a  pipe-stem  triangle  which  is  supported  about  1 
inch  above  a  wire  gauze  which  is  heated  by  a 
Bunsen  burner.  Repeat  this  treatment,  cool  in  the 
desiccator,  and  weigh.  Continue  this  operation  until 
constant  weight  is  obtained.  This  weight  should 
equal  that  of  the  carbonate  weighed  out  for  the 
analysis.  The  theoretical  percentage  of  calcium  oxide 
in  calcium  carbonate  is  56. 

DETERMINATION  OF  ZINC,  MANGANESE,  AND  CADMIUM 
BY  PRECIPITATION  WITH  SODIUM  CARBONATE. 

The  salts  of  these  metals  readily  form  double  salts  with  ammo 
nium  compounds.  They  cannot,  therefore,  be  completely  pre- 
cipitated by  sodium  carbonate  in  the  presence  of  considerable 
amounts  of  ammonium  salts.  The  latter  are  readily  removed 
by  evaporating  the  solution  to  dryness  and  volatilizing  the  ammo- 
nium compound  by  gently  heating  the  dry  residue. 

63.  Cadmium  Carbonate  is  almost  insoluble  in  ammonium  salts 
and  completely  insoluble  in  the  fixed  alkali  carbonates  and  in 
water.  On  burning  the  filter-paper,  the  cadmium  carbonate  left 
upon  it  is  reduced  to  metallic  cadmium,  which  is  very  volatile  at 
the  temperature  of  the  Bunsen  burner.  The  precipitate  should 
therefore  be  dried  and  removed  as  completely  as  possible  from 
the  paper.  The  loss  of  cadmium  may  be  almost  completely  pre- 
vented by  saturating  the  paper  with  a  strong  solution  of  ammo- 
nium nitrate,  drying  and  igniting.  The  oxygen  from  the  nitrate 
serves  to  reoxidize  any  cadmium  which  may  have  been  reduced. 

*  If  on  volatilizing  one  or  two  grams  of  the  ammonium  carbonate  a  weighable 
residue  is  left,  it  must  be  purified  by  sublimation. 


DETERMINATION  OF  METALS  AS  OXIDE.  69 

The  paper  during  the  ignition  should  also  be  held  in  the  oxidizing 
zone  of  the  Bunsen  burner,  for  the  same  reason.  The  error  may 
be  still  further  reduced  by  first  washing  the  paper  with  a  little 
dilute  nitric  acid  and  water,  and  evaporating  the  solution  to 
dryness  in  the  weighed  crucible.  The  paper  may  then  be  burned 
as  directed  previously.  The  precipitate  is  converted  by  ignition 
into  oxide,  which  is  not  decomposed  nor  volatilized  at  white  heat. 
If  the  precipitate  is  small  it  need  not  be  dried,  but  may  be  imme- 
diately dissolved  in  nitric  acid  and  the  solution  evaporated  to 
dryness  in  the  porcelain  crucible. 

64.  Manganess  Carbonate  is  quite  soluble  in  ammonium  salts. 
The  recently  precipitated  carbonate  is  white,  but  on  standing  it 
becomes  brown,  being  oxidized  by  the  oxygen  of  the  air.     It  is 
insoluble   in  solutions  of  potassium  or  sodium  carbonate.     On 
ignition  in  the  presence  of  air  the  carbonate  is  converted  into 
protosesquioxide  of  manganese  which  approximates  the  formula 
Mn304.     Pickering  has  shown  that  the  percentage  of  manganese 
in  the  ignited  oxide  varies  from  69.69  to  75,  according  to  the 
temperature  to  which  the  oxide  has  been  heated.     The  theoretical 
percentage  of  manganese  in  Mn304  is  72.05.     Only  small  amounts 
of  manganese  should  be  weighed  in  this  form  if  accurate  results 
are  desired.    .No  manganese  is  lost  on  burning  the  filter-paper. 
If  the  precipitate  is  large  and  not  removed  from  the  paper  some 
difficulty  will  be  found  in  securing  complete  combustion  of  the 
paper. 

65.  Zinc  is  precipitated  as  the  basic  carbonate.     Carbon  diox- 
ide is,  therefore,  liberated  during  the  reaction.     The  bicarbonate 
produced  holds  some  of  the  zinc  in  solution,  but  this  is  completely 
precipitated  on  boiling.     It  is,  therefore,  best  to  precipitate  the 
zinc  from  a  boiling  solution  and  keep  it  at  this  temperature  for 
some  time.     If  even  a  small  amount  of  ammonium  salts  are  pres- 
ent, the  precipitation  is  not  complete.     As  the  boiling  alkaline 
solution  acts  on  glass  quite  energetically,  it  is  best  to  use  a  porce- 
lain or  platinum  dish.     The  precipitate  is  washed  with  hot  water. 
On  ignition,  the  carbonate  is  converted  into  oxide.     In  the  pres- 
ence of  carbon  or  other  reducing  substances,  both  the  oxide  and 
carbonate  are  reduced  on  ignition  to  metallic  zinc  which  volatilizes. 
The  precipitate  must,  therefore,  be  treated  in  the  same  manner 


70  DETERMINATION  OF  METALS. 

as   the  cadmium   precipitate.     The   zinc    oxide   is   stable   when 
ignited  in  the  air  in  the  absence  of  reducing  agents. 

EXERCISE  16. 

Determination  of  Zinc  in  Zinc-ammonium  Sulphate, 
(NH4)2SO4.ZnSO4.6H20. 

Weigh  out  2  grams  of  crystallized  zinc-ammonium  sulphate.  Transfer 
to  a  platinum  dish  or  crucible.  Heat  very  gently  with  a  small  Bunsen  burner 
flame  or  on  the  hot  plate  until  the  water  is  expelled.  Keep  the  dish  covered 
to  prevent  spattering.  Increase  the  heat  somewhat  and  volatilize  the  ammo- 
nium sulphate.  When  no  more  fumes  come  off,  allow  to  cool  and  dissolve 
the  zinc  sulphate  in  water  and  a  little  hydrochloric  acid  if  necessary.  If  a 
crucible  has  been  used,  transfer  the  solution  to  a  platinum  or  porcelain 
dish,  heat  nearly  to  boiling  and  add  with  stirring  a  freshly  made  sodium 
carbonate  solution  until  the  zinc  is  entirely  precipitated.  Continue  the 
heating  for  about  fifteen  minutes,  allow  to  settle,  wash  several  times  by 
decantation,  finally  transfer  to  the  funnel  and  complete  the  washing  with 
hot  water.  Dry  the  precipitate,  detach  it  from  the  paper  and  place  on  a 
watch-crystal.  Replace  the  paper  in  the  funnel  and  wash  with  25  c.c.  of  water 
to  which  a  little  nitric  acid  has  been  added.  Evaporate  the  wash-water  to 
dryness  in  the  weighed  platinum  crucible.  In  the  meantime  moisten  the 
paper  with  a  saturated  solution  of  ammonium  nitrate  and  dry  in  the  steam- 
bath.  Burn  the  paper  on  the  platinum  wire,  being  careful  to  touch  it  only 
with  the  oxidizing  portion  of  a  Bunsen-burner  flame  which  is  entirely  free 
from  a  luminous  zone.  The  precipitate  is  now  transferred  to  the  crucible 
and  ignited  with  the  Bunsen  burner  and  finally  with  the  blast-lamp.  Theo- 
retical percentage  of  zinc  in  zinc-ammonium  sulphate  is  16.28. 

IGNITION    OF   SALTS   OF   VOLATILE  ACIDS. 

66.  Determination  of  the  Metals  by  Ignition  of  the  Salts  of 
Volatile  Acids. — As  has  already  been  observed,  it  is  possible  to 
ignite  the  salts  of  many  metals  and  obtain  oxides  of  definite  com- 
position, from  the  weight  of  which  the  percentage  of  the  metal 
present  may  be  calculated.  Calcium  oxalate  and  carbonate, 
cadmium,  manganese,  zinc,  and  lead  carbonates  are  examples  of 
such  salts  which  are  obtained  by  precipitation.  In  many  cases 
the  nitrates  and  nitrites  may  be  treated  in  this  manner,  but  not 
the  chlorides  since  they  are  quite  volatile  and  the  oxygen  is  not 
present  to  unite  with  the  metal  to  form  the  oxide. 

In  some  cases  the  oxygen  may  be  furnished  and  the  chlorine 
driven  off,  as  a  volatile  compound  by  igniting  the  salt  with  mer- 


DETERMINATION  OF  METALS  AS  OXIDE.  71, 

curie  oxide,  nitric  acid,  or  ammonium  carbonate  or  nitrate.  Zinc 
chloride  may  be  converted  into  oxide  by  treatment  with  excess 
of  mercuric  oxide  or  nitric  acid.  Magnesium  chloride  may  be 
converted  into  oxide  by  ignition  with  mercuric  oxide,  ammonium 
carbonate,  or  nitrate. 

In  the  absence  of  substances  of  this  character,  the  acid  with 
which  the  metal  is  combined  must  contain  oxygen  and  be  readily 
decomposed  and  volatilized  by  heat.  The  sulphates  and  phos- 
phates, though  containing  the  necessary  oxygen,  are  usually  very 
stable,  and  the  acid  is  with  difficulty  volatilized  by  heat. 

The  metals  whose  salts  of  volatile  oxygen  acids  may  be  decom- 
posed into  oxides  are  LEAD,  BISMUTH,  COPPER,  TIN,  IRON,  ALUMIN- 
IUM, CHROMIUM,  MANGANESE,  NICKEL,  ZINC,  and  MAGNESIUM. 

No  other  volatile  substance  except  the  metal  may  be  present. 

67.  Ignition  of  Salts  of  Organic  Acids. — Many  of  the  metallic 
salts  of  organic  acids  may  be  ignited  so  as  to  leave  the  metal  in  a 
weighable  form.  The  carbon,  hydrogen,  nitrogen,  and  some  of 
the  oxygen  of  the  organic  compound  are  either  volatilized  in  the 
free  condition  or  combined  with  each  other,  or  they  unite  with 
the  oxygen  of  the  air  to  form  simple  oxygen  compounds.  Most 
metals  retain  at  least  some  of  the  oxygen  of  the  organic  com- 
pound, or  are  oxidized  by  the  oxygen  of  the  air.  Under  these 
conditions  BARIUM,  STRONTIUM,  and  SODIUM  remain  as  CARBONATES 
with  a  partial  reduction  to  oxide,  which  is  converted  to  carbonate 
by  treatment  with  ammonium  carbonate,  so  that  these  metals 
are  weighed  as  carbonates.  Sodium  compounds  must  be  heated 
only  to  very  low  redness,  because  of  the  volatility  of  this  metal. 
It  is  best  not  to  attempt  the  complete  combustion  of  the  carbon 
by  heat.  The  charred  mass  should  be  extracted  with  water  and 
filtered,  the  solution  of  the  sodium  carbonate  being  evaporated 
to  dry  ness  in  a  weighed  dish. 

ORGANIC   COMPOUNDS   of   CALCIUM,   MAGNESIUM,   ALUMINIUM, 

CHROMIUM,     MANGANESE,     NICKEL,     COPPER,     LEAD,     and     BISMUTH 

may  be  ignited  in  the  air  until  the  organic  matter  is  burned  off 
and  the  oxide  of  the  metal  remains.  Many  organic  salts  froth  on 
being  heated.  The  crucible  should  therefore  have  a  tight  lid,  and 
the  heat  should  be  applied  gently  at  first.  If  this  confinement  is 
not  sufficient,  the  crucible  should  be  set  in  a  larger  covered  cruci- 


72  DETERMINATION  OF  METALS. 

ble,  since  the  error  due  to  loss  of  metal  by  the  mechanical  carry- 
ing away  of  fine  particles  by  the  products  of  the  combustion  may 
be  considerable.  The  ignition  of  the  salts  containing  lead  or 
bismuth  must  be  conducted  at  as  low  a  heat  as  possible,  as  these 
metals  are  readily  reduced  and  are  volatile  at  red  heat.  When 
the  organic  material  is  charred  and  the  carbon  ceases  to  burn 
readily  the  flame  is  removed  from  the  crucible  and  ammonium 
nitrate  added.  The  crucible  is  covered  and  gently  heated.  The 
ammonium  nitrate  fuses  and  oxidizes  the  remaining  carbon  and 
converts  the  metallic  lead  or  bismuth  into  nitrate  which  is  con- 
verted into  oxide  by  red  heat.  Concentrated  nitric  acid  may  be 
used  for  the  same  purpose.  When  copper  salts  are  ignited,  some 
of  the  copper  is  left  as  cuprous  oxide,  which  it  is  most  advanta- 
geous to  oxidize  by  igniting  with  mercuric  oxide. 

EXERCISE  17. 
Determination  of  Lead  by  Ignition  of  Lead  Nitrate,  Pb(N03)2. 

Heat  a  porcelain,  crucible  to  redness  with  the  Bunsen  burner  for  ten 
minutes.  Cool  in  the  desiccator  and  weigh.  Weigh  out  one-half  gram  of 
lead  nitrate  and  transfer  to  the  crucible.  If  the  salt  is  in  large  crystals, 
it  should  be  reduced  to  a  coarse  powder  by  grinding  in  a  clean  and  dry 
porcelain  mortar.  It  should  be  weighed  out  as  soon  as  powdered.  Heat 
gently,  at  first  with  the  lid  on  the  crucible.  When  decrepitation  ceases 
and  copious  nitrous  fumes  cease  to  be  evolved  the  lid  may  be  removed. 
Heat  to  dull  redness  until  no  more  fumes  are  evolved.  Cool  in  the  desiccator 
and  weigh  as  PbO.  Repeat  until  the  weight  is  constant.  Calculate  the 
percentage  of  lead  in  the  lead  nitrate. 


CHAPTER  VI. 

DETERMINATION    OF    METALS    AS    SULPHATE 
AND   AS   SULPHIDE. 

A  LARGE  number  of  the  metals  form  sulphates  which  are  of 
definite  composition  and  which  can  be  heated  high  enough  to 
expel  all  water  and  volatile  matter  without  suffering  decomposi- 
tion. Most  of  the  sulphates,  however,  are  soluble.  Only  three, 
those  of  BARIUM,  STRONTIUM,  and  LEAD,  are  insoluble  enough  for 
separation  of  the  metal  from  water  solution  by  precipitation. 

68.  Barium  Sulphate  is  the  most  insoluble,  requiring  about 
400,000  parts  of  water  to  dissolve  one  part  of  the  salt.  It  is 
somewhat  more  soluble  in  even  dilute  solutions  of  hydrochloric 
acid  and  of  ammonium  chloride.  It  readily  dissolves  in  concen- 
trated sulphuric  acid,  especially  when  hot,  from  which  it  is  com- 
pletely precipitated  by  the  addition  of  water.  Barium  sulphate 
has  the  very  peculiar  though  by  no  means  uncommon  property 
of  rendering  insoluble  some  very  soluble  salts,  notably  the  nitrates, 
chlorides,  sulphates,  and  chlorates  of  the  alkaline  earths  and 
alkali  metals,  ferric,  aluminium,  and  chromic  salts.  Potassium 
salts  give  more  trouble  than  sodium  salts.  Once  carried  down 
by  the  barium  sulphate,  these  salts  cannot  be  entirely  washed 
out.  The  solution  must  be  freed  from  nitrates  and  chlorates  by 
evaporation  with  hydrochloric  acid.  If  ferric  salts  are  reduced 
to  ferrous  salts,  iron  will  not  be  carried  down  by  the  barium  sul- 
phate. A  precipitate  contaminated  by  any  of  these  impurities 
may  be  dissolved  in  concentrated  sulphuric  acid  and  reprecipi- 
tated  by  diluting  with  much  water.  It  may  also  be  fused  with 
sodium  carbonate,  which  converts  the  sulphate  of  barium  into 
the  carbonate,  which  may  be  filtered  off,  washed,  redissolved,  and 
reprecipitated  with  sulphuric  acid. 

It  has  been  shown  that  the  contamination  of  the  precipitate 

73 


74  DETERMINATION  OF  METALS. 

takes  place  at  the  moment  of  its  formation.  The  most  favorable 
conditions  for  avoiding  the  contamination  have  been  found  to  be  a 
boiling-hot,  slightly  acid  solution  of  large  bulk,  and  the  addition 
of  the  precipitant  in  very  dilute  solution  drop  by  drop,  and  with  the 
most  vigorous  stirring.  If  from  failure  to  observe  any  of  these 
conditions  salts  are  carried  down,  washing  will  not  remove  them. 
On  account  of  its  slight  solubility,  barium  sulphate  tends  to  come 
down  in  such  a  finely  divided  condition  that  it  passes  through  the 
pores  of  the  filter-paper.  By  precipitating  as  directed  above 
and  keeping  the  solution  near  the  boiling-point  for  some  time 
the  precipitate  settles  out,  leaving  a  clear  solution  and  does  not 
pass  through  the  filter-paper. 

On  burning  the  paper  some  of  the  sulphate  is  reduced  to  sul- 
phide of  barium.  It  is  customary  to  add  a  drop  or  two  of  sul- 
phuric acid  in  order  to  reconvert  the  sulphide  into  sulphate,  and 
then  to  ignite  the  precipitate  and  volatilize  the  excess  of  sulphuric 
acid.  By  this  treatment  barium  pyrosulphate  is  produced, 
which  is  decomposed  slowly  and  only  at  a  high  temperature.  It 
has  been  shown  that  the  addition  of  sulphuric  acid  is  unnecessary 
since  the  barium  sulphide  is  oxidized  to  sulphate  by  ignition  in 
the  air. 

69.  Strontium  Sulphate  is    much  more  soluble  in  water  and 
acids  than  barium  sulphate,  1  part  of  the  former  being  dissolved 
by  about  7000  parts  of  cold  water,  and  10,000  parts  of  boiling 
water,  while  if  the  water  contains  sulphuric  acid,  from  11,000  to 
12,000  parts  are  required.     One  part  of  the  precipitate  is  dis- 
solved by  about  500  parts  of  dilute  nitric  or  hydrochloric  acid, 
while  it  is  practically  insoluble  in  95%  or  absolute  alcohol  and  in 
boiling  ammonium  sulphate  solution  (1  to  4).     The  precipitate 
should  therefore  be  washed  by  one  of  the  two  latter  solutions,  or 
by  dilute  sulphuric  acid.     On  ignition  in  presence  of  carbon  or 
reducing  gases  it  is  converted  into  sulphide.     This  is  volatile  in 
the  Bunsen  burner  flame,  coloring  it  red,  so  that  the  precipitate 
must  be  detached  as  completely  as  possible  from  the  paper.     The 
subsequent  treatment  is  similar  to  that  of  barium  sulphate. 

70.  Lead    Sulphate  is  quite  insoluble  in  water,  requiring  for 
its  solution  about  23,000  parts  of  cold  water,  but  if  the  water  con- 
tains sulphuric  acid  36,500  parts  are  required.     The  presence  of 


DETERMINATION  OF  METALS  AS  SULPHATE.  75 

alcohol  reduces  this  solubility.  Solutions  of  ammonium  salts, 
especially  the  nitrate,  tartrate,  and  acetate  dissolve  lead  sulphate 
quite  readily.  The  two  latter  should  be  rendered  strongly  alka- 
line with  ammonia.  It  is  also  dissolved  by  hot  solutions  of  caus- 
tic soda  or  potash.  Dilute  nitric  and  hydrochloric  acids  dissolve 
considerable  quantities  of  lead  sulphate  which  is  reprecipitated  on 
the  addition  of  dilute  sulphuric  acid.  These  acids  are  readily 
removed  by  evaporating  the  solution  after  the  addition  of  sul- 
phuric acid,  until  fumes  of  the  latter  appear.  On  diluting  the 
solution  with  considerable  water  and  adding  alcohol,  the  lead  is 
completely  precipitated. 

The  lead  precipitate  should  be  washed  by  alcohol  or  dilute 
sulphuric  acid,  preferably  by  the  former  as  the  paper  would  char 
on  drying  if  sulphuric  acid  were  left  in  it,  and  if  washed  out  by 
water  some  lead  would  be  lost.  On  burning  the  paper  some  of 
the  lead  sulphate  is  reduced  to  metallic  lead.  The  precipitate 
should  therefore  be  removed  from  the  paper,  the  latter  placed  in 
a  porcelain  crucible,  and  ignited  until  all  carbon  is  burned.  A 
drop  or  two  of  nitric  acid  and  the  same  amount  of  sulphuric  acid 
is  added  and  evaporated  off,  care  being  taken  to  avoid  spattering. 
The  remainder  of  the  precipitate  should  then  be  added  and  heated 
with  a  Bunsen  burner.  Sulphate  of  lead  may  very  advantage- 
ously be  filtered  off  on  a  Gooch  crucible  and  dried  on  the  hot  plate. 

EXERCISE  18. 
Determination  of  Barium  in  Barium  Chloride,  BaCl2.2H20. 

Weigh  out  about  one-half  gram  of  crystallized  barium  chloride.  Transfer 
to  a  500-c.c.  beaker  and  dissolve  in  about  250  c.c.  of  distilled  water.  Heat 
nearly  to  boiling  on  the  hot  plate  and  add,  while  stirring  vigorously,  dilute 
sulphuric  acid  drop  by  drop  until  no  further  precipitate  is  produced.  The 
acid  may  be  drawn  up  in  a  glass  tube  or  pipette  and  the  flow  of  acid  regu- 
lated by  pressing  the  index  finger  against  the  upper  end  of  the  tube.  After 
allowing  the  precipitate  to  settle  a  little,  it  can  easily  be  seen  if  a  drop 
of  acid  produces  any  precipitation.  The  heating  of  the  solution  is  con- 
tinued until  the  precipitate  has  settled  completely,  leaving  a  clear  solution. 
About  an  hour  is  usually  sufficient.  It  is  washed  several  times  by  decanta- 
tion,  using  about  75  c.c.  of  hot  water  each  time  and  stirring  up  the  precipi- 
tate thoroughly.  The  precipitate  is  transferred  to  the  funnel  and  the 
washing  with  hot  water  continued  until  the  wash-water  is  free  from  chlorides. 


73  DETERMINATION  OF  METALS. 

The  moist  precipitate  with  the  paper  may  be  transferred  to  a  weighed 
platinum  crucible  or  dish  and  ignited  over  the  Bunsen  burner  with  free 
access  of  air  and  finally  over  the  blast-lamp.  The  theoretical  percentage 
of  barium  in  barium  chloride  is  56.25. 

71.  Determination  of  Metals  by  Evaporation  with  Sulphuric 
Acid. — Many  metals  form  sulphates  which  are  stable  enough  to 
be  ignited  and  weighed,  but  which  are  soluble  and  therefore  can- 
not be  precipitated  and  washed.  POTASSIUM,  SODIUM,  CALCIUM, 

MAGNESIUM,     COBALT,     NICKEL,     and    MANGANESE    form    SUCh    SUl- 

phates.  A  salt  or  compound  of  one  of  these  metals  which  con- 
tains no  other  non-volatile  metal  and  which  contains  only  volatile 
acids  may  be  analyzed  by  ignition  with  excess  of  sulphuric  acid. 
The  sulphate  of  the  metal  will  remain,  from  the  weight  of  which 
the  percentage  of  the  metal  present  may  be  calculated.  In  this 
manner  the  salts  of  nitric  and  nitrous  acids,  the  halogen  acids, 
organic  acids,  etc.,  may  be  analyzed. 

ORGANIC  SALTS  of  SODIUM  and  POTASSIUM  are  treated  with  a 
moderate  excess  of  dilute  sulphuric  acid  in  a  covered  platinum 
crucible  and  gently  heated  to  dull  redness.  The  evolution  of 
white  fumes  indicates  an  excess  of  sulphuric  acid.  The  readily 
fusible  acid  sulphate  of  the  alkali  remains.  Small  lumps  of 
ammonium  carbonate  which  leave  no  residue  on  ignition  are 
thrown  into  the  hot  crucible  and  the  heating  continued.  This  is 
repeated  until  the  alkali  sulphate  no  longer  fuses.  The  crucible 
is  cooled  and  weighed,  and  the  addition  of  ammonium  carbonate 
and  the  heating  repeated  until  constant  weight  is  obtained.  The 
alkali-acid  sulphate  may  be  converted  into  the  neutral  sulphate 
by  the  application  of  a  white  heat,  but  some  loss  of  the  metal 
results. 

BARIUM,  STRONTIUM,  and  CALCIUM  may  be  determined  in 
salts  of  volatile  acids  by  the  addition  of  an  excess  of  sulphuric 
acid,  evaporating  to  dryness  and  igniting,  at  first  gently  and 
finally  to  full  redness.  To  remove  the  excess  of  sulphuric  acid, 
a  little  ammonium  carbonate  is  sprinkled  on  the  precipitate, 
which  is  again  heated  to  redness.  MAGNESIUM  may  be  deter- 
mined in  the  same  manner,  but  a  large  excess  of  sulphuric  acid 
should  be  avoided  and  the  residue  may  be  exposed  to  a  moderate 


DETERMINATION  OF  METALS  AS  SULPHATE.  77 

red  heat  only  or  loss  of  sulphuric  acid  will  result.  Magnesium 
sulphate  is  very  hygroscopic  and  must  be  weighed  rapidly. 

NICKEL,  COBALT,  and  MANGANESE  may  be  weighed  as  sul- 
phates, but  care  must  be  exercised  not  to  overheat,  as  sulphuric 
acid  may  be  lost.  If  this  has  occurred,  as  shown  by  blackening 
of  the  material,  dilute  sulphuric  acid  must  be  added,  and  after 
evaporating  off  the  excess  the  ignition  is  repeated.  The  platinum 
dish  must  not  be  heated  above  dull  redness. 

LEAD  may  be  determined  in  organic  salts,  and  those  of  volatile 
acids  by  treatment  in  a  weighed  porcelain  dish  or  crucible  with 
excess  of  dilute  sulphuric  acid.  Loss  by  spattering  is  avoided  by 
keeping  the  vessel  well  covered  and  evaporating  with  very  gentle 
heat.  If  the  residue  from  the  ignition  of  an  organic  compound  is 
not  perfectly  white,  the  addition  of  sulphuric  acid  and  the  ignition 
must  be  repeated.  Lead  sulphate  is  stable  when  exposed  to  a 
red  heat,  but  reducing  gases  must  not  be  allowed  to  come  in  con- 
tact with  it. 

EXERCISE  19. 
Determination  of  Magnesium  in  Magnesium  Carbonate. 

Weigh  one-half  gram  of  magnesium  carbonate  and  transfer  to  a  weighed 
platinum  crucible.  To  prevent  loss  by  spattering  expel  most  of  the  car- 
bonic acid  by  heating  the  crucible  at  first  gently,  then  strongly  with  the 
Bunsen  burner,  the  whole  operation  taking  about  ten  minutes.  Allow 
the  crucible  to  cool  quite  completely  and  then  add  about  3  c.c.  dilute  sul- 
phuric acid,  at  first  cautiously,  to  prevent  spattering  in  case  much  carbonic 
acid  stiil  remains.  Evaporate  the  solution  by  first  heating  on  the  hot 
plate  and  then  very  gently  with  the  Bunsen  burner  until  acid  fumes  cease 
to  be  evolved.  If  white  fumes  do  not  escape  enough  acid  has  not  been 
added.  Heat  for  a  few  minutes  to  duil  redness,  cool  in  the  desiccator,  and 
weigh  quickly,  as  magnesium  sulphate  is  hygroscopic.  Repeat  the  ignition 
with  the  Bunsen  burner  until  the  weight  is  constant.  The  residue  should 
dissolve  completely  in  water  and  should  form  a  neutral  solution,  otherwise 
it  has  been  too  highly  heated. 

Calculate  the  percentage  of  magnesium  carbonate  in  the  sample  tested. 
Ordinary  magnesium  carbonate  approximates  the  formula 

4MgC03.Mg(OH)2.5H20. 


78  DETERMINATION  OF  METALS. 


DETERMINATION   OF   METALS    AS    SULPHIDE. 

A  large  number  of  the  metals  form  sulphides  which  are  almost 
entirely  insoluble  in  water,  especially  when  it  contains  an  excess 
of  the  precipitant.  The  separation  of  metals  as  sulphides  from 
their  solution  is  therefore  very  complete,  and  as  in  some  case* 
the  precipitates  can  be  ignited  as  sulphides,  giving  very  pure 
products,  the  determination  of  the  metal  as  sulphide  is  in  many 
cases  of  the  highest  accuracy.  This  is  true  of  COPPER,  ZINC,  and 

MANGANESE. 

COPPER,  MERCURY,  LEAD,  SILVER,  BISMUTH,  CADMIUM,  ARSENIC, 

ANTIMONY,  and  TIN  may  be  precipitated  by  hydrogen  sulphide 
from  acid  solutions.  SILVER,  CADMIUM,  LEAD,  and  BISMUTH  may 
also  be  precipitated  by  ammonium  sulphide  in  alkaline  solution, 
though  the  precipitation  by  hydrogen  sulphide  in  moderately 
acid  solution  is  generally  used.  LEAD  and  SILVER  may  also  be 
precipitated  by  hydrogen  sulphide  or  ammonium  sulphide  in 
neutral  solution.  CADMIUM  may  be  precipitated  from  a  potas- 
sium cyanide  solution  by  hydrogen  sulphide. 

72.  Copper  may  be  precipitated  from  a  neutral  or  slightly 
acid  solution  by  ammonium  sulphocyanate  after  reduction  by 
sulphurous  acid,  ammonium  sulphite,  or  hypophosphorous  acid. 
The  copper  is  precipitated  as  cuprous  sulphocyanate  and  may  be 
dried  at  100°  and  weighed  as  such,  or  it  may  be  converted  into 
cuprous  sulphide  by  ignition  with  sulphur  in  a  stream  of  hydrogen. 
Many  copper  compounds  containing  no  non-volatile  constituent 
except  copper  may  be  determined  by  heating  a  weighed  portion 
with  sulphur  in  a  stream  of  hydrogen,  thus  converting  the  copper 
into  cuprous  sulphide.     The  oxides  of  copper,  the  sulphate,  nitrate, 
etc.,  may  be  analyzed  in  this  manner.     If  the  copper  solution  is 
free  from  oxidizing  substances,  it  is  advisable  to  heat  it  nearly  to 
boiling  before  passing  hydrogen  sulphide,  as  under  these  condi- 
tions the  precipitate  collects  and  settles  better. 

73.  Washing. — Some  of  the  sulphides  form  so-called  colloidal 
solutions  with  water,  while  others  are  readily  oxidized,  forming 
sulphates  which  dissolve  in  the  wash-water.      For  this  reason 
the  wash-water  must  contain  hydrogen  sulphide,  and  in  the  case 


DETERMINATION  OF  METALS  AS  SULPHIDE.  79 

of  cadmium  and  arsenic  hydrochloric  acid  should  be  present,  more 
being  necessary  for  arsenic  sulphide  than  for  cadmium  sulphide. 

74.  Removal  of  Free  Sulphur  and  Drying  at  100°. — The  sul- 
phides of  SILVER,  MERCURY,  CADMIUM,  and  ARSENIC  cannot  be 
ignited,  but  must  be  dried  at  100°  before  weighing.  A  Gooch 
crucible  is  most  advantageous  for  this  purpose,  though  a  weighed 
filter-paper  may  be  used.  As  hydrogen  sulphide  is  very  readily 
oxidized  even  by  the  oxygen  of  the  air,  the  precipitated  sulphides 
will  almost  invariably  contain  sulphur.  When  the  precipitate 
can  be  ignited,  this  contaminant  does  not  interfere  with  the  accuracy 
of  the  determination.  When  the  sulphide  is  dried  at  100°  the 
removal  of  the  free  sulphur  presents  some  difficulties.  The  solu- 
tion from  which  the  metal  is  to  be  precipitated  should  be  freed  as 
far  as  possible  from  oxidizing  agents.  The  precipitation  should 
be  carried  out  in  a  cold  solution,  which  should  be  protected  from 
the  air  as  much  as  possible  by  placing  it  in  an  Erlenmeyer  flask 
and  filtering  and  washing  immediately  after  precipitation.  Even 
under  these  circumstances  more  or  less  sulphur  is  apt  to  be  formed, 
so  that  the  precipitate  should  always  be  treated  for  the  removal 
of  this  element.  Carbon  disulphide  and  a  moderately  strong 
solution  of  sodium  sulphite  have  been  used  for  this  purpose.  If 
carbon  disulphide  is  used,  the  washed  precipitate  is  first  dried, 
then  digested  with  small  quantities  of  carbon  disulphide  until 
no  sulphur  is  obtained  by  evaporating  a  portion  of  the  carbon 
disulphide.  If  a  Gooch  crucible  is  used,  it  may  be  placed  in  a 
small  beaker  and  the  carbon  disulphide  poured  over  the  precipi- 
tate. The  precipitate  may  also  be  freed  from  water  by  washing 
with  alcohol,  after  which  the  carbon  disulphide  may  be  added. 
If  the  sodium  sulphite  solution  is  used,  the  precipitate  need  not  be 
dried,  but  must  be  washed  thoroughly  after  the  sulphur  has  been 
removed.  The  sodium  sulphite  solution  must  be  heated  very 
nearly  to  the  boiling-point.  The  solution  may  be  tested  for 
sulphur  by  acidifying  and  allowing  to  stand  for  a  few  moments. 
A  milky  precipitate  indicates  sulphur.  The  removal  of  the  sul- 
phur by  either  of  these  solutions  is  incomplete. 

On  drying  at  100°  BISMUTH  SULPHIDE  first  loses  water  and  then 
begins  to  oxidize  and  gain  in  weight.  This  precipitate  should 
therefore  be  weighed  after  drying  for  half  an  hour.  The  drying 


80  DETERMINATION  OF   METALS 

for  periods  of  half  an  hour  and  weighing  is  continued  until  the 
weight  is  constant  or  begins  to  increase.  The  lowest  weight 
found  is  that  of  the  dry  sulphide. 

75.  The  Sulphides  of  Tin  cannot  be  weighed  as  such,  but  are  con- 
verted into  the  dioxide  by  ignition  in  the  air.  The  heat  must  be 
applied  gently  at  first  or  some  of  the  tin  will  be  volatilized  as  sul- 
phide. The  stannic  sulphide  is  more  volatile  than  the  stannous 
sulphide.  When  sulphur  dioxide  is  no  longer  given  off  copiously 
the  precipitate  is  heated  more  intensely.  To  completely  expel 
the  oxides  of  sulphur  a  little  powdered  ammonium  carbonate  is 
sprinkled  over  the  precipitate,  which  is  again  ignited. 

Antimony  sulphide  is  ignited  in  the  same  manner  and  converted 
into  the  tetroxide  Sb204. 

EXERCISE  20. 
Determination  of  Copper  in  Copper  Sulphate,  CuS04.sH2O. 

Weigh  out  1  gram  of  pure  crystallized  copper  sulphate.  Transfer  to  a  3CKV 
c.c.  beaker  and  dissolve  in  about  200  c.c.  of  water.  Add  a  few  cubic  centimeters 
of  hydrochloric  acid,  heat  nearly  to  boiling,  and  pass  hydrogen  sulphide  until 
precipitation  is  complete.  Allow  the  precipitate  to  settle  for  a  few  minutes, 
decant  the  solution  through  the  filter,  and  transfer  the  precipitate.  Wash 
with  hydrogen-sulphide  water.  Before  discarding  any  portion  of  the 
filtrate,  test  for  copper  with  hydrogen  sulphide.  Copper  sulphide  is  very 
readily  oxidized  by  the  oxygen  of  the  air  to  copper  sulphate,  which  passes 
through  the  filter-paper.  The  hydrogen  sulphide  of  the  wash-water  is 
present  to  reprecipitate  this  dissolved  copper.  The  precipitate  should  be 
exposed  to  the  air  as  little  as  possible.  For  this  purpose  it  should  be 
washed  down  into  the  point  of  the  funnel  and  kept  well  covered  with  the 
wash-water  containing  hydrogen  sulphide. 

76.  Ignition  of  the  Precipitate  with  Sulphur  in  a  Stream  of  Hydrogen. — 
The  precipitate  is  dried  in  the  steam-oven,  detached  from  the  paper  and 
placed  in  a  weighed  Rose  crucible.  The  paper  is  then  burned  and  the 
ash  added  to  the  precipitate.  If  the  paper  crumbles  considerably,  it  may 
'be  placed  in  a  porcelain  dish  or  crucible  and  burned  by  heating  the  dish  or 
crucible.  Unburned  particles  of  carbon  sometimes  remain  unless  the 
dish  is  heated  strongly  for  some  time.  The  residue  is  transferred  to  the 
crucible,  the  contents  of  which  are  mixed  with  an  equal  bulk  of  powdered 
sulphur  which  leaves  no  residue  on  ignition.  A  gentle  stream  of  dry  hydro- 
gen is  now  led  in  through  the  porcelain  tube  which  passes  through  the  lid 
of  the  crucible.  The  hydrogen  is  freed  from  arsine  by  passing  through  a 
solution  of  mercuric  chloride.  The  arrangement  of  the  apparatus  is  shown 
in  Fig.  12,  page  63.  The  hydrogen  must  be  allowed  to  pass  until  the  air 
has  been  displaced  from  the  wash-bottles  as  well  as  from  the  crucible. 


DETERMINATION  OF  METALS  AS  SULPHIDE.  81 

About  ten  minutes  is  generally  sufficient  if  the  stream  of  hydrogen  passes 
quite  rapidly.  The  danger  of  explosion  is  greatly  reduced  if  a  plug  of 
glass  wool  or  cotton  is  loosely  inserted  in  the  glass  tube  nearest  to  the 
crucible.  The  crucible  is  at  first  heated  gently  with  the  Bunsen  burner, 
and  when  the  excess  of  sulphur  is  nearly  all  volatilized  it  is  heated  to  redness 
with  the  full  flame  of  the  burner.  After  a  few  minutes'  strong  heating 
the  burner  is  removed  and  the  crucible  is  allowed  to  become  very  nearly 
cold  in  the  stream  of  hydrogen.  It  is  then  placed  in  the  desiccator  and 
after  a  few  minutes  is  weighed. 

The  precipitate  is  again  mixed  with  sulphur,  heated  in  the  stream  of 
.hydrogen  and  weighed.  This  is  repeated  until  constant  weight  is  ob- 
tained. The  percentage  of  copper  in  crystallized  copper  sulphate  is  25.47. 
The  ignited  precipitate  is  cuprous  sulphide,  Cu2S. 

EXERCISE  21. 
Determination  of  Arsenic  in  Arsenious  Oxide,  As20g. 

Weigh  out  about  0.3  gram  of  pure  resublimed  arsenous  oxide.  Transfer  to 
a  small  beaker  and  dissolve  in  a  little  freshly  made  dilute  solution  of  caustic 
soda.  Transfer  the  solution  to  a  250-c.c.  Erlenmeyer  flask.  Acidify  with 
hydrochloric  acid  and  then  add  about  30  c.c.  of  the  dilute  acid.  Dilute 
to  about  200  c.c.  or  until  the  flask  is  nearly  full  and  pass  hydrogen  sulphide 
until  precipitation  is  complete.  Filter  immediately  through  a  Gooch  crucible 
which  has  been  dried  at  100°  to  constant  weight.  Wash  with  about  250  c.c. 
of  water  containing  a  few  cubic  centimeters  of  dilute  hydrochloric  acid* 
and  a  considerable  amount  of  hydrogen  sulphide.  Suck  the  water  out  as  com- 
pletely as  possible  with  the  filter-pump,  wash  two  or  three  times  with  alco- 
hol, place  the  crucible  in  a  small  beaker  and  pour  in  carbon  disulphide  until 
the  precipitate  is  covered.  The  carbon  disulphide  used  for  this  purpose 
should  leave  no  residue  on  evaporating  a  few  cubic  centimeters  on  a  watch- 
crystal.  If  a  residue  is  left,  the  carbon  disulphide  must  be  redistilled. 
Allow  to  stand  about  fifteen  minutes.  Pour  off  the  carbon  disulphide  and 
extract  again  with  a  fresh  portion.  Continue  this  operation  until  a  portion 
of  the  carbon  disulphide  evaporated  on  a  watch-crystal  leaves  no  deposit 
of  sulphur.  Dry  the  precipitate  in  the  steam-oven  and  weigh.  Theoreti- 
cal percentage  of  arsenic  in  arsenious  oxide  is  75.75. 

77.  Precipitation  of  Zinc  as  Sulphide. — Zinc  as  well  as  man- 
ganese and  iron  are  precipitated  by  ammonium  sulphide  in  alka- 
line solution.  Zinc  may  be  precipitated  by  hydrogen  sulphide  in 
a  solution  which  is  only  very  slightly  acid  with  a  mineral  acid  or 
moderately  acid  with  acetic  acid.  This  sulphide  is  so  finely 
divided  that  it  passes  through  the  filter-paper,  making  it  very 

*  The  acid  prevents  the  formation  of  a  "colloidal "  solution 


82  DETERMINATION  OF  METALS. 

difficult  to  filter  and  weigh.  If  some  mercuric  chloride  is  added 
to  the  zinc  solution,  a  precipitate  is  obtained  which  can  be  filtered 
and  washed  with  ease.  In  the  subsequent  ignition  the  mercury 
is  completely  volatilized.  The  zinc  sulphide  obtained  by  adding 
ammonium  sulphide  to  a  solution  of  zinc  made  strongly  alkaline 
with  caustic  soda  is  also  easily  washed,  but  it  generally  is  con- 
taminated with  alkali  and  silica. 

78.  The  Two  Sulphides  of  Manganese. — Manganese  forms  two 
sulphides,  one  of  which  is  green,  while  the  other  is  pink.     The 
pink  sulphide  resembles  the  sulphide  of  zinc  in  being  so  finely 
divided  that  filtration  is  difficult.     The  'green  sulphide  on  the 
other  hand  offers  no  difficulty  of  this  kind  in  manipulation.     It  is, 
therefore,  highly  desirable  to  obtain  this  variety  in  determining 
manganese.     This  is  accomplished  by  placing  an  excess  of  color- 
less ammonium  sulphide  together  with  some  ammonium  chloride 
in  an  Erlenmeyer   flask,  diluting  to  about  100  c.c.  and  heating 
nearly  to  boiling.     The  concentrated  Lnd  hot  manganese  solution 
is  then  added  all  at  once,  and  the  flask  vigorously  shaken.     The 
solution  is  kept  hot  and  occasionally  vigorously  agitated,  until 
the  sulphide  changes  from  pink  to  green  and  settles  readily,  leav- 
ing a  clear  supernatant  liquid.     Failure  to  obtain  this  result  is 
generally  due  to  the  presence  of  an  insufficient  amount  of  ammo- 
nium   sulphide.     The    precipitate    is    filtered    immediately,    and 
washed  with  water  containing  ammonium  sulphide  and  ammonium 
chloride. 

79.  Precipitation  of  Iron  as  Sulphide. — Iron  is  precipitated  as 
sulphide  from  solutions  containing  organic  matter,  which  prevents 
the  complete  precipitation  of  the  metal  in  the  usual  manner  as 
hydroxide.     Ammonium   chloride   must   be   present,   and   either 
the  yellow  or  colorless  ammonium  sulphide  may  be  used  for  pre- 
cipitation.    The  solution  must  stand  in  a  warm  place  until  the 
precipitate  has  completely  settled,  leaving  a  clear  supernatant 
fluid.     Ammonium  chloride  and  sulphide  must  be  present  in  the 
wash- water,  as  the  sulphide  oxidizes  to  sulphate,  which  passes 
through  the  filter-paper  unless  it  is  reprecipitated  by  the  ammo- 
nium  sulphide.     The   bulk   of   the   dried   precipitate   should   be 
removed  from  the  paper  before  burning  the  latter,  though  there 
is  no  danger  of  losing  iron.      The  oxide  formed  during  the  com- 


DETERMINATION  OF  METALS  AS  SULPHIDE.  83 

bustion  of  the  paper  is  reconverted  to  sulphide  by  ignition  in 
hydrogen  after  the  addition  of  sulphur. 

EXERCISE   22. 
Determination  of  Manganese  in  Potassium  Permanganate,  KMn04. 

Weigh  out  one-half  gram  of  pure  potassium  permanganate.  Transfer 
to  a  small  beaker,  add  a  few  cubic  centimeters  of  cone,  hydrochloric  acid, 
cover  with  a  watch-crystal,  and  evaporate  to  dryness  on  the  water-bath. 
Dissolve  in  a  few  c.c.  of  water  and  neutralize  the  solution  with  ammonia. 

Prepare  some  colorless  ammonium  sulphide  by  adding  100  c.c.  of  water 
to  50  c.c.  concentrated  ammonia  (sp.  gr.  0.90) ,  saturating  half  of  this  solu- 
tion with  hydrogen  sulphide  and  then  adding  the  other  half  of  the  solution. 
In  an  Erlenmeyer  flask  of  about  250  c.c.  capacity  is  placed  25  c.c.  of  the 
ammonium  sulphide  solution,  10  c.c.  of  a  five  times  normal  solution  of 
ammonium  chloride  or  about  3  grams  of  the  dry  salt.  The  solution  is 
diluted  to  about  100  c.c.  and  warmed  on  the  hot  plate  or  with  a  Bunsen 
burner.  As  soon  as  it  comes  to  a  boil  the  hot  manganese  solution  is  added, 
the  beaker  being  rinsed  with  a  little  water.  The  flask  is  shaken  vigorously 
and  the  solution  kept  nearly  at  the  boiling-point.  After  alternate  shaking 
and  heating  for  a  few  minutes  the  manganese  sulphide  which  came  down 
pink  turns  green  and  settles  readily,  leaving  a  clear  supernatant  liquid. 
Failure  to  secure  this  result  is  due  to  the  absence  of  a  sufficient  amount  of 
ammonium  sulphide,  the  reagent  being  weak  or  the  salt  was  boiled  away 
before  adding  the  manganese  solution.  If  the  addition  of  more  ammonium 
sulphide  and  heating  fails  to  produce  the  green  precipitate,  the  operation 
should  be  repeated,  using  more  ammonium  sulphide. 

The  green  sulphide  is  allowed  to  settle  for  half  an  hour,  the  solution  being 
kept  warm.  The  precipitate  is  then  filtered  off  and  washed  with  water  con- 
taining ammonium  sulphide  and  ammonium  chloride.  It  is  dried  and  de- 
tached from  the  paper,  which  is  burned,  the  ash  being  placed  on  top  of  the 
precipitate  in  a  weighed  porcelain  crucible.  The  precipitate  is  mixed  with 
two  or  three  times  its  bulk  of  sulphur  which  is  free  from  non-volatile  matter. 
It  is  ignited  in  a  stream  of  hydrogen  which  has  been  passed  through  a  solu- 
tion of  mercuric  chloride  to  free  it  from  arsine  and  then  dried  over  calcium 
chloride  or  concentrated  sulphuric  acid.  The  crucible  is  heated,  at  first 
gently,  until  the  sulphur  begins  to  burn,  and  then  with  the  full  flame  of  a 
Bunsen  burner,  giving  a  flame  at  least  15  cm.  long  when  burnmg  free. 
The  crucible  is  allowed  to  become  nearly  cold  in  the  stream  of  hydrogen. 
If  brown  particles  of  oxides  of  manganese  are  still  visible,  the  precipitate 
should  be  again  ignited  after  mixing  with  sulphur,  and  after  cooling  in 
the  stream  of  hydrogen  is  transferred  to  the  desiccator,  completely  cooled 
and  weighed.  No  further  change  in  weight  should  be  observed  on  again 
heating  with  sulphur  in  the  stream  of  hydrogen. 


CHAPTER  VII. 

DETERMINATION   OF   METALS  AS  PHOSPHATE,    ARSE- 
NATE,    CHROMATE,   AND   CHLORIDE. 

MANGANESE,  ZINC,  and  MAGNESIUM  may  be  precipitated  and 
weighed  as  phosphates.  Although  most  of  the  metals  form  phos- 
phates which  are  quite  insoluble,  only  those  of  the  three  metals 
mentioned  are  sufficiently  insoluble  or  stable  on  ignition  for 
quantitative  determination.  Phosphoric  acid  is  one  of  the  most 
non-volatile  acids,  but  on  account  of  its  weak  affinity  and  ten- 
dency to  form  acid  salts  it  is  difficult  to  procure  a  precipitate  of 
constant  composition.  It  may  also  be  reduced  more  or  less  com- 
pletely during  the  combustion  of  the  filter-paper. 

80.  Precipitation  of  Manganese  and  Zinc. — With  zinc  and 
manganese  an  amorphous  precipitate  is  first  formed.  This  is 
the  neutral  metallic  phosphate  M3(P04)2,*  and  must  be  converted 
into  the  double  ammonium  phosphate  MNH4P04.  This  is  ac- 
complished by  heating  the  amorphous  precipitate  with  the  solu- 
tion containing  a  large  amount  of  ammonium  chloride.  20  grams 
of  the  dry  salt  should  be  present  for  0.2  gram  of  the  metal.  5  to 
10  c.c.  of  a  cold  saturated  solution  of  microcosmic  salt  is  then 
added  and  the  solution  diluted  to  200  c.c.  The  solution  is  then 
cautiously  neutralized  with  ammonia,  and  on  the  first  appearance 
of  a  precipitate  it  is  heated  nearly  to  boiling  and  vigorously  stirred 
until  the  amorphous  phosphate  has  become  crystalline.  Ammo- 
nia is  added  drop  by  drop  and  the  stirring  continued  after  each 
addition  of  ammonia  until  the  precipitate  is  crystalline.  The 
addition  of  ammonia  and  the  stirring  is  continued  until  no  fur- 
ther precipitate  forms.  With  manganese  a  large  excess  of  ammo- 
nia should  be  avoided.  The  zinc  solution  should  be  made  exactly 
neutral.  The  heating  of  the  solution  must  be  continued  during 
the  addition  of  ammonia  until  the  phosphate  is  completely  pre- 

*  In  this  formula  M  may  be  either  Mn  or  Zn. 

84 


DETERMINATION  OF  METALS  AS  PHOSPHATE.  85 

cipitated  and  converted  into  the  crystalline  form.  A  platinum 
dish  is  most  suitable  for  this  purpose.  A  porcelain  dish  is  prefer- 
able to  a  beaker,  as  the  latter  is  attacked  by  the  hot  alkaline  solu- 
tion, contaminating  the  precipitate  with  silica.  The  solution 
should  contain  no  other  metals  than  the  alkalies.  The  precipitate 
is  washed  with  water  containing  ammonium  nitrate. 

81.  Magnesium  forms  a  crystalline  precipitate  much  more  read- 
ily than  zinc  and  manganese.     The  precipitation  is  carried  out 
in  a  cold  solution  which  must  be  allowed  to  stand  for  at  least 
twelve  hours  in  order  to  secure  complete  precipitation.     If  the 
solution  is  stirred  or  vigorously  shaken,  the  precipitation  is  com- 
plete in  an  hour  or  two.     If  the  amount  of  ammonium  salts  pres- 
ent is  considerable,  the  magnesium  is  precipitated  more  slowly, 
requiring  at  least  twenty-four  hours'  standing.     In  this  case  the 
composition  of  the  precipitate  is  also  different,  containing  rela- 
tively more  ammonia.     Such  a  precipitate   is  not   suitable   for 
weighing,  but  must  be  dissolved  in  the  least  amount  of  hydro- 
chloric acid  and  reprecipitated  by  the  addition  of  a  little  alkali- 
phosphate    solution   and   ammonia   drop  by  drop  with  vigorous 
stirring.     The  volume  of  the  solution  should  be  kept  small  on 
account  of  the  solubility  of  the  precipitate.     It  should  not  exceed 
100  c.c. 

82.  Removal    of  Ammonium  Salts. — If  one  does  not  wish  to 
make  two  precipitations  in  this  manner,   the  ammonium  salts 
must  first  be  removed.     A  very  convenient  method  of  accom- 
plishing this  object  is  that  suggested  by  J.  L.  Smith.     The  solu- 
tion is  evaporated  in  a  casserole  or  porcelain  dish  over  the  free 
flame  to  a  sirupy  consistency.     Dilute  nitric  acid  is  then  added 
in  small  portions,  and  the  heating  continued  until  nitrous  fumes 
are  no  longer  evolved.    A  little  hydrochloric  acid  is  then  added 
in  small  portions  and  the  heating  continued  until  the  excess  of 
nitric  acid  is  decomposed  and  chlorine  is  no  longer  evolved. 

The  ammonium  salts  may  also  be  removed  by  evaporating 
the  solution  to  dryness  in  a  porcelain  dish  and  gently  heating 
the  residue.  The  magnesium  remains  as  oxide  and  is  dissolved 
in  hydrochloric  acid.  The  solution  is  then  made  alkaline  with 
ammonia.  If  a  precipitate  of  magnesium  hydroxide  is  formed, 
the  solution  should  be  acidified  with  hydrochloric  acid  and 


86  DETERMINATION  OF  METALS. 

again  made  alkaline  with  ammonia.  This  operation  should  &e 
repeated  until  no  precipitate  occurs  on  making  the  solution 
alkaline.  Finally  the  solution  is  acidified  and  a  solution  of  di- 
sodium  phosphate  added  in  sufficient  amount  to  precipitate  all  of 
the  magnesium.  It  is  then  made  alkaline  by  the  addition  of 
ammonia  with  constant  stirring  until  no  further  precipitate  is 
formed.  The  solution  must  be  allowed  to  stand  twelve  hours 
before  filtering  off  the  precipitate.  A  few  drops  of  the  clear 
liquid  should  give  a  distinct  test  for  phosphoric  acid  with  mag- 
nesia mixture,  otherwise  enough  of  the  precipitant  has  not  been 
added.  The  precipitate  is  washed  with  dilute  ammonia,  one  part 
of  strong  ammonia  to  ten  parts  of  water. 

83.  Ignition  of  Phosphate  Precipitates. — All  of  the  phosphate 
precipitates  lose  ammonia  and  water  on  ignition  and  are  con- 
verted into  pyrophosphates  according  to  the  equation 

2NH4MP04  =2NH3  +11,0  +Mff>r 

If  too  much  ammonium  salts  have  been  present  in  the  solution,  the 
precipitate  will  contain  more  ammonia  than  corresponds  to  the 
formula  NH4MP04.  Some  metaphosphate  of  the  metal,  M(P03)2, 
will  be  formed  on  the  ignition  of  such  a  precipitate.  The  meta- 
phosphate loses  phosphorous  pentoxide,  and  is  converted  into 
pyrophosphate  only  after  very  prolonged  and  intense  ignition. 
Such  a  precipitate  could  be  considered  constant  only  when  it  does 
not  lose  weight  after  a  half-hour's  ignition  with  the  blast-lamp. 
For  this  reason  the  excess  of  ammonium  salts  is  removed  before 
precipitation,  as  already  directed.  The  evolution  of  ammonia 
and  water  is  quite  rapid  if  the  precipitate  is  heated  strongly, 
resulting  hi  appreciable  loss  of  material  due  to  spattering.  The 
precipitate  should  therefore  at  first  be  heated  very  gently  with 
the  Bunsen  burner  with  the  lid  on  the  crucible.  When  no  more 
ammonia  is  evolved  the  crucible  may  be  heated  with  the  blast- 
lamp. 

As  the  phosphates  fuse  readily,  the  complete  combustion  of  the 
filter-paper  is  secured  with  some  difficulty.  It  is  best  to  dry  the 
precipitate  and  remove  it  as  completely  as  possible  from  the 
paper,  and  burn  the  latter  on  the  platinum  wire,  using  the  oxid- 


DETERMINATION  OF  METALS  AS  PHOSPHATE.  87 

izing  flame  of  the  Bunsen  burner  to  assist  the  combustion.  If 
the  ash  which  falls  into  the  crucible  contains  carbon  which  cannot 
be  burned  by  simple  ignition,  it  may  be  moistened  with  a  drop  or 
two  of  concentrated  nitric  acid  which  is  volatilized  by  gentle  heat 
and  the  ignition  continued.  The  difficulty  experienced  in  burn- 
ing the  carbon  is  frequently  due  to  insufficient  washing  of  the 
precipitate,  so  that  it  still  contains  sodium  acid  phosphate. 

If  the  platinum  crucible  is  used  for  the  ignition  of  phosphates, 
it  is  sometimes  damaged  by  the  reduction  of  the  phosphates  and 
the  union  of  the  phosphorus  with  the  platinum.  The  likelihood 
of  damage  seems  to  be  much  reduced  if  the  moist  precipitate  and 
paper  are  introduced  into  the  crucible  and  the  ignition  conducted 
by  gradually  bringing  the  crucible  to  redness  with  the  Bunsen 
burner.  The  reducing  gases  from  the  flame  must  be  absolutely 
excluded.  Ignition  of  the  precipitate  in  a  porcelain  crucible  is 
entirely  satisfactory. 


EXERCISE  23. 
Determination  of  Magnesium  in  Magnesium  Sulphate,  MgS04.7H2O. 

Weigh  out  1  gram  of  the  pure  recrystallized  salt.  Transfer  to  a  small 
beaker  and  dissolve  in  50  c.c.  of  water.  Add  5  c.c.  dilute  hydrochloric 
acid  and  15  c.c.  of  disodium  phosphate  solution.  Add  dilute  filtered  ammo- 
nia slowly  with  vigorous  stirring,  being  careful  to  avoid  rubbing  the  sides 
of  the  beaker  with  the  stirring-rod,  as  very  firmly  adhering  crystals  would 
form  where  the  beaker  is  rubbed.  Add  a  few  c.c  of  ammonia  in  excess, 
allow  it  to  stand  for  twelve  hours,  then  transfer  to  a  funnel  containing  a 
small  paper  and  wash  with  dilute  filtered  ammonia  (1  part  of  strong  ammonia 
to  10  parts  of  water).  The  filtrate  may  be  tested  for  chlorides  by  acidify- 
ing with  nitric  acid  and  adding  a  drop  of  silver  nitrate  solution.  As  the 
ammonia  frequently  contains  chlorides,  the  washing  should  be  continued 
until  the  filtrate  gives  no  more  turbidity  with  silver  nitrate  than  the  dilute 
ammonia. 

The  precipitate  is  dried  in  the  steam-oven,  removed  from  the  paper 
quite  completely  by  means  of  a  brush  and  transferred  to  a  watch-crystal 
The  paper  is  burned  on  the  platinum  wire,  heating  the  charred  portions 
from  tune  to  time  in  the  oxidizing  flame  of  the  Bunsen  burner.  The  ash 
is  allowed  to  drop  into  a  weighed  porcelain  crucible.  If  unburned  carbon 
still  remains,  the  open  crucible  should  be  placed  on  its  side  and  heated 
with  the  Bunsen  burner.  If  necessary,  the  combustion  is  completed  by 
allowing  the  crucible  to  cool,  adding  a  few  drops  of  concentrated  nitric 


88  DETERMINATION  OF  METALS. 

acid  and  placing  the  lid  on  the  crucible  and  heating  gently  until  the  nitric 
acid  is  volatilized.  When  the  carbon  has  been .  completely  burned  the 
main  portion  of  the  precipitate  is  added  and  the  covered  crucible  very 
gently  heated  until  no  more  ammonia  fumes  escape.  It  is  then  heated  with 
the  full  flame  of  the  Bunsen  burner  and  finally  for  five  or  ten  minutes  with 
the  blast-lamp.  It  is  then  weighed  and  again  heated  with  the  blast-lamp 
and  weighed.  This  is  repeated  until  constant  weight  is  obtained.  Theo- 
retical percentage  of  magnesium  in  crystallized  magnesium  sulphate  is  9.88. 

84.  Determination  of  Arsenic  as  Magnesium  Pyroarsenate. — 
When  present  in  its  higher  state  of  oxidation  arsenic  may  be 
precipitated  as  magnesium  ammonium  arsenate,  which  is  ignited 
and  weighed  as  magnesium  pyroarsenate,  Mg2As207.  Solution 
of  compounds  of  arsenic  in  hydrochloric  acid  and  potassium 
chlorate,  aqua  regia,  or  strong  nitric  acid  converts  the  arsenic 
into  arsenic  acid.*  The  solution  is  filtered  if  necessary,  evapo- 
rated to  a  small  bulk,  neutralized  with  ammonia,  and  magnesia 
mixture  added  with  stirring.  Alcohol  is  added  to  the  extent  of 
one-fourth  of  the  volume  of  the  solution.  After  standing  twelve 
or,  still  better,  as  long  as  forty-eight  hours  the  precipitate  is 
filtered  off  and  washed  with  a  mixture  of  2  volumes  of  strong 
ammonia,  2  volumes  of  water,  and  1  volume  of  alcohol. 

The  precipitate  is  dried,  detached  from  the  paper  and  placed 
on  a  watch-crystal.  The  paper  is  returned  to  the  funnel  and  the 
remaining  portions  of  the  precipitate  dissolved  in  hot  dilute  nitric 
acid,  and  the  paper  washed  with  hot  water.  If  it  is  not  large,  the 
entire  precipitate  may  be  dissolved  in  nitric  acid.  The  solution 
is  evaporated  to  dryness  in  a  weighed  porcelain  crucible  and  the 
remainder  of  the  precipitate  added.  With  the  lid  on,  the  crucible 
is  heated  very  gently  until  the  ammonia  is  expelled.  There  is 
great  danger  of  loss  of  arsenic  before  the  ammonia  is  expelled 
because  of  its  reducing  action  on  the  arsenic.  The  presence  of 
nitric  acid  tends  to  prevent  this,  while  by  heating  in  a  stream  of 
oxygen  the  time  required  to  expel  the  ammonia  may  be  reduced 
to  ten  minutes.  Moistening  the  precipitate  with  nitric  acid  is 
almost  as  efficient.  When  the  ammonia  is  completely  expelled 
the  crucible  is  heated  to  redness  with  the  Bunsen  burner.  The 

*  Note  for  student:    Write  the  equations  for  these  reactions. 


DETERMINATION  OF  METALS  AS  CHLORIDE.  89 

precipitate  is  then  magnesium  pyroarsenate,  Mg2As207.  The  pre- 
cipitate may  also  be  collected  on  a  Gooch  crucible  and  ignited 
with  the  precautions  given. 

85.  Determination  of  Lead,  Barium,  and  Chromium  as  Chro- 
mates. — Lead  and  barium  form  very  insoluble  chromates  which 
are  of  definite  composition  and  may  be  completely  dried  without 
suffering  decomposition.    These  precipitates  may  therefore  serve 
to  determine  not  only  lead  and  barium,  but  also  chromium.    The 
precipitation  is  carried  out  in  a  solution  acid  with  acetic  acid. 
In  determinations  of  lead  and  barium,  potassium  dichromate  is 
used  as  the  precipitant,  wiiile  chromic  acid  is  precipitated  by  lead 
acetate  or  barium  chloride.     Solutions  acid  with  hydrochloric  or 
nitric  acid  are  rendered  acid  with  acetic  acid  by  addition  of  sodium 
acetate.     The  precipitate  is  washed  with  water,   filtered  on  a 
Gooch  crucible,  and  dried  on  the  hot  plate,  or  it  may  be  gently 
ignited  with  the  Bunsen  burner,  care  being  taken  not  to  allow  it 
to  fuse. 

86.  Determination  of  Potassium  as  Platino-chloride.  —  Potas- 
sium may  be  precipitated  as  potassium  platino-chloride,  collected 
on  a  Gooch  crucible  which  has  been  dried  at  100°  and  weighed, 
or  it  may  be  collected  on  a  filter-paper,  washed  with  80%  alcohol, 
dissolved  in  hot  water,  and  the  solution  evaporated  to  dryness  on 
the  water-bath  in  a  dish  and  weighed.    The  precipitate  may 
also  be  decomposed  by  ignition  with  a  reducing  agent,  such  as 
oxalic  acid,  or  in  a  stream  of  hydrogen.     The  potassium  then 
remains  as  chloride  and  is  washed  out  of  the  reduced  spongy 
platinum  by  hot  water.     The  platinum  may  then  be  gently  ignited 
and  weighed.     Potassium  platino-chloride  is  soluble  in  100  parts 
of  cold  water  and  20  parts  of  hot  water,  in  40,000  parts  of  absolute 
alcohol  and  25,000  parts  of  80%  alcohol.     The  presence  of  sodium 
platino-chloride  and  excess  of  the  precipitant,  platinum  tetra- 
chloride,  or,  more  correctly,  chlorplatinic  acid,  decreases  the  solu- 
bility.    As  the  precipitate  is  washed  with  alcohol,  salts  insoluble  in 
this  liquid  must  be  absent.     Practically  the  solution  may  contain 
only  chlorides  of   sodium  and  potassium  but  sufficient  platino- 
chloride  solution  must  be  added  to  convert  both  of  these  salts 
into  the  double  platinum  salt. 

87.  Lindo-Gladding   Method. — If  the  Lindo-Gladding  method 


90  DETERMINATION  OF  METALS. 

of  determining  potassium  is  used,  sulphates  may  be  present,  but 
the  heavy  metals  should  be  absent.  If  chlorides  are  not  present, 
sodium  chloride  should  be  added,  and  after  the  addition  of  an 
excess  of  platino-chloride  solution  the  solution  is  evaporated  to  a 
sirupy  consistency.  The  sodium  platino-chloride  and  the  excess 
of  the  precipitant  is  washed  out  by  means  of  80%  alcohol.  The 
sulphates  are  washed  out  with  a  solution  of  ammonium  chloride 
made  by  dissolving  100  grams  in  500  c.c.  of  water  and  shaking 
up  the  solution  with  10  grams  of  potassium  platino-chloride.  The 
ammonium  chloride  is  now  washed  out  with  80%  alcohol.  While 
the  Lindo-Gladding  method  has  been  extensively  used  by  Ameri- 
can chemists,  the  German  workers  still  prefer  to  remove  sulphates 
by  precipitation  with  barium  chloride. 

88.  Determination  of  Silver  as  Chloride. — Silver  as  well  as 
mercury  and  bismuth  may  be  precipitated  as  chloride  and  weighed 
as  such.  The  neutral  or  slightly  acid  solution  of  silver  is  precipi- 
tated by  the  addition  of  hydrochloric  acid  or  sodium  chloride.  It 
should  be  heated  nearly  to  boiling  before  precipitation  and  the 
chloride  added  while  stirring  the  solution  vigorously.  For  this 
purpose  it  is  convenient  to  place  the  solution  in  an  Erlenmeyer 
flask.  The  addition  of  a  considerable  excess  of  chloride  should  be 
avoided,  as  silver  chloride  is  appreciably  soluble  both  in  hydro- 
chloric acid  and  in  sodium  chloride.  After  each  addition  of  the 
reagent,  and  vigorous  shaking  or  stirring  of  the  solution,  the  pre- 
cipitate should  be  allowed  to  settle  and  a  drop  of  chloride  added  to 
the  clear  liquid.  If  a  precipitate  appears,  more  of  the  precipitant 
must  be  added.  When  the  silver  has  been  completely  precipi- 
tated the  solution  should  be  heated  on  the  hot  plate  or  the  water- 
bath  with  occasional  stirring  until  the  solution  is  clear.  The  pre- 
cipitate is  filtered  off  on  a  Gooch  crucible  which  has  been  dried  on 
the  hot  plate  or  in  an  air-bath  at  a  temperature  above  120°.  Silver 
chloride  loses  chlorine  when  exposed  to  light,  acquiring  a  blue 
color.  It  should  therefore  be  protected  as  much  as  possible  during 
the  manipulation  from  the  action  of  strong  light. 

It  may  also  be  filtered  off  on  paper  and  washed,  as  already  de- 
scribed. The  precipitate  is  dried  and  removed  from  the  paper 
as  completely  as  possible  and  placed  on  a  watch-crystal.  The 
paper  is  now  burned  in  a  weighed  porcelain  crucible.  The  silver 


DETERMINATION  OF  METALS  AS  CHLORIDE.  91 

chloride  left  on  the  paper  is  reduced  to  metallic  silver.  It  is 
reconverted  to  chloride  by  the  addition  of  a  drop  of  nitric  acid 
and  one  of  hydrochloric  and  evaporating  to  dryness.  As  silver 
chloride  is  quite  volatile,  the  amount  left  on  the  paper  should  be 
very  small.  The  main  portion  of  the  precipitate  is  now  trans- 
ferred to  the  crucible  and  dried  by  gently  heating  with  the  Bunsen 
burner.  The  precipitate  must  not  be  allowed  to  fuse.  The  Bun- 
sen  burner,  having  a  small  flame,  should  be  held  in  the  hand,  and 
immediately  removed  when  the  precipitate  begins  to  fuse  around 
the  edge.  The  results  are  very  accurate  if  the  determination  is 
properly  carried  out. 

89.  Washing   the  Precipitate   without   Filtration. — The  silver 
chloride  may  also  be  washed  and  transferred  to  a  crucible  without 


FIG.  14. 

the  use  of  filter-paper.  The  precipitation  is  carried  out  in  an 
Erlenmeyer  flask.  When  all  of  the  silver  has  been  precipitated 
the  flask  is  shaken  vigorously,  until  the  liquid  is  perfectly  clear 
and  the  precipitate  has  collected  into  a  compact  mass.  The 
liquid  is  carefully  decanted,  and  the  precipitate  washed  by  decan- 
tation.  The  flask  is  then  filled  with  water,  and  a  weighed  porce- 
lain crucible  inverted  over  the  mouth  of  the  flask.  By  quickly 
inverting  the  flask  the  precipitate  falls  into  the  crucible,  which 
is  placed  on  the  desk  while  the  flask  is  held  upright  with  the 
mouth  in1  the  bottom  of  the  crucible.  By  gently  tapping  the 
flask  any  adhering  particles  of  the  precipitate  fall  into  the  crucible. 
By  tilting  the  flask  slightly,  bubbles  of  air  enter  and  the  water 
flows  quietly  into  the  crucible.  When  the  latter  is  full  the  flask 
may  be  removed  by  quickly  passing  it  to  one  side  of  the  crucible. 


92  DETERMINATION  OF  METALS. 

The  water  in  the  crucible  is  decanted  as  much  as  possible,  the 
portion  remaining  being  evaporated  on  the  water-bath.  The 
precipitate  is  then  heated  in  the  usual  manner  and  weighed. 

90.  Determination    of   Mercury  as  Mercurous   Chloride. — For 
precipitation  as  chloride,  mercury  must  be  in  the  mercurous  con- 
dition.    The  cold  and  highly  dilute  solution  is  mixed  with  sodium 
chloride  solution  until  precipitation  is  complete.    After  allowing 
the  precipitate  to  settle,  filter  on  a  Gooch  crucible  dried  at  100°. 
Wash  with  cold  water.     Weigh  after  drying  at  100°.     If  the  solu- 
tion contains  much  nitric  acid  it  should  be  nearly  neutralized 
with  -sodium  carbonate  before  precipitation  of  the  mercury.     If 
the    metal  is  present   in    the    solution  as  mercuric    mercury,  it 
may  be  reduced  by  means  of  phosphorous  acid.     Hydrochloric 
acid  together  with  excess  of  phosphorous  acid  is  added  to  the 
cold  solution,  which  is  allowed  to  stand  for  twelve  hours  cold  or 
heated  not  higher  than  60°.    The  precipitate  of  mercurous  chlo- 
ride is  filtered  off,  washed,  dried,  and  weighed  as  already  directed. 

91.  Determination   of  Bismuth    as    Oxychloride. — Bismuth  is 
precipitated   and    weighed    as   the    oxychloride,    BiOCl.     If   the 
solution  is  strongly  acid,  potassium,  sodium,  or  ammonium  hydrox- 
ides are  added  until  not  over  J  per  cent  of  free  acid  remains.    If 
chlorides  are  not  present,  ammonium  chloride  is  added  and  the 
solution  is  then  largely  diluted  by  the  addition  of  water.    After 
standing  some  time  water  is  added  to  a  portion  of  the  clear  liquid. 
If  a  precipitate  forms,  more  water  must  be  added  to  the  entire 
solution.     When  all  of  the  bismuth  has  been  precipitated  the 
oxychloride  is  filtered  off  on  a  Gooch  crucible  which  has  been 
dried  at  100°.     It  is  washed  with  water  containing  a  few  drops 
of  hydrochloric  acid,  dried  at  100°,  and  weighed.     If  sulphuric 
or  phosphorous  acids  are  present,  small  quantities  of  these  acids 
are  apt  to   contaminate   the   precipitate.     The   solution   should 
contain  only  nitric  and  hydrochloric  acids.     Because  of  the  ten- 
dency of  the  precipitate  to  lose  chlorine,  the  results  are  not  quite 
as  reliable  as  when  the  bismuth  is  weighed  as  oxide.    A  little 
hydrochloric  acid  is  added  to  the  wash-water  to  prevent  this  loss. 


DETERMINATION  OF  METALS  AS  CHLORIDE.  93 


EXERCISE  24. 
Determination  of  Silver  in  Silver  Nitrate,  AgNO,. 

Prepare  a  Gooch  crucible  with  a  layer  of  asbestos  about  £  cm.  thick. 
Wash  until  no  fine  particles  of  asbestos  come  through  with  the  wash-water, 
Dry  on  the  hot  plate  or  in  an  air-bath  at  120°  to  150°  and  weigh.  Usually 
two  hours'  drying  is  sufficient. 

Weigh  out  ^  gram  of  silver  nitrate  and  transfer  to  a  250-c.c.  Erlen- 
meyer  flask.  Dissolve  in  100  to  150  c.c.  of  water  and  heat  nearly  to 
boiling.  Add  dilute  hydrochloric  acid,  a  few  drops  at  a  time,  shaking  the 
flask  vigorously  after  each  addition.  To  avoid  adding  a  large  excess  of 
the  acid  notice  after  each  addition  if  a  precipitate  is  produced.  When 
sufficient  hydrochloric  acid  has  been  added  the  flask  should  be  wanned 
and  vigorously  shaken  until  the  curdy  precipitate  is  well  collected  and  the 
solution  is  very  nearly  clear.  Decant  the  clear  liquid  on  the  weighed 
Gooch  crucible,  wash  two  or  three  times  by  decantation,  using  hot  water 
which  contains  a  few  drops  of  concentrated  nitric  acid.  Finally  transfer  the 
precipitate  to  the  crucible,  using  the  "  policeman "  to  clean  the  flask  if 
necessary.  Dry  the  Gooch  crucible  as  before  and  weigh.  The  silver  chlo- 
ride should  be  exposed  to  the  light  as  little  as  possible.  The  theoretical 
percentage  of  silver  in  silver  nitrate  is  63.50. 


REVIEW. — At  this  point  the  student  is  advised  to  review  the  deter- 
minations of  the  metals  by  tabulating  the  methods  as  follows:  In  the  first 
column  write  the  name  or  symbol  of  each  metal  studied.  In  the  next 
column  headed  "Precipitated  as"  write  the  formulas  of  the  precipitates 
in  which  each  element  may  be  separated  from  solution.  In  the  next 
column  headed  "Remarks,  sources  of  error,  etc.,"  indicate  briefly  the 
special  precautions  to  be  observed  in  each  precipitation.  In  the  next 
column  headed  "  Weighed  as "  give  the  formulas  of  the  dried  or  ignited 
precipitates.  In  the  next  column  headed  "Methods  of  drying"  state 
whether  the  precipitate  is  dried  at  100°  on  the  hot-plate  or  by  ignition. 


DETERMINATION  OF  ACIDS. 

CHAPTER  VIII. 

DETERMINATION   OF  THE  HALOGENS,  SUL- 
PHUR,  AND  NITROGEN. 

THE  gravimetric  determination  of  acids  is  more  difficult  than 
the  determination  of  metals  because  the  acid  radicals  are  as  a  rule 
more  volatile  and  unstable  than  the  metallic  compounds.  In 
the  determination  of  metals  various  acid  radicals  have  formed  a 
part  of  precipitates  which  can  be  washed  and  weighed.  Among 
these  are  the  carbonates  of  the  alkaline-earth  metals;  the  sul- 
phates of  lead  and  barium,  the  sulphides  of  mercury,  cadmium, 
silver,  and  arsenic,  the  phosphates  of  magnesium,  zinc,  and  manga- 
nese, the  chromates  of  barium  and  lead,  and  the  chlorides  of  silver, 
mercury,  and  bismuth.  In  a  good  many  cases  it  is  found  possible 
to  use  a  given  precipitate  for  the  determination  of  either  the  metal 
or  the  acid  radical. 

DETERMINATION    OF   THE    HALOGENS. 

92.  Precipitation  as  the  Silver  Salt.  —  Where  a  given  acid 
forms  a  weighable  precipitate  with  several  metals,  choice  is  made 
of  the  most  insoluble  and  most  stable  precipitate.  The  CHLORIDE 
OF  SILVER  has  been  found  to  be  the  most  advantageous  form  in 
which  to  precipitate  and  weigh  either  silver  or  the  chlorine  radical. 
In  the  determination  of  a  chloride  or  of  hydrochloric  acid  the 
process  is  very  nearly  identical  with  the  determination  of  silver 
as  given  on  page  90.  The  chloride  solution  must  be  slightly 
acid.  For  this  purpose  the  neutral  or  alkaline  solution  is  acidified 
with  nitric  acid,  adding  a  drop  or  two  in  excess  to  the  neutral 
solution.  The  solution  is  heated  to  about  60°  and  silver  nitrate 
solution  added  with  constant  stirring  until  no  more  precipitate  is 
formed.  Th«  solution  is  then  heated  nearly  to  boiling  and  stirred 
vigorously  until  the  precipitate  coagulates,  leaving  a  clear  superna- 

94 


DETERMINATION  OF   THE  HALOGENS.  95 

tant  liquid.  Failure  to  secure  this  result  may  be  due  to  the  absence 
of  an  excess  of  silver  nitrate.  The  precipitate  is  filtered  off  on  a 
weighed  Gooch  crucible  and  washed  with  hot  water  containing 
a  little  silver  nitrate.  The  presence  of  silver  nitrate  reduces  very 
much  the  solubility  of  silver  chloride  in  water.  When  the  other 
impurities  have  been  washed  out  the  silver  nitrate  is  removed 
by  washing  with  as  little  distilled  water  as  possible.  The  pre- 
cipitate is  then  dried  on  the  hot  plate,  or  in  an  air-bath  at  about 
150°.  Silver  chloride  should  be  protected  from  the  light  as  much 
as  possible,  as  it  turns  dark,  losing  chlorine.  The  method  gives 
very  accurate  results.  IODIDES  and  BROMIDES  as  well  as  the 
free  acids  may  be  determined  in  the  same  manner. 

93.  Ignition  of  Paper. — If  a  Gooch  crucible  is  not  at  hand,  the 
silver  halides  may  be  filtered  off  on  paper,  washed  in  the  manner 
already  described,  and  dried.     The  precipitate  is  then  separated 
from  the  paper  as  completely  as  possible  and  placed  on  a  watch- 
crystal.     The  paper  is  burned  on  the  platinum  wire  or  in  a  weighed 
porcelain  crucible.     The  SILVER  CHLORIDE  or  BROMIDE  remaining 
on  the  paper  is  reduced  to  metallic  silver.     The  ash  of  the  paper 
in  the  crucible  is  treated  with  a  drop  or  two  of  nitric  acid  and  a 
drop  of  hydrochloric  or  hydrobromic  acid  to  reconvert  the  silver 
into  the  proper  halide  salt.     The  excess  of  acid  is  evaporated  off 
by  gentle  heat  and  the  remainder  of  the  precipitate  added.     The 
crucible  is  now  heated  very  gently  with  the  Bunsen  burner,  care 
being  taken  not  to  fuse  the  precipitate,  which  volatilizes  slightly 
when  fused.     The  SILVER  IODIDE  which  remains  on  the  filter-paper 
is  not  reduced  when  the  latter  is  burned,  but  on  account  of  its 
volatility  it  should  be  removed  as  completely  as  possible.     The 
silver  precipitate  may  also  be  washed  without  filtration  as  described 
in  Chapter  VII,  page  91. 

94.  Determination   of   the    Halogens   in   Metallic    Salts. — The 
chlorine  in  MERCURIC  CHLORIDE,   STANNOUS  CHLORIDE,  PLATINIC 

CHLORIDE,  the  CHLORIDE   of  ANTIMONY,   and  the    GREEN    CHLORIDE 

of  CHROMIUM  cannot  be  determined  by  precipitation  with  silver 
nitrate.  The  precipitate  is  contaminated  in  the  case  of  the  stannous 
salt  with  stannic  oxide  and  silver  oxide,  in  the  case  of  the  mer- 
curic and  antimony  salts  with  metallic  mercury  or  antimony. 
All  of  the  chlorine  is  not  precipitated  from  the  chromium  solution. 


96  DETERMINATION  OF  ACIDS. 

Platinous  chloride  contaminates  the  precipitate  of  silver  chloride 
from  platinic  chloride.  The  tin  in  stannous  chloride  is  therefore 
precipitated  by  boiling  the  concentrated  neutral  solution  with 
ammonium  nitrate.  The  mercury  and  antimony  are  precipitctul 
with  hydrogen  sulphide,  tartaric  acid  having  been  added  to  the  anti- 
mony solution.  The  chlorine  in  platinic  chloride  may  be  determined 
by  fusing  the  salt  with  sodium  carbonate,  filtering  off  the  platinum 
from  the  solution  of  the  fusion  and  determining  the  chlorine  in 
the  filtrate.  The  chromium  in  the  green  chloride  is  precipitated 
by  means  of  ammonia,  filtered  off  and  washed,  and  the  chlorine 
precipitated  with  silver  nitrate  in  the  filtrate.  The  corresponding 
salts  of  BROMINE  and  IODINE  are  treated  in  a  similar  manner. 

The    INSOLUBLE    CHLORIDES,    of    LEAD,    SILVER,   and    MERCURY 

must  be  decomposed  by  alkalies  for  the  separation  of  the  metal. 
Silver  chloride  may  be  fused  in  a  porcelain  crucible  with  sodium 
and  potassium  carbonates.  Lead  chloride  may  be  treated  in 
the  same  manner  or  digested  with  alkali  bicarbonates  and  water, 
while  mercurous  chloride  may  be  decomposed  by  digestion  with  a 
solution  of  sodium  or  potassium  hydroxide.  The  INSOLUBLE 
BROMIDES  and  IODIDES  may  be  treated  in  a  similar  manner. 

SEPARATION   OF  CHLORINE,    BROMINE,  AND  IODINE. 

95.  Separation  of  Iodine  as  Fallacious  Iodide  from  Chlorine  and 
Bromine. — Very  excellent  methods  have  recently  been  worked 
out  for  the  separation  of  chlorine,  bromine,  and  iodine.  Iodine 
may  be  precipitated  either  as  palladous  or  thallous  iodide  from 
solutions  which  contain  chlorine  or  bromine.  For  the  precipita- 
tion as  palladous  iodide  the  solution  is  made  slightly  acid  with 
nitric  acid  and  palladous  nitrate  added  in  slight  excess.  The 
solution  is  allowed  to  stand  for  twenty-four  to  forty-eight  hours. 
The  brownish-black  precipitate  of  palladous  iodide,  PdI2,  is  fil- 
tered off  on  a  Gooch  crucible  or  on  a  tared  paper,  washed  with 
warm  water,  and  dried  at  100°  to  constant  weight.  The  paper 
may  also  be  burned,  and  the  precipitate  ignited  in  a  Rose  crucible 
in  a  stream  of  hydrogen.  It  is  then  reduced  to  metallic  palla- 
dium, and  may  be  weighed  as  such. 

If  only  hydrobromic  or  hydrochloric  acid  is  present  in  the 
filtrate,  it  may  be  precipitated  with  silver  nitrate,  after  removal 
of  the  excess  of  palladium  with  hydrogen  sulphide.  The  excess 


SEPARATION  OF  THE  HALOGENS.  97 

of  hydrogen  sulphide  must  be  removed  by  means  of  ferric  sul- 
phate or  hydrogen  peroxide,  and  the  precipitated  sulphur  filtered 
off  after  digesting  some  time  on  the  water-bath.  If  sufficient 
material  is  at  hand,  the  iodine  may  be  determined  by  means  of 
palladous  nitrate  or  chloride  in  one  portion,  and  the  iodide 
together  with  the  chloride  or  bromide  precipitated  and  weighed  as 
the  silver  salt  in  another  equal  portion.  The  weight  of  silver 
iodide  is  then  calculated  from  the  weight  of  the  palladium  pre- 
cipitate and  subtracted  from  the  weight  of  the  combined  silver 
precipitate. 

96.  Separation  of  Iodine   from   Chlorine  as  Thallous  Iodide. — 
Iodine  may  also  be  separated  from  chlorine  by  precipitation  as 
thallous  iodide,  according  to  Jannasch  and  Aschoff.     Both  thallous 
iodide  and  chloride  are  slightly  soluble  in  cold  water,  the  iodide 
less  than  the   chloride,   however.    The  presence   of  ammonium 
sulphate  increases  the  solubility  of  the  chloride  and  decreases  the 
solubility  of  the  iodide,  which  is  almost  absolutely  insoluble  if 
alcohol  is  present.     To  the  solution  of  the  chloride  and  iodide, 
which  should  have  a  volume  of  about  50  c.c.,  the  same  volume  of 
20%  ammonium  sulphate  solution  is  added  and  30  c.c.  of  alcohol. 
A  4%  solution  of  thallous  sulphate  is  now  added  until  the  iodide 
is  precipitated.     The  solution  is  gently  warmed,  well  stirred,  and 
allowed  to  stand  for  twelve  hours.     The  precipitate  of  thallous 
iodide,  Til,  is  filtered  off  on  a  Gooch  crucible,  and  washed  with  a 
mixture  of  5  parts  ammonium  sulphate,  70  parts  of  water,  and  30 
parts  of  alcohol.     It  is  dried  at  100°.     After  expelling  the  alcohol 
from  the  filtrate  the  chlorine  may  be  precipitated  with  silver 
nitrate  after  diluting  considerably  to  prevent  the  precipitation  of 
silver  sulphate.    For   the   same  reason  the   solution  should  be 
digested  hot  for  some  time. 

97.  Separation   of  Chlorine,   Bromine,   and  Iodine. — Jannasch 
and  Aschoff  have  also  worked  out  a  method  of  separating  the 
three  halogens  which  depends  on  the  fact  that  iodine  is  liberated 
from  hydriodic  acid  by  nitrous  acid,  while  neither  hydrochloric 
nor  hydrobromic  acid  is  oxidized  under  these  conditions.     The 
liberated  iodine  may  be  removed  from  the  solution  by  means  of 
carbon  disulphide,  or  it  may  be  volatilized  with  steam  after  dilut- 
ing the  solution  so  that  neither  hydrobromic   nor  hydrochloric 


98 


DETERMINATION  OF   ACIDS. 


acid  is  expelled.  Ferric  sulphate  and  a  number  of  other  reagents 
have  been  found  to  liberate  iodine  and  not  chlorine  or  bromine. 
The  hydrobromic  acid  may  be  oxidized  by  potassium  permangan- 
ate and  acetic  acid,  and  the  bromine  driven  out  of  the  solution  by 
means  of  steam.  The  chlorine  may  then  be  precipitated  with  sil- 
ver nitrate. 

A  suitable  arrangement  of  apparatus  is  shown  in  Fig.  15.    The 
ground-glass  joint  of  the  tubes  entering  the  flask  K  is  very  essen- 


FIG.  15. 

tiaJ,  as  neither  cork  nor  rubber  is  unacted  on  by  the  halogens. 
The  most  satisfactory  substitute  for  a  ground-glass  joint  is  a 
cork  stopper  coated  with  paraffine.  The  stoppers  in  the  absorp- 
tion-flask as  well  as  the  Peligot  tube  are  of  this  character.  The 
distilling-flask  K  should  have  a  capacity  of  at  least  1  liter,  and 
the  absorption-flask  a  capacity  of  500  c.c.  W  is  a  tin  or  metallic* 
vessel  for  generating  steam. 

The  neutral  solution  of  the  halogens  is  placed  in  the  flask  K] 
diluted  to  about  750  c.c.  with  water,  5  c.c.  of  dilute  sulphuric 
acid  added,  as  well  as  1  gram  of  sodium  nitrite  dissolved  in  a  little 
water.  The  flask  is  closed  immediately.  The  absorption-flask 
should  contain  a  mixture  of  50  c.c.  of  caustic  soda  *  solution  and 

*  If  caustic  soda  free  from  the  halogens  is  not  at  hand,  it  may  be  made  by  dis- 
solving metallic  sodium  in  distilled  water. 


SEPARATION  OF  THE   HALOGENS.  99 

an  equal  volume  of  hydrogen  peroxide  solution.  The  bulbs  of 
the  Peligot  tube  should  be  nearly  filled  with  a  similar  solution, 
and  a  little  dilute  caustic  soda  solution  should  be  placed  in  the 
Erlenmeyer  flask. 

98.  Distillation  of  the  Iodine. — On  heating  the  contents  of  the 
flask  K  to  boiling  and  passing  a  current  of  steam,  the  iodine  is 
driven  over  into  the  absorption-flask,  which  is  kept  cold  by  means 
of  the  cold  water  in  the  beaker.     The  distillation  is  continued 
until  the  solution  in  K  is  colorless,  which  requires  about  fifteen 
minutes.     All  of  the  iodine  will  then  be  driven  out.     On  being 
absorbed  by  the  caustic  soda,  part  of  the  iodine  is  converted  into 
iodate  according  to  the  equation 

GNaOH  +3I2  =NaI03  +5NaI  +3H20. 

The  hydrogen  peroxide  present  converts  the  iodate  into  iodide: 
NaI03  +3H202  =NaI  +3H20  +302. 

99.  Determination  of  the  Iodine  as  Silver  Iodide. — While  the 
steam  is  still  passing,  the  delivery-tube  is  withdrawn  from  the 
absorption-flask  and  rinsed  with  a  little  water.     The  contents  of 
the  flask  as  well  as  the  Peligot  tube  are  transferred  to  a  porcelain 
dish,  50  c.c.  of  hydrogen  peroxide  added,  and  the  solution  warmed 
for  some  time  on  the  water-bath.     Silver  nitrate  is  added  until  a 
permanent  precipitate  of  silver  hydroxide  is  formed.     On  digesting 
the  precipitate  the  brown  silver  hydroxide  is  converted  into  the 
yellow  iodide.     If  the  brown  color  disappears  entirely,  more  silver 
nitrate  must  be  added.     Finally  the  solution  is  acidified  with 
nitric  acid,  warmed,  and  the  silver  iodide  filtered  off  on  a  Gooch 
crucible,  washed  with  hot  water,  dried  on  the  hot  plate  or  in  the 
air-oven  at  150°  and  weighed. 

100.  Distillation  of  the  Bromine. — The  contents  of  the  dis- 
tilling flask  are  made  faintly  alkaline  with  caustic  soda  and  con- 
centrated to  a  volume  of  500  c.c.    The  solution  is  cooled,  60  c.c. 
of  33%  acetic  acid  is  added,  and  1  to  1J  grams  of  potassium  per- 
manganate dissolved  in  a  little  water.     The  absorption  flask  and 
Peligot  tube  are  filled,  and  the  distillation  conducted  exactly  as 
directed  for  the  iodine.     The  bromine  is  expelled  from  the  solution 
with  more  difficulty  than  in  the  case  of  the  iodine,  45  to  75  minutes 


100  DETERMINATION  OF  ACIDS. 

being  generally  required.  The  distillation  must  be  continued  foi 
some  time  after  bromine  vapors  are  no  longer  visible. 

The  solution  of  the  bromine  is  transferred  to  a  porcelain  dish, 
50  c.c.  of  hydrogen  peroxide  solution  added,  and  warmed  on  the 
water-bath.  The  bromine  is  then  precipitated,  washed,  and 
weighed  in  the  same  manner  as  the  iodine. 

10 1.  Determination    of    the    Chlorine. — The    potassium    per- 
manganate in  the  distilling-flask  is  decomposed  by  neutralizing 
the  solution  with  caustic  soda,  adding  a  little  methyl  alcohol  and 
warming.    The  manganese  dioxide  is  filtered  off  and  washed  with 
warm  water.     The  chlorine  in  the  filtrate  is  precipitated  in  the 
usual  manner. 

102.  Determination   of   Chloric   Acid. — Chloric    acid   may   be 
determined  by  precipitation  with  silver  nitrate  after  reduction 
with  pure  zinc  and  sulphuric  acid. 

103.  Determination  of  Hydrocyanic  Acid. — Hydrocyanic   acid 
may  be  precipitated  and  weighed  as  silver  cyanide.     The  solution 
should  be  dilute  and  slightly  acid  with  nitric  acid.     Excess  of 
silver  nitrate  is  added  to  the  cold  solution  with  vigorous  stirring. 
The  precipitate  may  be  collected  on  a  weighed  filter-paper  or,  still 
better,  on  a  Gooch  crucible  and  dried  at  100°.     It  may  also  be  col- 
lected on  an  unweighed  filter-paper  and  converted  into  metallic 
silver  by  simple  ignition  until  the  weight  is  constant.     This  opera- 
tion should  be  carried  out  in  a  porcelain  crucible.     Heating  to 
redness  for  one-quarter  hour  is  generally  sufficient  to  completely 
decompose  the  cyanide. 

104.  Separation  of  Hydrocyanic  Acid  from  Chlorides,  Bromides, 
and  Iodides. — If  chlorides,  bromides,  or  iodides  are  present  in  the 
solution  of  the  cyanide,  the  precipitated  silver  salt  of  the  halogen 
must  be  separated  from  the  silver  cyanide.      For  this  purpose 
the  precipitate  is  digested  with  a  solution  of  mercuric  oxide  in 
dilute  acetic  acid.     The  silver  cyanide  is  converted  into  soluble 
mercuric  cyanide  and  silver  acetate: 

2AgCN  +Hg(C2H302)2  =Hg(CN)2  +2AgC2H302. 

A  solution  of  mercuric  oxide  in  100  c.c.  of  water  and  5  c.c.  dilute 
acetic  acid  is  sufficient  to  dissolve  the  silver  cyanide  from  0. 1  gram 
of  potassium  cyanide.  The  halogen  salts  of  silver  are  not  decom- 


DETERMINATION  OF  SULPHUR.  101 

posed  by  the  mercury  solution,  which  is  filtered  off  and  the  dis- 
solved silver  precipitated  as  chloride  after  the  addition  of  nitric 
acid.  The  silver  chloride  must  be  ignited  in  a  stream  of  hydrogen 
in  a  porcelain  crucible.  Some  mercury  is  carried  down  with  the 
silver  and  cannot  be  entirely  removed  by  drying  the  silver  chloride. 
The  presence  of  the  halogens  do  not  interfere  in  the  volumetric 
determination  of  cyanogen. 

DETERMINATION    OF    SULPHUR. 

105.  Determination  of  Sulphur  by  Fusion  with  Alkali  Carbon- 
ates and  Nitrates. — Sulphur  in  sulphides  and  in  most  of  its  com- 
pounds is  usually  converted  into  sulphuric  acid  and  precipitated 
and  weighed  as  barium  sulphate.  Many  methods  of  oxidation 
have  been  proposed  for  converting  the  various  sulphur  compounds 
into  sulphuric  acid  or  soluble  sulphates.  Perhaps  the  most  gen- 
erally applicable  method  is  fusion  with  sodium  carbonate  and 
potassium  nitrate.  The  finely  powdered  and  weighed  material 
is  intimately  mixed  with  six  parts  of  dry  sodium  carbonate  and 
four  parts  of  potassium  nitrate.  The  proportion  of  potassium 
nitrate  should  be  reduced  if  the  percentage  of  sulphur  is  small,  as 
when  much  nitrate  is  present  the  action  on  the  platinum  crucible 
is  considerable.  This  mixture  is  transferred  to  the  crucible,  and 
gradually  heated  until  fused.  The  illuminating-gas  used  for  the 
Bunsen  burner  frequently  contains  sulphur,  which  may  be  ab- 
sorbed by  the  fusion  mixture.  To  prevent  this  it  is  advisable  to 
place  the  crucible  in  a  hole  cut  in  a  piece  of  asbestos  board.*  The 
crucible  should  not  be  heated  higher  than  necessary  to  keep  the 
contents  fused.  When  cool,  dissolve  the  residue  in  hot  water, 
filter,  and  wash  with  water  containing  a  little  sodium  carbonate. 
Acidify  the  filtrate  with  hydrochloric  acid  and  evaporate  to  dry- 
ness  in  a  porcelain  dish.  Add  a  little  dilute  hydrochloric  acid 
and  evaporate  to  dryness  again  to  completely  remove  the  nitric 
acid.  Dissolve  the  chlorides  in  water  and  a  drop  or  two  of  dilute 
hydrochloric  acid.  Filter  and  wash  the  paper  free  from  chlorides. 
The  filtrate  should  have  a  bulk  of  about  250  c.c.  Heat  to  boiling, 
add  barium  chloride  solution  slowly  with  constant  stirring,  digest 

*  As  alcohol  is  free  from  sulphur  the  crucible  is  frequently  heated  with  a 
spirit  lamp. 


102  DETERMINATION  OF  ACIDS. 

hot  until  the  solution  is  clear;    filter,  wash  free  from  chlorides, 
ignite  and  weigh. 

This  method  has  been  largely  used  for  determining  sulphur  hi 
sulphides,  either  minerals  or  laboratory  products.  If  the  sul- 
phides lose  sulphur  on  heating,  the  fusion  mixture  should  consist 
of  4  parts  of  sodium  carbonate,  8  parts  of  potassium  nitrate,  and 
24  parts  of  pure  and  perfectly  dry  sodium  chloride.  As  the 
reagents  used  may  contain  sulphur,  a  blank  determination  is  nec- 
essary in  careful  work.  For  this  purpose  the  fusion  and  precipi- 
tation must  be  carried  out  exactly  as  in  the  actual  determination. 
The  amount  of  barium  sulphate  obtained  in  this  manner  is  deducted 
from  that  obtained  in  the  analysis  of  the  unknown. 

106.  Fusion   of  Sulphur  Compounds  with  Sodium  Peroxide. — 
Sulphur  may  also  be  oxidized  to  sulphuric  acid  by  fusion  with 
sodium  peroxide.    The  finely  ground  material  is  mixed  with  five 
or  six  times  its  weight  of  powdered  sodium  peroxide,  and  the 
mixture  fused  in  a  nickel  or  copper  crucible.    The  crucible  must 
be  covered  with  a  closely  fitting  lid,  and  must  be  very  cautiously 
heated,  as  the  action  is  quite  violent  in  the  beginning.    The  flame 
of  the  Bunsen  burner  must  be  small  and  not  touching  the  crucible. 
After  a  few  minutes  the  heat  is  increased  just  sufficiently  to  keep 
the  contents  of  the  crucible  in  the  state  of  fusion,  which  is  main- 
tained for  a  few  minutes.    The  crucible  is  then  allowed  to  cool, 
placed  on  its  side  in  a  beaker,  and  water  added  from  a  wash-bottle 
while  a  watch-crystal  is  held  over  the  beaker.    When  the  fused 
mass  is  dissolved  the  crucible  and  lid  are  taken  out  with  a  clean 
pair  of  tongs  and  washed  with  water.     The  solution  is  filtered, 
and  the  residue  washed  with  water  containing  a  little  sodium 
carbonate.     If  the  residue  is  small,  and  especially  if  it  is  free  from 
iron,  this  filtration  may  be  omitted.     The  filtrate  is  acidified  with 
hydrochloric  acid,  and  if  silica  was  present  in  the  original  material 
the  solution  is  evaporated  to  dryness  finally  on  the  water-bath, 
where  it  is  heated  for  half  an  hour.     The  residue  is  treated  with 
water  and  a  few  drops  of  hydrochloric  acid  and  filtered.     The 
sulphuric  acid  in  the  filtrate  is  precipitated  and  weighed  as  barium 
sulphate.    As  the  sodium  peroxide  is  not  always  free  from  sulphur, 
a  blank  determination  must  be  made  as  in  the  preceding  method 

107.  Oxidation    of    Sulphur    Compounds   with    Fuming   Nitric 
Acid. — Digestion  with  red  fuming  nitric  acid  is  one  of  the  best  of 


DETERMINATION  OF  SULPHUR.  103 

the  methods  in  which  a  liquid  oxidizing  substance  is  used.  Most 
sulphides  and  sulphur  compounds  may  be  oxidized  in  an  Erlen- 
meyer  flask  in  the  neck  of  which  a  small  funnel  is  placed  to  pre- 
vent the  too  rapid  escape  of  the  fumes.  With  substances  which 
are  readily  decomposed,  giving  off  sulphuretted  hydrogen,  it  is 
advisable  to  conduct  the  operation  in  a  glass-stoppered  bottle. 
A  small,  flat-bottomed  glass  tube  or  weighing-bottle,  which  may 
easily  be  inserted  into  the  bottle,  is  provided.  The  material  to 
be  analyzed  is  weighed  out  and  placed  in  this  small  tube,  which  is 
then  lowered  into  the  bottle  so  as  not  to  spill  the  contents.  The 
fuming  nitric  t  acid  is  now  added  from  a  graduate.  30  c.c.  is 
sufficient  for  quantities  of  material  which  will  yield  not  more 
than  1  gram  of  barium  sulphate.  The  stopper  is  then  inserted  in 
the  bottle  and  held  firmly  with  the  hand,  while  the  bottle  is  tilted 
so  that  the  acid  comes  in  contact  with  the  sulphide.  When  the 
first  violent  reaction  has  ceased  shake  the  bottle  a  little,  cool  the 
bottle  and  contents  by  holding  it  under  a  tap  of  running  water 
for  a  few  minutes,  and  then  cautiously  remove  the  stopper  so  as 
to  relieve  the  pressure  without  loss  of  material  by  spattering. 
The  stopper  is  replaced  in  a  slanting  position  to  allow  exit  for  the 
fumes,  and  the  bottle  is  placed  on  the  water-bath.  If  after  half 
an  hour's  digestion  particles  of  sulphur  remain  floating  on  the 
liquid,  strong  hydrochloric  acid,  potassium  chlorate,  or  liquid 
bromine  may  be  added  in  small  portions,  the  potassium  chlorate 
and  liquid  bromine  being  the  most  efficient.  Care  should  be 
taken  to  prevent  the  fuming  nitric  acid  or  bromine  from  coming 
in  contact  with  the  hands,  as  serious  burns  are  very  quickly 
produced.  The  digestion  on  the  water-bath  is  then  continued 
until  the  sulphur  is  oxidized. 

108.  Decomposition  of  Insoluble  Sulphates.  —  The  complete 
disintegration  of  the  material  is  frequently  indicated  by  a  change 
in  color  or  physical  appearance.  The  nitrates  of  the  metals  are 
generally  insoluble  in  the  concentrated  acid.  Insoluble  sulphates, 
of  such  metals  as  barium  and  lead,  may  also  be  produced.  In  the 
case  of  minerals,  gangue,  quartz,  etc.,  will  be  left.  The  concentrated 
acid  is  diluted  with  water  and  digested  for  a  few  minutes  to  dis- 
solve soluble  salts.  If  BARIUM  is  present,  the  insoluble  material 
must  be  filtered  off,  washed  thoroughly,  and  fused  with  five  parts 
of  mixed  sodium  and  potassium  carbonates.  The  melt  is  dis- 


104  DETERMINATION   OF  ACIDS. 

solved  in  water,  and  the  insoluble  carbonates  filtered  off  and  thor- 
oughly washed.  The  filtrate  is  acidified  with  hydrochloric  acid 
and  added  to  the  nitric  acid  solution.  If  LEAD  is  present,  the  sul- 
phate of  this  metal  may  be  decomposed  by  digesting  with  a  solu- 
tion of  acid  sodium  or  potassium  carbonate,  filtering,  washing, 
and  treating  the  filtrate  as  before. 

109.  Removal  of  the  Nitric  Acid.  —  The  nitric  acid  solution, 
now  containing  all  of  the  sulphur  as  sulphuric  acid,  is  evaporated 
to  dryness  after  the  addition  of  a  little  sodium  chloride,  if  alkalies 
have  not  already  been  introduced.  The  evaporation  is  repeated 
after  the  addition  of  dilute  hydrochloric  acid,  until-  the  nitric  acid 
is  entirely  removed.  The  residue  is  dissolved  in  water,  a  few 
drops  of  hydrochloric  acid  are  added,  the  solution  is  filtered  if 
necessary,  and  the  sulphuric  acid  determined  as  barium  sulphate. 

no.  Other  Oxidizing  Solutions  Used  for  Sulphur  Compounds. — 
The  digestion  with  LIQUID  BROMINE  or  AQUA  REGIA  made  by  mixing 
one  part  of  strong  hydrochloric  acid  with  three  parts  of  strong 
nitric  acid  is  carried  out  in  a  similar  manner.  The  bromine  may 
either  be  added  gradually  to  the  material  mixed  with  a  little 
nitric  acid  or  to  the  dry  material.  These  methods  are  largely  used 
for  the  determination  of  sulphur  in  pyrites  or  crude  sulphur. 
(See  Exercise  36,  p.  172.)  The  dry  material  may  also  be  mixed 
with  powdered  POTASSIUM  CHLORATE  in  an  Erlenmeyer  flask, 
and  concentrated  hydrochloric  acid  added  in  small  portions. 
Finally  the  flask  is  gently  heated  on  the  water-bath.  The  sub- 
sequent treatment  is  identical  with  that  given  after  treatment 
with  nitric  acid.  A  very  powerful  oxidizing  agent  consists  of  a 
saturated  solution  of  potassium  chlorate  in  concentrated  nitric 
acid. 

in.  Oxidation  of  Sulphur  Compounds  by  Means  of  Chlorine.— 
If  lead  is  present,  the  oxidation  of  the  sulphur  by  means  of  chlo- 
rine in  alkaline  solution  is  advantageous,  because  the  lead  is  pre- 
cipitated as  peroxide.  Iron  is  also  precipitated  as  ferric  hydrox- 
ide. On  passing  the  chlorine  for  a  considerable  time,  however, 
the  iron  passes  into  solution  as  ferrate,  producing  a  red  tint.  Heat- 
ing the  liquid  for  a  few  minutes  with  powdered  quartz  serves  to 
precipitate  the  iron.  The  finely  powdered  SULPHIDE  or  CRUDE 
SULPHUR  is  heated  for  some  time  with  a  dilute  solution  of  caustic 
potash  which  is  free  from  sulphuric  acid.  Free  sulphur  as  well  as 


DETERMINATION  OF  NITROGEN.  105 

the  sulphides  of  arsenic  and  antimony  dissolve  in  the  caustic 
potash.  Chlorine-gas  is  now  led  into  the  warm  solution  for  some 
time.  Most  of  the  metals  together  with  the  gangue  remain  in  the 
precipitate,  which  is  filtered  off  and  well  washed.  The  filtrate  is 
acidified  with  hydrochloric  acid,  again  filtered  if  necessary  to 
remove  silica,  and  the  sulphuric  acid  determined  as  barium  sul- 
phate in  the  usual  manner. 

112.  Determination   of   Tellurium   and   Selenium. — These   ele- 
ments are  precipitated  from  acid  solutions  by  means  of  sulphur 
dioxide  or  sodium  or  potassium  sulphite.     If  the  solution  contains 
nitric  acid  it  must  be  evaporated  to  dryness  after  the  addition  of 
hydrochloric  acid  and  sodium  or  potassium  chloride.     The  solu- 
tion should  be  heated  nearly  to  boiling,  and  kept  at  this  temp- 
erature for  a  considerable  time  to  insure  complete  precipitation. 
If   the   solution  is  dilute  or  a  small   amount  of  the  element  is 
present,  it  must  be  allowed  to  stand  for  several  days  in  a  warm 
place.     The  precipitate   is   washed   by   decantation   with   water 
containing  sulphur  dioxide,  and  finally  transferred  to  a  weighed 
filter-paper,  or  to  a  Gooch  crucible,  and  dried  at  100°. 

DETERMINATION  OF  NITROGEN. 

Nitrogen  unites  with  hydrogen  to  form  a  fairly  strong  base, 
and  with  oxygen  and  hydrogen  to  form  one  of  the  strongest  acids. 
The  methods  used  for  determining  the  nitrogen  when  it  exists  in 
one  of  these  forms  are  not  generally  applicable  to  the  determina- 
tion of  nitrogen  in  the  other  form.  The  methods  most  commonly 
used  are  volumetric.  In  this  chapter  the  gravimetric  methods, 
which  are  at  times  convenient,  will  be  given. 

113.  Determination   of  Ammonia  as  Ammonium   Chloride. — 
Ammonia  may  be  weighed  as  ammonium  chloride,  which  can  be 
dried  at  100°.     A  solution  which  contains  nothing  but  free  ammo- 
nia, ammonium  chloride,  or  the  ammonium  salt  of  a  very  readily 
volatilized  acid,  such  as  carbonic  acid  or  hydrogen  sulphide,  may 
be  acidified  with  pure  hydrochloric  acid  and  evaporated  to  dryness 
on  the  water-bath  in  a  weighed  platinum  dish.     Many  impurities, 
such  as  silica  and  the  alkaline  earth  metals  and  the  alkalies,  do  not 
interfere,  since  after  weighing  the  residue  in  the  dish  the  ammo- 
nium chloride  may  be  volatilized  by  gentle  heat,  and  the  non- 
volatile residue  weighed.     Carried  out  in  this  manner  the  deter- 


106  DETERMINATION  OF  ACIDS. 

mination  of  ammonia  is  quicKly  and  accurately  made.  It  is 
a  convenient  method  when  so  few  determinations  are  required 
that  the  preparation  of  volumetric  solutions  is  not  warranted. 

1 14.  Determination  of  Ammonia  as  Ammonium  Platmo-chloride. 
— Ammonia   may  also   be   precipitated   as   ammonium   platino- 
chloride  which  is  washed  with  alcohol  and  dried  at  100°,  as  de- 
scribed for  potassium.    The  precipitate  may  also  be  decomposed 
by  ignition  into  metallic  platinum.    The  heat  must  be  applied 
cautiously  to  the  covered  crucible,  otherwise  the  escaping  ammo- 
nium chloride  will  carry  away  platinum. 

115.  Determination  of  Nitric   Acid  by  Means  of  Nitron. — No 
inorganic  compound  of  nitric  acid  is  known  which  is  sufficiently 
insoluble  to  be  used  for  the  quantitative  determination  of  this 
acid.    Recently  an  organic  base  has  been  made  which  forms  an 
insoluble  nitrate.*     This  base  is  1.4  diphenyl-3.5  endanilodihy- 
drotriazol,    having  the   formula   C20H16N4.      For   convenience   in 
ordinary  use  and  because  it  precipitates  nitric  acid,  the  name 
" nitron"  has  been  given  to  the  base.     For  use  as  a  reagent  a  10 
per  cent  solution  in  5  per  cent  acetic  acid  is  made. 

At  ordinary  temperatures  nitron  precipitates  nitric  acid  from 
solutions  of  1  in  60,000,  while  at  0°  C  it  will  precipitate  1  part  in 
80,000.  Unfortunately  nitron  also  precipitates  a  number  of  other 
common  acids,  such  as  hydrobromic  when  present  to  the  amount 
of  1  part  in  800  of  the  solution,  hydriodic  acid  (1  in  20,000), 
chromic  acid  (1  in  6000),  chloric  acid  (1  in  4000),  perchloric  acid 
(1  hi  50,000),  sulphocyanic  acid  (1  in  15,000),  and  nitrous  acid 
(1  in  4000).  These  acids  must  therefore  be  first  removed  from 
the  solution.  Hydrobromic  and  hydriodic  acids  may  be  removed 
by  adding  chlorine  water  and  boiling  out  the  liberated  halogen, 
or  in  the  case  of  hydriodic  acid  it  may  be  oxidized  to  the  iodate. 
These  acids  may  also  be  removed  by  means  of  silver  sulphate. 
Nitrous  and  chromic  acids  may  be  reduced  by  means  of  hydrazine 
sulphate.  To  remove  the  nitrous  acid  the  neutral  concentrated 
solution  (5-6  c.c.)  is  dropped  slowly  on  one-fourth  gram  of  finely 
powdered  hydrazine  sulphate.  It  is  advisable  to  place  the  hydra- 
zine sulphate  in  a  small  flask  in  order  to  avoid  loss  by  spattering 

*  M.  Busch  Ber.,  38,  861  (1905).     J.  Chem.  Soc.,  88,  II,  282,  418  (1905). 


DETERMINATION  OF  NITROGEN.  107 

and  to  facilitate  cooling  the  mixture  by  means  of  cold  water,  as 
a  small  amount  of  nitric  acid  is  formed  if  the  mixture  is  allowed 
to  become  warm. 

The  volume  of  the  solution  of  the  nitrate  or  nitric  acid  should 
be  small  because  of  the  slight  solubility  of  the  precipitate.  About 
0.1  gram  of  nitric  acid  should  be  present  in  a  volume  of  80  to  100  c.c. 
which  has  been  acidified  with  10  drops  of  sulphuric  acid.  The 
solution  is  brought  almost  to  the  boiling-point  and  10  to  12  c.c.  of 
the  nitron  reagent  is  added.  The  hot  solution  gives  a  crystal- 
line precipitate.  The  beaker  is  then  immersed  in  ice-water,  where 
it  is  allowed  to  stand  for  one  and  one-half  to  two  hours.  The  pre- 
cipitate is  filtered  off  on  a  Gooch  crucible  which  has  been  dried 
at  110°.  It  is  transferred  to  the  crucible  by  means  of  the  mother- 
liquor  and  is  washed  with  small  portions  of  ice-water,  using  not 
more  than  10  to  12  c.c.,  the  precipitate  being  sucked  dry  after 
each  addition  of  ice-water.  The  precipitate  is  then  dried  at  110° 
until  constant,  which  requires  about  three-fourths  of  an  hour. 
The  precipitate  has  the  formula  C20HltJN4.HN03,  and  contains 
16.8%  HN03  or  14.4%  N205. 

116.  Volatilization  of  N205  by  Fusion  of  Nitrates  with  Silica  or 
Potassium  Chromate. — Some  very  excellent  methods  of  determin- 
ing nitric  acid  depend  on  its  conversion  into  ammonia,  nitric  oxide, 
or  nitrogen.  Most  of  these  methods  will  be  described  in  the 
chapters  on  volumetric  and  gas  analysis.  It  may  also  be  deter- 
mined indirectly  by  heating  the  dry  nitrate  with  a  weighed  amount 
of  silica  or  potassium-acid  chromate  until  all  of  the  nitric  acid  is 
expelled.  The  loss  in  weight  is  due  to  expulsion  of  N205.  The 
method  can  obviously  not  be  applied  to  material  which  ma}^  lose 
anything  but  nitric  acid.  It  has  been  extensively  applied  to 
sodium  and  potassium  nitrates  which  can  be  dried  at  130°  and  100° 
respectively.  The  silica  used  must  be  finely  powdered  and  ignited 
before  being  weighed.  About  seven  times  as  much  silica  as  nitrate 
must  be  taken.  The  material  is  intimately  mixed  and  heated 
to  redness  in  a  platinum  crucible  for  two  to  four  hours  or  until 
constant  weight  is  obtained. 

Instead  of  the  silica  a  mixture  of  equal  parts  of  neutral  and  acid 
potassium  chromate  may  be  used.  This  material  is  fused  in  a  plat- 
inum crucible,  allowed  to  cool,  and  finely  powdered.  About  3  grams 


108  DETERMINATION   OF  ACIDS. 

of  the  chromate  mixture  are  weighed  out  and  placed  in  a  platinum 
crucible.  .8000  gram  of  the  dried  nitrate  is  added  and  thoroughly 
mixed  with  the  chrornate  by  means  of  a  platinum  or  glass  rod. 
The  crucible  is  heated  gently  so  as  to  gradually  bring  the  contents 
to  a  state  of  fusion.  When  no  more  acid  fumes  are  evolved,  the 
crucible  is  cooled  in  the  desiccator  and  weighed.  It  is  reheated 
until  constant  weight  is  obtained. 


CHAPTER  IX. 

DETERMINATION  OF   CARBONIC,   BORIC,    AND 
PHOSPHORIC   ACIDS. 

DETERMINATION  OF  CARBON  DIOXIDE  BY  LOSS. 

Two  general  methods  are  in  use  for  the  determination  of 
carbon  dioxide.  By  the  first  method  the  carbonate  is  decom- 
posed by  means  of  a  strong  mineral  acid  or  other  means,  and  the 
carbon  dioxide  expelled.  The  amount  present  is  ascertained  by 
weighing  the  apparatus  before  and  after  the  expulsion  of  the 
carbon  dioxide.  By  the  second  method  the  gas  evolved  by  the 
decomposition  of  the  carbonate*  is  led  into  a  caustic-potash  solu- 
tion, which  is  weighed  before  and  after  the  absorption  of  the  car- 
bon dioxide.  The  gain  in  weight  represents  the  amount  of  carbon 
dioxide  present. 

Various  forms  of  apparatus  have  been  devised  for  the  deter- 
mination of  carbon  dioxide  by  loss  when 
the  carbonate  is  decomposed  by  means  of 
acid.  Two  of  these  will  be  described,  one 
of  which  may  be  purchased  complete, 
while  the  other  rnay  be  put  together 
from  the  usual  laboratory  apparatus. 

117.  The  Schrotter  Apparatus  shown 
in  Fig.  16  may  be  purchased  ready  for  use. 
A  weighed  amount  of  the  carbonate  is 
placed  in  the  small  bulb  e  and  the  stopper 
a  firmly  inserted.  The  tube  c  is  filled  with 
dilute  nitric  or  10%  hydrochloric  acid, 
and  b  is  about  half  filled  with  concen- 
trated sulphuric  acid.  The  entire  appa- 
ratus is  then  carefully  weighed.  By  open- 
ing the  stop-cock  d,  acid  is  allowed  to 
flow  into  e  at  such  a  rate  that  the  bubbles  of  gas  can  readily 
be  counted  while  passing  through  the  concentrated  sulphuric 

109 


FIG.  16. 


110 


DETERMINATION  OF  ACIDS. 


acid  in  6.  When  all  of  the  acid  has  been  allowed  to  flow  out  of 
c,  and  carbon  dioxide  is  no  longer  evolved,  the  stop-cock  d  is 
closed  and  the  solution  hi  e  is  gently  warmed  on  the  water-bath 
or  a  hot  plate  to  expel  the  carbon  dioxide.  The  stop-cock  d  is 
now  opened  and  air  is  sucked  through  the  apparatus  at  a  moderate 
rate  until  the  carbon  dioxide  has  been  displaced.  After  being 
allowed  to  come  to  the  atmospheric  temperature,  the  apparatus 
is  again  weighed.  The  loss  in  weight  represents  the  amount 
of  carbon  dioxide  evolved. 

118.  Sources  of  Error  in  the  Determination  of  Carbon  Dioxide 
by  the  Schrotter  Apparatus.  —  Though  the  manipulation  of  this 
apparatus  is  very  simple,  it  is  impossible  to  obtain  results  accu- 
rate within  more  than  J%.  The  error  involved  in  weighing  a 
glass  apparatus  of  large  surface  is  considerable.  The  gas  which 
leaves  the  apparatus  is  dried  over  concentrated  sulphuric  acid, 
while  that  which  enters  is  not  dry.  It  is  customary  to  ascertain 
when  all  of  the  carbon  dioxide  has  been  displaced  by  air  by  the 
taste  of  the  gas  which  is  sucked  through  by  the  mouth.  The 
ordinary  chemical  tests  for  carbon  dioxide  cannot  be  used 
unless  the  air  drawn  through  is  freed  from  this  gas.  The  accu- 
racy of  the  method  would  undoubtedly  be  increased  if  the  air 
drawn  through  were  first  passed  through 
tubes  containing  soda-lime  and  concen- 
trated sulphuric  acid.  Care  must  also  be 
taken  to  have  the  apparatus  at  the  same 
temperature  each  time  it  is  weighed. 

119.  Simple  Laboratory  Apparatus  for 
Determining  Carbon  Dioxide.  —  Various 
forms  of  apparatus  which  can  be  made 
from  the  usual  laboratory  material  have 
been  suggested.  A  very  simple  device  is 
shown  in  Fig.  17.  The  small  flask  A  is 
closed  with  a  two-holed  rubber  stopper, 
through  which  the  thistle-tube  passes  as 
well  as  the  bent-glass  tube  6,  the  latter 
also  passing  to  the  bottom  of  the  short  test- 
The  thistle-tube  is  closed  with  a  glass  rod, 
piece  of  rubber  tube  has  been 


FIG.  17. 

tube  or  small  bottle  B. 

over  the  end  of  which  a  short 


DETERMINATION  OF  CARBON  DIOXIDE.  Ill 

slipped.  The  weighed  amount  of  the  carbonate  is  placed  in  the 
flask,  and  covered  with  a  little  water.  The  rubber  stopper  is 
firmly  inserted,  and  a  few  cubic  centimeters  of  dilute  hydrochloric 
acid  placed  in  the  thistle-tube.  If  the  carbonate  is  completely 
decomposed  by  sulphuric  acid,  it  may  be  used  instead  of  the  dilute 
nitric  or  1C%  hydrochloric  acid.  The  tube  B  is  half  filled  with  con- 
centrated sulphuric  acid.  The  apparatus  is  tested  for  gas  leaks 
by  drawing  out  a  little  air  from  the  bottle  A,  by  applying  suction 
to  the  tube  a.  The  sulphuric  acid  rises  in  the  tube  6,  and  should 
remain  at  the  same  height  for  several  minutes.  The  apparatus 
is  now  carefully  weighed,  and  the  carbonate  decomposed  by  allow- 
ing the  acid  in  the  thistle-tube  to  flow  into  the  flask  in  small  por- 
tions. When  all  of  the  acid  has  been  introduced,  the  flask  A  is 
gently  warmed  to  expel  the  carbon  dioxide.  The  carbon  dioxide 
is  displaced  with  air  by  applying  suction  at  a.  In  careful  work 
the  air  must  be  purified  before  entering  the  apparatus  by  passing 
through  soda-lime  and  concentrated  sulphuric  acid,  these  absorp- 
tion-tubes being  connected  to  the  thistle-tube  by  means  of  a  one- 
holed  rubber  stopper. 

120.  Determination  of  Carbon  Dioxide  by  Fusion  of  Carbonates 
with  Anhydrous  Borax  or  Microcosmic  Salt. — The  carbon  dioxide 
in  all  anhydrous  carbonates  may  be  determined  by  fusion  with 
anhydrous   borax.    The   carbon   dioxide   is   completely   expelled 
and  its  amount  computed  from  the  loss  in  weight.    About  4 
grams  of  vitrified  borax  are  taken  for  1  gram  of  the  carbonate. 
The  borax  is  placed  in  a  platinum  crucible,   and  heated  with 
the  Bunsen  burner  until  the  material  is  fused.    It  is  allowed 
to  cool  in  the  desiccator  and  then  weighed.    As  there  is  dan- 
ger of  loss  by  the  cracking  of  the  borax  if  cooled  suddenly,  the 
flame  of  the  Bunsen  burner  is  turned  down   gradually  before 
placing   the   crucible  in   the  desiccator.    As   the  borax  retains 
water  somewhat  persistently,  it  is  advisable  to  fuse  and  weigh  it 
again.    The  anhydrous  material  may  be  kept  in  a  state  of  fusion 
for  one-quarter  to  one-half  hour  without  loss  in  weight.     The 
borax  will  be  volatilized  if  heated  with  the  blast-lamp.    The  weighed 
quantity  of  the  dry  carbonate  is  placed  on  top  of  the  borax,  and 
the  latter  again  fused.    As  there  is  danger  of  loss  by  the  cracking 
of  the  borax  on  being  heated,  the  crucible  should  be  covered  until 
the  borax  is  melted.     It  must  be  kept  melted  until  the  carbonate 


112  DETERMINATION  OF  ACIDS. 

is  dissolved  and  no  more  bubbles  escape.  A  few  generally  remain 
in  the  fused  mass,  and  cannot  be  expelled  except  by  very  pro- 
longed heating.  After  being  again  cooied  in  the  desiccator,  the 
crucible  is  weighed.  The  loss  in  weight  is  du3  to  carbon  dioxide. 
The  borax  glass  may  generally  be  removed  by  pressing  the  crucible 
between  the  thumb  and  fingers,  thus  loosening  the  fused  mass, 
which  will  drop  out  on  gently  tapping  the  inverted  crucible. 

Microcosmic  salt  may  be  substituted  for  borax  in  this  deter- 
mination.* This  salt  melts  very  easily,  needirg  only  a  small 
flame  of  the  Bunsen  burner.  The  heating  is  continued  until  all 
of  the  ammonia  and  water  has  been  expelled.  It  is  cooled  and 
weighed  and  reheated  until  the  weight  is  constant.  The  weighed 
carbonate  is  then  added  and  the  contents  of  the  crucible  heated 
very  cautiously  to  avoid  loss  by  the  foaming  caused  by  the  escape 
of  carbon  dioxide.  When  no  more  carbon  dioxide  escapes,  the 
crucible  is  cooled  and  weighed.  It  is  again  heated  until  the 
weight  is  constant. 

The  action  of  the  borax  on  the  carbonate  produces  metaborates 
from  the  pyrob  orate,  while  the  fused  microcosmic  salt  is  changed 
from  the  metaphosphate  to  the  normal  phosphate,  according  to 
the  following  equations  : 

Na2B407  +  CaC03  =  2NaB02  +  Ca  (B02)2  +  C02  ; 
NaP03   +CaC03  =  NaCaP04+C02. 


DIRECT  WEIGHING  OF  CARBON   DIOXIDE. 

The  most  reliable  and  accurate  method  of  determining  carbon 
dioxide  is  that  in  which  the  gas  is  absorbed  and  weighed.  The 
carbonate  is  decomposed  by  sulphuric  or  hydrochloric  acid. 
The  gas  evolved  is  dried,  freed  from  hydrochloric  acid,  and 
absorbed  in  soda-lime  or  caustic-potash  solution.  The  last  por- 
tions of  carbon  dioxide  are  swept  out  of  the  apparatus  by  means  of 
a  stream  of  air  which  has  been  freed  from  carbon  dioxide. 

121.  Apparatus.  —  Many  forms  of  apparatus  have  been  proposed 

and  used  for  this  purpose.     The  simplest  and  most  serviceable 

form  is  that  given  by  FRESENIUS.     The  characteristic  part  of  this 

apparatus  consists  of  a  long  glass  tube  which  acts  as  a  condenser, 

and  also  holds  the  drying  and  purifying  material.     In  other  forms 

*  O.  Lutz  and  A.  Tschischikow,  Jour.  russ.  phys.-chem.  Gesellsch.,  36,  1274. 


DETERMINATION  OF   CARBON  DIOXIDE. 


113 


of  apparatus  the  drying  materials  are  placed  in  a  series  of  U-tubes 
which  are  connected  by  rubber  tubing,  stoppers,  etc.  The  diffi- 
culties in  making  absolutely  gas-tight  joints  are  numerous,  espe- 
cially when  the  permeability  of  rubber  to  carbon  dioxide  is  consid- 
ered. The  small  number  of  joints  in  the  apparatus  of  Fresenius 
gives  it  a  decided  advantage  over  other  forms.  In  all  forms  of 
apparatus  the  carbonate  is  placed  in  a  small  flask  closed  with  a 
two-holed  stopper.  Through  one  hole  a  dropping-funnel  is  passed 
by  means  of  which  the  acid  is  introduced,  and  through  the  other 
the  exit  tube  for  the  gas  is  passed.  The  end  of  the  dropping- 
funnel  is  drawn  out  to  a  capillary,  which  is  bent  upwards  to  pre- 
vent loss  of  carbon  dioxide. 

The  arrangement  of  the  apparatus  of  Fresenius  is  shown  in  Fig. 
18.  The  Erlenmeyer  flask  should  be  of  about  150  c.c.  capacity. 
The  glass  tube  should  be  about  70  cm.  long  and  at  least  1.2  cm. 
in  diameter.  It  is  inclined  slightly  so  as  to  allow  any  water 
which  condenses  in  its  lower  half  to  flow  back  into  the  flask.  A 


FIG.  18. 

wad  of  glass  wool  or  cotton  is  pushed  down  to  the  centre  of  the 
tube,  the  upper  half  of  which  is  then  filled  with  granulated  cal- 
cium chloride,  large  lumps  being  first  introduced  and  then  the 
finer  material.  If  this  calcium  chloride  has  not  already  been 


114  DETERMINATION  OF  ACIDS. 

saturated  with  carbon  dioxide,  a  slow  stream  of  the  dry  gas  is 
passed  through  the  tube  for  some  time  and  then  completely  dis- 
placed with  air. 

122.  Drying   and   Absorption    of    the    Carbon    Dioxide.  —  The 
GEISSLER  CAUSTIC-POTASH  BULBS  are  filled  two-thirds  full  with  a 
solution  of  1  part  of  caustic  potash  and  2  parts  of  water.     The 
straight  calcium- chloride   tube  is  filled  with  soda-lime  and  fused 
calcium  chloride,  so  .that  the  gas  leaves  the  apparatus  after  pass- 
ing over  the  calcium  chloride.    SODA-LIME  is  sometimes  used  foi 
absorbing  the  carbon  dioxide.     It  is  placed  in  U-tubes,  which 
are  weighed  before  and  after  absorption  of  the  carbon  dioxide. 
Unless  the  soda-lime  is  freshly  prepared  and  somewhat  moist  the 
absorption  of  large  amounts  of  carbon  dioxide  is  somewhat  slow, 
so  that  two  U-tubes  are  usually  required.     A  calcium-chloride 
tube  must  also  be  attached  and  weighed.     Whether  caustic  potash 
or  soda-lime  is  used,  an  unweighed  guard  tube  filled  with  calcium 
chloride  must  be  attached.     A  small  tube  filled  with  soda-lime  is 
also  fitted  to    the    dropping-funnel    to    remove    carbon    dioxide 
from  the  air  which  is  drawn  through  the  apparatus  to  sweep  out 
the  carbon  dioxide. 

123.  Decomposition  of   the  Carbonate. — Dilute  sulphuric  acid 
should  be  used  for  decomposing  the  carbonate,  provided  insoluble 
sulphates  are  not  formed.     When  sulphuric  acid  cannot  be  used, 
dilute  hydrochloric  or  nitric  acids  may  be  employed.     If  sul- 
phides or  sulphites  are  present,  chromic  acid  or  a  chromate  is 
added  to  prevent  the  evolution  of  sulphuretted  hydrogen  or  sul- 
phur dioxide  and  sulphuric  or  nitric  acid  must  be  used  to  decom- 
pose the  carbonate.     When  hydrochloric  acid  is  used  or  chlorides  are 
present,  the  carrying  of  the  acid  to  the  absorption  bulbs  may  be 
prevented  by  substituting  for  part  of  the  calcium  chloride  in  the 
long  drying-tube  pieces  of  pumice-stone  which  have  been  saturated 
with  copper-sulphate  solution  and  dried  at  125°.     The  method  of 
setting  up  the  apparatus  and  the  details  of  manipulation  are 
given  in  Chapter  XVI,  p.  183,  in  the  determination  of  carbon  diox- 
ide in  dolomite. 


DETERMINATION  OF   BORIC  ACID. 


115 


DETERMINATION  OF  BORIC  ACID    BY  THE    METHOD 

OF  GOOCH. 

Considerable  difficulty  has  been  met  with  hi  determining  boric 
acid,  since  no  insoluble  salt  of  this  acid  is  known  which  can  be 
washed  and  weighed  accurately  without  the  expenditure  of  a 
great  deal  of  labor.  The  acid  can  be  readily  and  completely 
separated  from  all  other  acids  and  bases  by  distillation  with 
methyl  alcohol.  The  distillate  may  then  be  evaporated  to  dryness 
and  ignited  with  a  known  amount  of  calcium  oxide.  No  boric 
acid  is  lost  and  the  amount  present  is  given  by  the  increase  hi 
weight  of  the  lime.  The  details  of  this  method  were  first  pub- 
lished by  Gooch.* 

124.  Apparatus. — The  apparatus  recommended  by  Gooch  con- 
sists of  a  retort  made  of  a  150-c.c.  or  200-c.c.  pipette.  One  of 
the  stems  of  the  pipette,  which  should  have  an  internal  diameter 
of  at  least  0.7  cm.,  is  bent  into  the  form  of  a  gooseneck,  and  is  con- 


FIG.  19. 

nected  by  means  of  a  rubber  stopper  with  an  upright  condenser, 

which  in  turn  is  connected  by  means  of  a  loosely  fitting  stopper 

*  Am.  Chem.  Jour.,  vol.  9,  p.  23. 


116  DETERMINATION  OF  ACIDS. 

with  an  Erlenmeyer  flask  of  about  250  c.c.  capacity.  The  other 
stem  of  the  pipette  is  bent  at  right  angles  and  is  connected  by 
means  of  a  short  piece  of  rubber  tubing  with  a  small  dropping- 
funnel.  An  oil-  or  paraffine-bath  is  provided  of  such  a  size  that 
the  retort  may  be  immersed  in  the  liquid. 

125.  The  Solution    of   the    Borate,  which  should  not  contain 
more  than  0.2  gram  of  B203,  is  acidified  with  nitric  or  acetic  acid.* 
Hydrochloric  acid  should  be  absent,  as  a  considerable  amount  of 
this  acid  would  pass  over  into  the  lime  and  be  weighed  with  it 
unless  the  solution  is  neutralized  and  then  acidified  with  acetic 
acid.     It  is  better,  however,  to  remove  the  chlorides  with  silver 
nitrate.    The  filtrate  may  be  distilled  directly  or  the  excess  of 
silver  nitrate   removed   with   sodium     hydroxide   or   carbonate. 
Substances  insoluble  in  nitric  acid  are  fused  with  sodium  carbon- 
ate.    If  fluorine  is  present,  it    is  removed  by  precipitation    as 
calcium  fluoride  in  the  water  solution  of  the  fused  material. 

126.  Distillation  of  the  Boric  Acid  with  Methyl  Alcohol. — From 
1  to  2  grams  of  calcium  oxide  are  placed  in  a  platinum  dish  and 
heated  over  the    blast-lamp  until    the  weight    is    constant.     It 
is  then   rinsed  with  a  little  water  into  the  Erlenmeyer   flask. 
A    large  excess    of  the  acetic    or  nitric    acid   used   to  acidify 
the  solution  of  the  borate  should  be  avoided,  since  these  acids 
volatilize  and  neutralize  the  lime.     Their  salts  also  form  viscous 
liquids  on  evaporation.     A  drop  or  two  of  phenolphthalein  should 
therefore  be  added  to  the  solution  and  a  drop  or  two  of  the  acid 
added  after  the  alkaline  color  of  the  indicator  has  been  removed. 
The  smaller  amounts  of  calcium  oxide  may  be  used  when  acetic 
acid  is  employed.     The  solution  of  the  borate  is  transferred  to 
the  retort,  which  is  lowered  so  that  at  first  only  the  bottom  touches 
the  oil  or  parafnne,  which  is  heated  to  130°-140°.     As  the  liquid 
distils  off   the  retort  is   lowered  until  half  of  it  is  immersed  and 
the  distillation  continued  until  the  water  is  entirely  removed. 
10  c.c.  of  methyl  alcohol  is  then  introduced  through  the  dropping- 
funnel  and  the  solution  distilled  to  dryness.      This  is  repeated 
five  times.     If  nitric  acid  has  been  used  to  acidify  the  solution, 
introduce  into  the  retort  2  c.c.  of  water  between  the  second  and 
third  and  between  the  fourth  and  fifth  additions  of  methyl  alco- 

*The  small  amounts  of  these  acids  which  distil  over  into  the  lime  are  ex- 
pelled during  the  subsequent  ignition. 


DETERMINATION  OF  PHOSPHORIC   ACID.  117 

hol  and  distil  to  dryness.      If  acetic  acid  has  been  used,  add  a 
few  drops  of  this  acid  with  the  fourth  portion  of  methyl  alcohol. 

127.  Weighing  the  Boric  Acid.  —  After  the  sixth  distillation 
with  the  methyl  alcohol  transfer  the  solution  hi  the  Erlenmeyer 
flask  to  the  platinum  dish  in  which  the  lime  was  ignited  and 
weighed.    Evaporate  the  solution  to  dryness,  being  careful  to 
avoid  spattering.     Finally  heat  the  dish  with  the  blast-lamp  until 
constant  weight  is  obtained.     The  increase  in  weight  is  due  to  B203. 
The  residue  in  the  retort  may  be  tested  for  boric  acid  by  means 
of  turmeric-paper.     If  nitric  acid  has  been  used  the  nitrous  acid 
should  first  be  oxidized  by  means  of   bromine  and  the  excess  of 
the  latter  expelled. 

DETERMINATION   OF   PHOSPHORIC  ACID. 

128.  Precipitation  of  Phosphoric  Acid  as  Magnesium-arnmon- 
ium    Phosphat3.  —  Phosphoric    acid    may    be    most    accurately 
weighed     as     magnesium     pyrophosphate.     The    properties    of 
magnesium-ammonium  phosphate    as    well    as    the    magnesium 
pyrophosphate    produced    by  ignition  of    this    precipitate,  have 
already  been  discussed  under  the  Determination    of    Magnesium 
in  Chapter  VII,  page  85.       Only  the  alkali  metals  may  be  pres- 
ent in  the  solution,  as  the  phosphates  of  the  other  metals  are  in- 
soluble in  ammonia  and  would  contaminate  the  precipitate.    Only  a 
moderate  amount  of  ammonium  salts  should  be  present,  and  the 
solution   should  be   slightly   acid  with  hydrochloric   acid.     The 
phosphoric  acid  is  precipitated  by  adding  with  constant  stirring 
magnesia  mixture  in  slight  excess.     This  mixture  is  made  by 
dissolving  55  grams  of  crystallized  magnesium  chloride  and  70 
grains  of  ammonium  chloride  in  water,  adding  300  c.c.  of  dilute 
ammonia  or  88  c.c.  of   concentrated  ammonia  (sp.  gr.  0.90)  and 
diluting  to  1  liter.     After  standing  for  several  days,  the  solution 
is  filtered  as  required  for  use.     10  c.c.  of  this  solution  is  sufficient 
to  precipitate  0.15  gram  of  P205.      If    the  phosphate  solution  is 
not  distinctly  alkaline  after  the  addition  of  the  magnesia  mixture, 
it  should  be  made  so  -by  the  addition  of  ammonia.     After  stand- 
ing for  at  least  six  hours  the  precipitate  is  filtered  off,  washed 
with  water  containing  one-tenth  its  volume  of  strong  ammonia, 
and  the  precipitate  ignited  as  directed  for  the  determination  of 
magnesium. 


118  DETERMINATION  OF  ACIDS. 

129.  Separation  of  Phosphoric  Acid  as  Phosphomolybdate. — If 

metals  other  than  the  alkalies  are  present,  the  phosphoric  acid  must 
be  precipitated  from  a  nitric-acid  solution  as  phosphomolybdate. 
All  of  the  phosphoric  acid  is  precipitated  while  the  metals  are 
held  in  solution  by  the  nitric  acid,  in  which  the  phosphorous  pre- 
cipitate is  insoluble.  The  percentage  of  phosphoric  acid  in  the 
precipitate  is  not  constant,  but  varies  within  quite  wide  limits 
with  the  conditions  of  the  precipitation.  Although  by  rigidly 
fixing  these  conditions  a  precipitate  of  sufficiently  constant  com- 
position for  direct  weighing  can  be  obtained,  more  reliable  results 
are  obtained  by  dissolving  the  precipitate  in  ammonia,  repre- 
cipitating  with  magnesia  mixture  and  weighing  as  magnesium 
pyrophosphate. 

130.  Removal  of    Silicic  and  Arsenic  Acids. — Before  precipi- 
tating the  phosphoric  acid  as  molybdate,  silicic  acid  must  be 
removed  from  the  solution  by  evaporating  to  dryness  and  heating 
at  100°  for  some  time.    The  dried  residue  is  dissolved  in  water 
and  nitric  acid.    Arsenic  acid  must  also  be  absent  except  in 
traces,  as  this  acid  forms  a  precipitate  with  molybdic  acid,  which 
is  very  similar  to  the  phosphomolybdate.    It  is  also  reprecipi- 
tated  with  the  magnesia  mixture.     It  may  readily  be  separated 
from  phosphoric  acid  by  passing  hydrogen  sulphide  through  the 
hot  hydrochloric-acid  solution.     As  HYDROCHLORIC  ACID  inter- 
feres with  the  precipitation  of  the  phosphoric  acid  as  molybdate, 
it  must  be  expelled  by  evaporation  with  nitric  acid  after  precipita- 
tion of  the  arsenic,  or  the  separation  of  the  arsenic  acid  may  be 
delayed  until  the  phosphoric  acid  has  been  precipitated  by  means 
of  magnesia  mixture.     This  precipitate,  which  will  generally  con- 
tain only  a  part  of  the  arsenic  acid,  is  dissolved  in  a  little  hydro- 
chloric acid,  the  solution  warmed  to  70°,  and  hydrogen  sulphide 
passed  for  a  considerable  time.    The  precipitate  is  filtered  off 
and  washed  and  the  nitrate  evaporated  to  a  volume  of  50  to  75  c.c. 
The  solution  is  allowed  to  cool,  a  few  drops  of  magnesia  mixture 
are  added,  and  then  ammonia,  with  constant  stirring  until  a  slight 
excess  is  present.     This  precipitate  is  filtered  off,  washed,  ignited, 
and  weighed  as  usual. 

131.  Precautions. — The  precautions  *  to  be  observed  in  pre- 
cipitating the  phosphoric  acid  as  molybdate  are  as  follows: 

*  Zeit.  Anal.  Ch.,  28,  141 . 


DETERMINATION  OF  PHOSPHORIC  ACID.  119 

Hydrochloric,  sulphuric,  and  boric  acids  should  be  absent  or 
present  only  in  small  amounts. 

Free  nitric  acid  should  be  present  only  to  the  extent  of  26 
molecules  to  one  molecule  of  the  phosphoric  acid.  If  more  than 
80  molecules  of  free  nitric  acid  are  present  to  one  molecule  of 
phosphoric  acid,  the  precipitation  is  incomplete. 

The  presence  of  ammonium  nitrate  hastens  the  precipitation. 

Twice  as  much  molybdic  acid  must  be  present  as  is  necessary 
to  form  the  precipitate;  that  is,  24  molecules  to  1  molecule  of 
phosphoric  acid. 

132.  Precipitation   of  the   Phosphomolybdate.  —  The  solution 
of  the  phosphate  should  be  concentrated  so  that  about  0.1  gram 
P205  is  present    in  25  c.c.,  0.2  gram  P205  being   sufficient  for  a 
determination.      Enough   of   a   concentrated   ammonium-nitrate 
solution  (750  gr.  per  liter)  is  added  to  constitute  15%  of  the  total 
volume  after  sufficient  molybdate  solution  has  been  added  to 
have  80  c.c.  present  for  each  0.1  gram  of  P205.    The  solution  is 
then  heated  on  the  water-bath  to  80°-90°  for  about  ten  minutes, 
and  is  then  allowed  to  stand  for  about  one  hour,  when  the  precipi- 
tate may  be  filtered  off  and  washed.    The  addition  of  the  ammo- 
nium nitrate  may  be  omitted,  but  more  of  the  molybdate  solution 
must  be  added,  and  the  solution  heated  on  the  water-bath  to  60° 
for  from  four  to  six  hours.     In  either  case  the  filtrate  should  be 
tested  for  phosphoric  acid  by  adding  more  molybdate  solution 
and  warming.     A  white  precipitate  of  molybdic  acid  must  not  be 
mistaken  for  the  yellow  phosphate  precipitate. 

133.  Washing  the    Precipitate. — Various  solutions  have  been 
used  for  washing  the  precipitate,  which  is  not  absolutely  insoluble, 
so  that  the  minimum   amount   of   wash-water  should  be  used. 
Wagner  recommends  a  solution  made  by  dissolving  150  grams  of 
ammonium  nitrate  in  water,  adding  10  c.c.  concentrated  nitric 
acid,  and  diluting  the  solution  to  one  liter.    Fresenius  recom- 
mends the  use  of  a  solution  made  by  adding  to  100  c.c.  molybdate 
solution  20  c.c.  nitric  acid  (sp.  gr.  1.2)  and  80  c.c.  water.     A  solu- 
tion made  by  diluting  the  molybdate  solution  with  an  equal  vol- 
ume of  water  is  also  frequently  used. 

134.  Direct  Weighing  of  the  Phosphomolybdate.  —  When  it  is 
desired  to  weigh  the  molybdate  precipitate,  the  following  slight 
variations  in  the  method  of  procedure  already  given  must  be  ob- 


120  DETERMINATION  OF  ACIDS. 

served:  Silica  must  first  be  completely  removed  from  the  solution 
of  the  phosphoric  acid.  After  the  molybdate  has  been  added, 
the  solution  should  be  heated  to  about  40°  to  effect  complete 
precipitation.  Serious  error  results  if  it  is  heated  to  a  tempera- 
ture near  100°.  About  four  hours'  digestion  at  40°  is  generally 
sufficient.  The  precipitate  is  filtered  off  on  a  Gooch  crucible  and 
washed,  first  with  a  dilute  solution  of  molybdate  until  the  metals 
present  are  removed,  and  then  with  water  containing  1%  of  nitric 
acid  until  the  molybdic  acid  is  removed.  The  precipitate  is 
then  dried  in  an  air-bath  heated  to  120°.  It  contains  1.63%  of 
phosphorus. 

135.  Reprecipitation  of  the  Phosphoric  Acid  as  Ammonium- 
magnesium  Phosphate. — When  it  is  desired  to  weigh  the  phos- 
phorus as  magnesium  pyrophosphate,  the  molybdate  precipitate 
is  washed  with  the  solutions  given  in  par.  133  until  the  metals 
present  are  removed.  The  beaker  in  which  the  precipitation  was 
made,  and  from  which  the  yellow  precipitate  need  not  be  wholly 
removed,  is  placed  under  the  funnel,  the  paper  is  pierced,  and  the 
precipitate  washed  into  the  beaker  with  a  fine  jet  of  water.  The 
paper  is  moistened  with  a  little  ammonia  and  washed  with  hot 
water,  a  few  drops  of  ammonia  being  added  if  necessary  to  dis- 
solve the  precipitate.  Not  more  than  50  c.c.  of  wash-water  should 
be  used.  If  the  precipitate  does  not  dissolve  on  stirring  the  solu- 
tion, a  little  more  ammonia  should  be  added.  Neutralize  the 
solution  with  strong  hydrochloric  acid,  and  if  the  yellow  phos- 
phomolybdate  begins  to  precipitate,  add  ammonia  until  dis- 
solved. A  white  flocculent  precipitate  insoluble  in  ammonia  is 
probably  silica,  and  should  be  filtered  off.  The  solution  is  again 
rendered  slightly  acid  with  hydrochloric  acid  and  enough  mag- 
nesia mixture  added  to  precipitate  the  phosphoric  acid  present, 
using  10  c.c.  for  0.1  gram  P205.  The  solution  is  neutralized  by 
adding  ammonia  while  stirring  constantly.  A  moderate  excess  of 
ammonia  is  added  and  the  solution  is  allowed  to  stand  for  at  least 
four  hours.  The  precipitate  is  filtered  off,  washed  with  dilute 
ammonia,  ignited,  and  weighed  as  usual.  Heating  with  the 
blast-lamp  for  ten  minutes  is  advisable,  in  order  to  expel  any 
molybdic  acid  which  may  have  been  carried  down  with  the  mag- 
nesia precipitate. 


ANALYSIS  OF  ALLOYS.* 

CHAPTER  X. 

ALLOYS  OF  SILVER,  LEAD,   COPPER,  BISMUTH, 
CADMIUM,  AND  TIN. 

136.  Reasons  for  Analysis  of  Pure  Salts. — In  the  determina- 
tions  given   in   the    preceding   chapters    pure    compounds   were 
taken    for    analysis.      This    method    of    procedure    serves    two 
purposes.      In    the    first    place    the    exact    composition    of    the 
compounds    being  known,   the   error    in   a  given    determination 
can  easily  be  ascertained.     In  the  second  place  no    error    can 
arise    from    the    contamination    of    a    precipitate    with    foreign 
material  other  than  the  reagents  used.     When  the  student  has 
acquired  proficiency  in   the   comparatively  simple  processes   of 
analyzing  pure  compounds,  he  must  undertake  the  determination 
of  elements  when  existing  in  combination  with  each  other.     It 
is  evident  that  the  determination  of  iron  by  precipitation  with 
ammonia  is  impracticable  if  aluminium  or  chromic  salts  are  present. 

137.  Difficulty  of  Complete  Separation  of  Elements. — Although 
calcium  is  the  only  metal  which  can  be  quantitatively  precipitated 
as  oxalate,  yet  it  is  by  no  means  true  that  this  element  can  be 
separated  as  oxalate  from  a  solution  containing  all  other  metals 
without  carrying  many  of  these  down  with  it.     This  is  due  to 
the  fact  that  most  of  the  oxalates  are  soluble  with  difficulty. 
All  of  the  sulphates  except  those  of  barium,  strontium,  and  lead 
are  quite  soluble,  and  yet  when  such  soluble  sulphates  as  those  of 
iron  or  aluminium  are  present  the  determination  of  sulphuric 
acid  by  precipitation  with  barium  chloride  presents  difficulties 

*  The  alloys  and  minerals  whose  analyses  are  given  in  this  and  the  following 
chapters  have  been  selected  so  as  to  afford  practice  in  the  separations  required 
in  the  analysis  of  commonly  occurring  substances.  The  elements  mentioned  as 
occurring  in  a  given  alloy  or  mineral  are  those  commonly  found.  Some  of  these 
are  at  times  absent,  while  others  not  listed  may  be  present. 

121 


122  ANALYSIS  OF  ALLOYS. 

which  have  been  the  subject  of  investigation  by  many  ingenious 
workers  for  more  than  half  a  century.  As  the  great  majority  of 
the  substances  with  which  the  chemist  is  called  upon  to  deal  are 
complex  rather  than  simple,  a  careful  study  of  the  separation  of 
the  elements  is  of  the  greatest  importance.  Only  by  the  closest 
attention  to  details  can  success  be  attained  hi  the  analysis  of 
complex  substances.  The  importance  of  testing  precipitates  for 
impurities  and  the  solution  for  unprecipitated  portions  of  an 
element  cannot  be  too  strongly  urged.  Only  in  this  manner  can 
the  accuracy  of  an  analysis  be  assured. 

138.  Limit  of  Accuracy  in  Analysis. — If  a  complete  analysis 
is  made  the  sum  of  all  the  constituents  must  be  very  close  to 
100%.  A  summation  which  is  within  .5%  can  generally  be  ob- 
tained if  the  analysis  is  conducted  with  care  and  reliable  methods 
are  used.  In  general  the  analysis  of  an  unknown  substance 
should  be  conducted  in  duplicate.  If  the  duplicate  results  do 
not  agree  within  .2  or  at  most  .3%,  a  third  analysis  should  be 
made.  As  the  error  of  most  determinations  is  at  least  .1%,  it  is 
unnecessary  to  calculate  results  to  more  than  hundredths  of  per 
cent.  As  the  error  in  each  determination  of  the  analysis  of  a 
given  substance  may  be  either  plus  or  minus,  the  practice  of 
dividing  the  difference  between  the  summation  and  100$  among 
the  various  determinations  is  not  justifiable. 

It  is  in  some  cases  possible  to  analyze  a  substance  in  such  a 
manner  that  the  results  are  accurate  to  the  hundredth  of  a  per 
cent.  Such  results  may  be  computed  to  the  .001  of  a  per  cent. 
This  practice  is  common  in  the  analysis  of  metals.  Large  quan- 
tities of  the  metal  are  taken,  so  that  considerable  quantities  of 
the  impurities  which  are  present  in  small  amounts  are  obtained 
for  determination.  The  results  may  then  be  accurate  to  the  hun- 
dredth of  a  per  cent.  This  does  not  imply  a  higher  degree  of 
accuracy  in  the  determination  of  a  given  element  than  .1  of  a  per 
cent.  For  example;  if  iron  were  present  in  copper  to  the  extent 
of  .5%,  a  determination  of  the  iron  which  is  accurate  to  .01%  of 
the  impure  copper  would  represent  an  error  of  ^  of  the  amount 
of  iron  present  in  the  copper.  In  giving  the  results  of  such  analy- 
ses the  percentage  of  the  main  constituent  is  obtained  by  difference, 
so  that  the  summation  is  exactly  100%. 


SEPARATION  OF  SILVER.  123 

139.  Separation  of  Silver  from  Other  Metals. — Silver  is  readily 
separated  from  other  metals  by  precipitation  as  chloride.    Silver 
chloride  being  more  insoluble  in  dilute  nitric  acid  than  in  water 
the  precipitate  is  washed  with  dilute  nitric  acid.    This  also  serves 
to  dissolve  compounds  of  metals  such  as  bismuth  and  antimony, 
which  tend  to  form  insoluble  basic  salts.     As   silver  chloride  is 
soluble  in  mercuric  nitrate  enough  hydrochloric  acid  must  be  added 
to  convert  all  of  the  mercury  present  into  chloride.     This  metal 
must  be  present  in  the  mercuric  condition.     The  small  amount 
of  silver  remaining  in  solution  is  subsequently  precipitated  with 
the    mercury  as  sulphide  and  may  be  separated  by  volatilization 
of  the  mercury.     If  lead  is  present  the  solution  must  be  dilute 
and  hot,  and  no  more  hydrochloric  acid  added  than  just  sufficient 
to  precipitate  the  silver.     The  silver  chloride  must  be  thoroughly 
washed  with  hot  water.     If  sodium  acetate  is  added  the  lead  is 
more  easily  kept  in  solution. 

EXERCISE  25. 
Analysis  of  a  Silver  Coin. 

Alloy  of  Copper  and  Silver. 

Prepare  two  Gooch  crucibles  for  weighing  silver  chloride.  They  may 
be  marked  1  and  2  with  a  blue  pencil  *  and  the  marks  made  permanent  by 
heating  to  redness  with  the  blast-lamp.  While  the  crucibles  are  drying 
on  the  hot  plate  the  following  operations  should  be  carried  out: 

140.  Dissolving  the  Alloy. — Clean  a   dime   thoroughly  by  rubbing  with 
sand  or  "  Sapolio."     Cut  into  pieces  and  weigh  carefully  a  piece  of  about  one- 
half  gram.     Place  in  a  200-c.c.  beaker  and  add  about  5  c.c.  concentrated 
nitric  acid  and  an  equal  bulk  of  water.     Cover  the  beaker  with  a  watch- 
crystal  and  warm  on  the  water-bath  until  the  metal  is  converted  into  nitrate. 
Dissolve  the  residue  in  water.     A  little  gold  is  sometimes  present  and  will 
remain  undissolved  at  this  point.     The  solution  of  silver  and  copper  is 
decanted  into  a  300-c.c.  Erlenmeyer  flask  and  the  gold  washed  by  decanta- 
tion  five  or  six  times,  using  about  25  c.c.  of  water  each  time.     The  gold  is 
now  transferred  to  a  small  filter-paper,  which  is  washed  with  hot  water 
until  free  from  silver.     The  moist  paper  is  transferred  to  a  weighed  porcelain 
crucible,  burned  in  the  usual  manner,  and  the  gold  weighed.  As  the  amount  of 
gold  is  always  small,  it  is  best  to  collect  on  the  same  paper  the  gold  from  all  of 
the  weighed  portions  of  the  dime,  which  are  dissolved  separately  in  nitric  acid 
to  obtain  duplicate  determinations  of  the  silver  and  copper.     The  paper  is 
washed  free  from  silver  after  each  precipitate  has  been  transferred  to  it. 

*  A  pencil  marie  for  marking  on  glass,  porcelain,  etc  ,  is  used  for  this  purpose 


124  ANALYSIS  OF  ALLOYS. 

141.  Silver. — The  volumes  of   the  solutions  of  silver  and  copper  in  the 
Erlenmeyer  flasks  should  not  much  exceed  200  c.c.     By  proper  manipula- 
tion this  result  can  easily  be  secured.     Heat  nearly  to  boiling  and  precipi- 
tate the  silver  by  adding  hydrochloric  acid,  drop  by  drop,  with  vigorous 
shaking  of  the  flask.     Add  only  a  slight  excess  of  hydrochloric  acid  and 
digest  the  precipitate  on  the  water-bath  with  occasional  vigorous  shaking 
until  the  solution  is  very  nearly  clear.     Decant  the  clear  liquid  through  the 
weighed  Gooch  crucible  and  wash  the  precipitate  two  or  three  times  by 
decantation  with  about  50  c.c.  of  hot  water  to  which  a  few  drops  of  nitric 
acid  have  been  added.     The  precipitate  should  be  digested  each  time  for  a 
few  minutes  on  the  water-bath.      Finally  transfer  the  precipitate  to  the 
crucible  and  wash  with  hot  water  containing  a  little  nitric  acid  until  the 
wash-water  gives  no  precipitate  with  hydrogen  sulphide.      Dry  the  pre- 
cipitate on  the  hot  plate  and  weigh. 

142.  Copper. — Ignite   and   weigh   a   Rose   crucible.      Heat   the   copper 
solution,  the  volume  of  which  should  be  about  400  c.c.,  nearly  to  boiling  and 
saturate    with    hydrogen    sulphide.     Filter   immediately   and    wash    with 
water  containing  hydrogen  sulphide.     Test  the  filtrate   and   wash -water 
for  copper  by  passing  hydrogen  sulphide.     Dry  the  precipitate,  detach 
from  the  paper,  burn  the  latter,  and  ignite  the  precipitate  in  the  Rose  cruci- 
ble in  a  stream  of  dry  hydrogen  after  the  addition  of  sulphur. 

143.  Division  of  Coin   Volumetrically. — If   there  is   available   a   250-c.c. 
flask  and  a  50-c.c.  pipette  which  have  been  carefully  calibrated  against 
each  other,  the  division  and  weighing  of  the  coin  may  be  carried  out  in  the 
following  manner.     The  coin  is  weighed  after  thorough  cleansing.     It  is 
then  placed  in  a  200-c.c.  beaker  and  treated  with  10  c.c.  concentrated  nitric 
acid  and  an  equal  bulk  of  water.     The 'beaker  is  covered  with  a  watch- 
crystal  and  warmed  on  the  water-bath  until  the  metal  is  converted  into 
nitrate.     The  watch-crystal  is  rinsed  off  and  the  solution  is  evaporated  to 
dryness  on  the  water-bath.     The  residue  is  dissolved  in  water,  the  gold,  if 
present,  filtered  off  and  weighed,  the  filtrate  and  washings  being  allowed 
to  flow  into  the  250-c.c.  flask.    Finally  the  solution  in  the  flask  is  diluted 
to  the  mark  with  water,  thoroughly  shaken,  and  50-c.c.  portions  taken  out 
with  the  pipette.     The  analysis  of  these  portions  is  carried  out  as  already 
directed. 

The  duplicate  determinations  of  silver  and  copper  should  agree  within 
about  .2%.  American  silver  coins  contain  very  nearly  90%  of  silver  and 
10%  of  copper. 

144.  Economizing    Time. — In    carrying    out    this    and   subse' 
quent  exercises  the  work  should  be  carefully  planned  to  econo- 
mize time.     Crucibles  should  be  prepared  and  weighed  during 
the  intervals  when  precipitates  are  digesting,  filtering,  or  drying, 
so  that  the  latter  may  be  transferred  and  weighed  without  delay. 


SEPARATION  OF  TIN.  125 

Two  precipitations  and  nitrations  can  be  carried  out  at  the  same 
time  just  as  readily  as  one.  As  experience  is  gained,  a  greater 
and  greater  number  of  analyses  can  be  carried  on  simultaneously. 
Careful  and  systematic  marking  of  beakers,  funnels,  etc.,  is  abso- 
lutely necessary.  All  weights  must  be  recorded  in  the  note-book 
under  systematic  headings  and  numbering  of  duplicate  analyses. 

145.  Separation  of  Tin  as  Stannic  Oxide. — Tin  is  usually  sepa- 
rated from  other  metals  as  the  dioxide  Sn02.      On  treating  an 
alloy  containing  tin  with  nitric  acid,  most  of  the  metals  form 
nitrates,  while  the  tin  is  converted  into  the  dioxide,  which  remains 
undissolved  on  treating  the  residue  with  water.     If  ANTIMONY  or 
ARSENIC  is  present  more  or  less  of  these  elements,  depending  on 
the  amount  present,  will  remain  with  the  tin.     Besides  these  ele- 
ments, which  one  would  expect  from  the  nature  of  their  oxides  to 
remain  undissolved  by  water  or  dilute  acids,  small  portions  of 
copper,  lead,  bismuth,  iron,  manganese,  and  zinc  remain  with  the  stannic 
oxide  and  cannot  be   removed  by  digestion  with  dilute  nitric 
acid.     In  the  absence  of  more  than  traces  of  arsenic  and  antimony 
the  stannic  oxide  is  filtered  from  the  nitric-acid  solution,  washed 
with  dilute  nitric  acid,  ignited  and  weighed. 

146.  Purification  of  the  Stannic  Oxide. — The  weighed  precip- 
itate   is    then    fused  in    a   porcelain  crucible  with  six  times  its 
weight  of  equal  parts  of  sodium  carbonate  and  sulphur,  or  with 
the  same  weight  of  dry  sodium  thiosulphate.     By  this  treatment 
the  metals  present  are  converted   into   sulphides.     On  treating 
the  fused  mass  with  hot  water  the  sulphides  of  tin,  arsenic,  and 
antimony  and  a  small  amount  of  copper  if  this  element  is  present 
dissolve   as   thio  salts    of  sodium,    while   the   sulphides    of    the 
other   metals   remain   and    may   be   filtered   off.        On    treating 
the  insoluble  sulphides  with  warm  dilute  hydrochloric  acid,  cop- 
per sulphide  remains  undissolved  and  may  be  ignited  and  weighed 
as  CuO.     In  the  filtrate  the  lead  may  be  precipitated  with  sul- 
phuric acid  and  weighed  as  sulphate.     The  iron  is  precipitated 
with  ammonia,  while  the  zinc  is  separated  from  the  manganese 
by  passing  hydrogen  sulphide  through  the  slightly  acid  solution. 
The  manganese  may  be  precipitated  with  hydrogen  peroxide  and 
weighed  as  Mn304.     The  weight  of  any  or  all  of  these  metals 
computed  as  oxide  is  deducted  from  the  weight  of  the  impure 
stannic  oxide. 


126  ANALYSIS  OF  ALLOYS. 

If  several  of  these  metals  are  present  in  the  alloy  it  is  perhaps 
easier  and  fully  as  accurate  to  omit  weighing  the  impure  stannic 
oxide,  but  fuse  it  immediately  as  already  described  for  its  purifi- 
cation. The  insoluble  sulphides  are  dissolved  in  a  little  nitric 
acid  and  added  to  the  nitric  acid  solution  of  the  alloy.  The  solu- 
tion of  sodium  sulphostannate  is  nearly  decolorized  by  boiling 
with  the  addition  of  pure  caustic  soda  and  hydrogen  peroxide. 
The  tin  is  precipitated  as  sulphide  by  acidifying  with  hydro- 
chloric acid.  The  precipitate  is  filtered  off  and  washed  with 
water  containing  ammonium  acetate  and  a  little  acetic  acid.  It 
is  then  treated  with  a  little  concentrated  ammonium  carbonate 
solution  to  dissolve  arsenic  sulphide.  After  washing  with  water  the 
sulphides  of  tin  and  antimony  are  dissolved  in  warm  concentrated 
hydrochloric  acid.  Any  antimony  which  is  present  may  be  pre- 
cipitated by  digesting  the  diluted  solution  with  metallic  iron.  The 
tin  in  the  filtrate  from  the  antimony  is  precipitated  with  hydrogen 
sulphide  and  washed.  The  precipitate  is  dried  and  detached 
from  the  paper,  which  is  burned.  It  is  then  placed  in  a  porcelain 
crucible  and  gently  heated  until  sulphur  dioxide  ceases  to  be 
given  off.  It  is  then  strongly  heated  after  the  addition  of  ammo- 
nium carbonate  and  weighed  as  stannic  oxide,  Sn02 

147.  Separation  of  Lead  as  Sulphate. — Lead  is  most  frequently 
separated  from  other  metals  by  precipitation  as  sulphate.     The 
nitric  acid  solution  is  evaporated  with  excess  of  sulphuric  acid 
until  the  nitric  acid  is  completely  expelled,  which  is  indicated  by 
the  evolution  of  dense  white  fumes  of  sulphuric  acid.     On  dilu- 
tion with  water  the  lead  separates  out  almost  completely.     In 
the  absence  of  much  bismuth  or  iron,  a  more  complete  precipita- 
tion of  the  lead  may  be  effected  by  the  addition  of  an  equal  bulk 
of  alcohol  to  the  dilute  sulphuric  acid  solution.     Besides  BARIUM 
and  STRONTIUM  sulphates  which  may  contaminate  the  precipitate, 
BISMUTH  may  be  present  as  an  insoluble  basic  sulphate.     To  keep 
the  bismuth  in  solution  considerable  sulphuric  acid  must  be  present 
and  the  precipitate  must  be  washed  with  dilute  (10%;  sulphuric 
acid,  which  is  finally  removed  by  washing  with  dilute  alcohol 

148.  Separation    of    Lead    as    Chloride    from    Bismuth.  —  The 
precipitate  of  lead  sulphate  may  be  tested  for  bismuth  by  warming 
with   concentrated   hydrochloric    acid.     After   allowing   to    cool, 


SEPARATION  OF  LEAD.  127 

50  c.c.  absolute  alcohol  is  added  to  the  solution  of  lead  chloride, 
the  volume  of  which  should  be  about  5  c.c.  The  bismuth  chlo- 
ride dissolves  in  the  alcohol  and  may  be  filtered  off  from  the  pre- 
cipitated lead  chloride.  A  small  amount  of  the  lead  dissolves 
in  the  alcohol  with  the  bismuth.  The  latter  may  be  precipitated 
by  largely  diluting  the  alcoholic  solution.  This  method  may 
also  be  used  for  separating  the  lead  and  bismuth  in  the  nitric- 
acid  solution  of  lead  and  bismuth  alloys.  Before  the  addition  of 
the  alcohol  the  nitric  acid  is  expelled  by  evaporating  twice  with 
20  c.c.  concentrated  hydrochloric  acid.  The  lead  chloride  may 
be  collected  on  a  Gooch  crucible,  washed  with  alcohol,  and  dried 
for  three  hours  at  150°  and  weighed.  About  1  mg.  of  lead  chloride 
remains  in  the  alcoholic  solution.  The  bismuth  is  precipitated 
as  BiOCl  by  largely  diluting  the  alcoholic  solution  and  neutralizing 
most  of  the  hydrochloric  a3i.l  with  ammonia  if  a  large  excess  is 
present.  It  may  be  washed  with  water  containing  a  few  drops  of 
hydrochloric  acid,  dried  at  110°  and  weighed. 

149.  Separation  of  Lead  from  Antimony  and  Barium. — If  con- 
siderable antimony  is  present  the  alloy  is  dissolved  in  a  rather 
small  amount  of  nitric  acid  (4  c.c.)  with  the  addition  of  tartaric 
acid  (10  grams)  and  water  (5  c.c.).  The  lead  is  precipitated  by 
the  addition  of  4  c.c.  concentrated  sulphuric  acid  and  diluting 
the  solution  to  about  250  c.c. 

If  barium  is  present,  as  when  minerals  are  analyzed,  so  that 
the  sulphate  of  lead  is  contaminated  with  barium  sulphate  a 
separation  may  be  effected  by  digesting  the  precipitate  with 
ammonium  carbonate  solution.  The  sulphate  of  lead  is  converted 
into  carbonate,  while  the  barium  sulphate  is  unaffected.  The 
precipitate  is  filtered  off  and  washed  with  ammonium  carbonate 
solution,  then  with  water  until  the  wash-water  no  longer  con- 
tains sulphates.  The  lead  carbonate  is  then  dissolved  with  dilute 
acetic  or  nitric  acid.  In  this  method  of  separation  a  little  lead  is 
lost  on  account  of  the  slight  solubility  of  lead  carbonate  in  solu- 
tions of  ammonium  salts.  It  may  be  recovered  by  passing 
hydrogen  sulphide  through  the  ammonium  carbonate  solution. 


128  ANALYSIS  OF  ALLOYS. 

EXERCISE  26. 
Analysis  of  Soft  Solder. 

AUoy  of  Lead  and  Tin,  Generally  containing  Small  Amounts  of  Arsenic^ 
Antimony,  Iron,  and  Zinc. 

150.  Solution  of  the  Alloy. — One  gram  of  the  alloy  is  weighed  out  and 
transferred  to  a  beaker  of  about  500  c.c.  capacity.     10  c.c.  of  concentrated 
nitric  acid  and  5  c.c.  of  water  are  added.     The  beaker  is  covered  with  a 
watch-crystal  and  heated  on  the  water-bath  until  the  alloy  is  completely 
decomposed  and  the  nitrous  fumes  are  entirely  expelled.     100  c.c.  of  water 
is  added  and  the  solution  boiled  for  five  minutes  and  allowed  to  settle  for 
one  hour.     The  stannic  oxide  is  filtered  off  and  washed  with  hot  water. 
The  moist  precipitate  may  be  introduced  into  a  weighed  porcelain  crucible, 
the  paper  burned  in  the  usual  manner,  and  finally  heated  to  redness  for  ten 
minutes. 

151.  Tin. — When  the  precipitate  has  been  brought  to  constant  weight, 
it  is  fused  with  six  times  its  weight  of  a  mixture  of  equal  parts  of  sulphur 
and  sodium  carbonate.     The  fused  mass  is  dissolved  in  hot  water  and  the 
solution  filtered.     The  insoluble  sulphides  are  washed  with  hot  water  and 
treated  with  a  little  dilute  hydrochloric  acid  and  the  paper  washed  with  water. 
If  COPPER  is  present  it  will  remain  on  the  paper  and  the  small  amount 
present  may  be  weighed  as  CuO  after  burning  the  paper  in  a  porcelain  cruci- 
ble and  igniting  the  precipitate.     The  LEAD  is  precipitated  by  the  addition 
of  a  few  drops  of  sulphuric  acid  and  25  c.c.  of  alcohol  to  the  solution,  which 
should  not  exceed  50  c.c.     After  standing  one  hour,  the  precipitate  is 
filtered  off  on  a  Gooch  crucible,  washed  with  alcohol,  dried  on  the  hot  plate, 
and  weighed.     The  filtrate  is  evaporated  until  the  alcohol  is  completely 
expelled.     Any  IRON  present  is  precipitated  with  ammonia  and  weighed. 
Hydrogen  sulphide  is  passed  through  the  filtrate  to  precipitate  any  ZINC 
present,  which  is  filtered  off.     The  filtrate  from  the  insoluble  sulphides 
will  contain  the  tin  as  a  thiostannate  and  part  of  the  ANTIMONY  present 
in  the  alloy  as  a  thioantimonate.     The  solution  is  boiled  after  the  addi- 
tion of  caustic  soda  and  hydrogen  peroxide  until  it  is  nearly  decolorized. 
On  acidifying  and  passing  hydrogen  sulphide  both  metals  are  precipi- 
tated as  sulphides.    If  antimony  is  present  the  metals  should  be  separated 
by  the  method  given  in  section  178,  page  141.     The  weight  of  the  im- 
purities found,  computed  as  oxides,  is  deducted  from  the  weight  of  the 
stannic  oxide. 

152.  Lead. — The  filtrate  from  the  stannic  oxide  is  transferred  to  a  porce- 
lain dish,  5  c.c.  concentrated  sulphuric  acid  added,  and  evaporated  until 
fumes  of  sulphuric  acid  are  evolved.  Cool  the  dish  by  floating  it  in  cold 
water  and  add  cautiously  75  c.c.  of  water.  Stir  thoroughly  and  add  25  c.c. 
of  alcohol.  Allow  the  solution  to  stand  for  at  least  one  hour,  filter  off  the 
lead  sulphate  on  a  weighed  Gooch  crucible,  wash  with  alcohol  until  free 
from  acid,  dry  on  the  hot  plate,  and  weigh. 


ANALYSIS  OF  ROSE'S  METAL.  129 

153.  Arsenic  and  Antimony. — The  alcohol  is  completely  expelled  from 
the  filtrate  by  evaporation  and  any  arsenic  present  precipitated  by  passing 
hydrogen  sulphide.     If  this  precipitate  is  of  an  orange  color  instead  of  pure 
yellow,  antimony  is  present.     It  should  be  filtered  off  and  washed  with 
water  containing  a  little  hydrochloric  acid  until  free  from  iron  and  hydrogen 
sulphide.     It  is  then  washed  with  small  portions  of  concentrated  ammo- 
nium  carbonate  solution  until  the  arsenic  sulphide  is  entirely  dissolved. 
The  arsenic  is  reprecipitated  by  acidifying  the  solution  with  hydrochloric 
acid  and  passing  hydrogen  sulphide.     It  is  filtered  off  on  a  Gooch  crucible 
and  washed  with  water  containing  hydrogen  sulphide  and  a  little  hydro- 
chloric   acid.      The  water    is    removed   by   alcohol    and    the  precipitate 
digested  with  carbon  disulphide  until  sulphur  is  entirely  removed.     The 
arsenic  sulphide  is  dried  at  100°  and  weighed.     If  antimony  is  absent  the 
treatment  with  ammonium  carbonate   is   omitted,  the  precipitate   being 
filtered  off  on  a  Gooch  crucible,  washed,  dried,  and  weighed.    If  antimony 
is  present  it  is  ignited  and  weighed  as  directed  in  section  179,  page  141. 

154.  Iron. — A  few  drops  of  bromine  water  are  added  and  the  solution  is 
boiled  to  oxidize  the  iron  and  to  expel  the  hydrogen  sulphide.    The  iron  is 
then  precipitated  by   making  the  filtrate  alkaline   with   filtered   ammonia 
and  warming   for  a   few   minutes.       It   is   filtered   off  on   a  small  paper 
and   dissolved  by  adding  a  few  drops  of  dilute  hydrochloric  acid.     The 
paper  is   washed  with  about   75   c.  c.  of  water  in  small  portions.       The 
iron  is  reprecipitated  and  filtered  on  the  same  paper  after  moistening 
with  a  few  drops  of  ammonia.     After  washing  free  from  chlorides,  the 
moist  paper  is  transferred  to  the  weighed  platinum  crucible  and  ignited. 

155.  Zinc. — Hydrogen  sulphide  is  passed  into  the  combined  filtrates  to 
precipitate  any  zinc  present,  which  is  filtered  off,  washed,  and  weighed  as 
sulohide. 

EXERCISE  27. 

Analysis  of  Rose's  Metal. 

Alloy  of  Lead}  Bismuth,  and  Tin,  Generally  containing  Small  Amounts  of 
Copper,  Arsenic,  Antimony,  Iron,  and  Zinc. 

One  gram  of  the  metal  is  weighed  out  and  decomposed  with  nitric 
acid  as  directed  in  Exercise  26.  The  STANNIC  OXIDE  is  weighed  and  the 
impurities  determined  as  directed  in  the  same  exercise. 

156.  Lead. — To   the   filtrate  from  the  stannic  oxide  containing  the  ni- 
trates of   lead  and  bismuth,  5  c.c.  concentrated  sulphuric  acid  is  added. 
The  solution  is  evaporated  in  a  porcelain  dish  until  sulphuric-acid  fumes  are 
given  off.     The  dish  may  be  placed  on  the  hot  plate,  sand-bath,  or  wire 
gauze  and  the  liquid  heated  to  just  below  the  boiling-point  to  avoid  spatter- 
ing.   When  the  acid  becomes  concentrated,  the  heat  may  be  somewhat 
increased.    The  hot  concentrated  solution  is  diluted  by  slowly  pouring  it  with 
constant  stirring  into  about  100  c.c.  of  water  and  digested  hot  for  about  half 
an  hour  with  occasional  stirring.     The  lead  sulphate  is  then  filtered  off  on  a 


130  ANALYSIS  OF  ALLOYS. 

Gooch  crucible,  washed  with  10%  sulphuric  acid  until  the  wash-wafer  no  longer 
gives  a  precipitate  on  making  it  alkaline  with  ammonia,  adding  ammonium 
carbonate,  and  warming.  The  sulphuric  acid  is  then  washed  out  with  alcohol. 
The  precipitate  is  dried  and  weighed.  It  is  tested  fora  possible  contamination 
with  bismuth  as  follows :  It  is  dissolved  in  5  to  10  c.c.  of  warm  concentrated 
hydrochloric  acid  and  50  c.  c.  of  absolute  alcohol  are  added  to  the  solution. 
After  standing  for  a  few  moments,  the  solution,  containing  the  bismuth  as 
chloride,  is  filtered  off.  By  nearly  neutralizing  with  ammonia,  and  largely 
diluting  with  water,  the  bismuth  is  precipitated  as  oxychloride  and  may  be 
washed  with  water  containing  a  few  drops  of  hydrochloric  acid,  dried,  and 
weighed. 

157.  Bismuth. — In  the  filtrate  from  the  lead  sulphate,  the  bismuth  is 
precipitated  by  just  neutralizing  with  filtered  ammonia,  adding  a  few  drops 
of  ammonium  carbonate,  and  warming  the  solution  gently  for  about  fifteen 
minutes.     The  precipitate  is  filtered  off  and  washed  a  few  times  with  water. 
To  free  the  precipitate  from  a  small  amount  of  basic  sulphate  it  is  dissolved 
in  a  small  amount  of  dilute  nitric  acid  and  reprecipitated.    The  precipitate 
is  washed  with  water  containing  a  little  ammonium  nitrate  and  dried.  It 
is  removed  from  the  paper  as  completely  as  possible  and  placed  on  a 
watch-crystal.     The  paper  is  replaced  in  the  funnel,  moistened  with  a 
few  drops  of  dilute  nitric  acid,  and  washed  with  small  amounts  of  warm 
water.    The  wash-water  is  evaporated  to  dryness  in  a  fairly  large  weighed 
porcelain  crucible,  and  the  residue  ignited  until  the  nitric  acid  is  com- 
pletely expelled.     The  main  portion  of  the  precipitate  is  now  added, 
heated  with  the  Bunsen  burner  and  weighed  as  Bi,,O.r 

158.  Copper. — If  copper  is  present  in  the  alloy,  it  will  be   contained  in 
the  two  filtrates  from  the  bismuth  precipitate.     Combine  these  nitrates, 
acidify  with  hydrochloric  acid,  and  concentrate  to  a  convenient  bulk.     Pass 
hydrogen  sulphide  through  the  warm  solution,  filter,  and  wash  with  water 
containing  hydrogen  sulphide.     Even  if  copper  is  absent,  a  small  black 
precipitate  of  bismuth  sulphide  will  be  obtained  at  this  point  because  of  the 
slight  solubility  of  the  bismuth  hydroxide  or  carbonate.     The  precipitate 
may  be  tested  for  bismuth  by  treating  with  a  little  dilute  hydrochloric  acid 
and  diluting  the  filtrate.      A  white  precipitate  indicates  bismuth.      The 
copper  sulphide  being  insoluble  in  dilute   hydrochloric  acid  remains  on 
the  paper  and  may  be  ignited  together  with  the  paper  and  weighed  as 
oxide.     If  ARSENIC,  ANTIMONY,  IRON,  or  ZINC  are  present  they  are  separated 
and  determined  by  the  methods  given  in  Exercises  26  or  28. 

159.  Separation  of  Copper  and  Cadmium. — Cadmium  may  be 
separated  from  copper  by  passing  hydrogen  sulphide  through  or 
adding  ammonium  sulphide  to  a  solution  of  the  two  metals  in 
potassium  cyanide.     In  such  a  solution  copper  sulphide  is  soluble, 
while  cadmium  sulphide  is  not.     Silver,  bismuth,  and  lead  must 
be  absent,  as  these  metals  would  be  precipitated  with  the  cad- 


ANALYSIS  OF  WOOD'S  METAL.  131 

mium.  Cadmium  may  also  be  separated  electrolytically  from 
copper.  The  latter  metal  may  be  deposited  from  a  solution  con- 
taining 5%  of  acid  by  a  current  having  a  tension  not  exceeding 
1.85  volts.  Under  these  conditions  cadmium  is  not  precipitated. 

EXERCISE  28. 
Analysis  of  Wood's  Metal. 

Alloy  of  Lead,  Bismuth,  Tin,  and  Cadmium,  Generally  containing  Small 
Amounts  of  Copper,  Arsenic,  Antimony,  Iron,  and  Zinc. 

One  gram  of  the  metal  is  weighed,  dissolved  in  nitric  acid,  and  the  STANNIC 
OXIDE  weighed  and  purified  by  the  method  given  in  Exercise  26.  The 
filtrate  from  the  tin  is  evaporated  to  dryness  on  a  water-bath.  The  nitrates 
are  converted  into  chlorides  by  evaporating  twice  on  the  water-bath  to  a 
small  bulk  after  the  addition  of  20  c.c.  of  concentrated  hydrochloric  acid. 

160.  Lead. —  After    coding,   25  c.c.    absolute   alcohol  are  added.     The 
mixture  is  stirred  and  after  standing  some  time  the  chloride  of  lead  is  fil- 
tered off  on  a  Gooch  crucible,  and  washed  with  an  ice-cold  mixture  of 
4  parts  of  95%  alcohol  and  1  part  of  concentrated  hydrochloric  acid. 
It  is  dried  on  the  hot  plate  or  at  150°  for  three  hours  and  weighed. 

161.  Bismuth. — The  filtrate  is  diluted  with  about  one-half  liter  of  water 
and  nearly  neutralized  with  ammonia  (about  40  c.c.  of  dilute  ammonia  will 
be  required).     After  standing  twenty-four  hours  the  bismuth  oxychloride 
is  filtered  off  on  a  Gooch  crucible,  washed  with  water  containing  a  few  drops 
of  dilute  hydrochloric  acid,  dried  at  110°,  and  weighed  as  BiOCl. 

The  bismuth  may  also  be  precipitated  as  bismuth  hydroxide  by  vola- 
tilizing most  of  the  alcohol,  neutralizing  with  ammonia,  and  warming  gently. 
If  IRON  is  present  this  precipitate  will  be  reddish.  In  that  case  it  is  best 
to  dissolve  it  in  hydrochloric  acid  and  precipitate  the  bismuth  as  oxy- 
chloride. The  bismuth  hydroxide  is  ignited  and  weighed  as  oxide,  Bi203. 

162.  Cadmium. — The  filtrate  from  the  bismuth    oxychloride   is  evapo- 
rated to  a  bulk  of  200  or  300  c.c.     If  the  bismuth  has  been  precipitated  by 
means  of  ammonia,  the  filtrate  is  first  acidified  with  hydrochloric  acid  and 
evaporated  to  a  moderate  bulk.     The  solution  is  saturated  with  hydrogen 
sulphide  and  the  precipitate  filtered  off  and  washed  with  water  containing 
hydrogen  sulphide.     If  the  cadmium  sulphide  is  dark  colored  or  black, 
traces  of  lead  or  bismuth  sulphides  may  be  present  because  of  incomplete 
separations,  or  copper  may  have  been  present  in  the  alloy.     Any  arsenic 
which  may  have  been  in  the  alloy  or  a  trace  of  tin  or  antimony  will  also 
be  present  in  this  precipitate. 

163.  Arsenic,    Antimony,  and  Tin.— It  should  be  tested  for  these  three 
elements  by  pouring  over  it  a  few  drops  of  warm  potassium  or  sodium 
sulphide  and  washing  two  or  three  times  with  warm  water,  being  careful 


132  ANALYSIS  OF  ALLOYS. 

to  stir  up  the  precipitate  with  the  stream  of  water  from  the  wash-bottle. 
A  precipitate  formed  on  acidifying  the  filtrate  indicates  the  presence  of 
arsenic,  antimony,  or  tin.  If  the  characteristic  orange  color  of  antimony 
is  absent,  the  supernatant  liquid  should  be  decanted  and  the  precipitate 
warmed  with  a  little  concentrated  hydrochloric  acid.  If  it  dissolves  com- 
pletely, arsenic  is  absent  and  the  tin  may  be  reprecipitated  by  diluting  and 
passing  hydrogen  sulphide.  After  washing,  the  moist  precipitate  with  the 
paper  may  be  burned  and  the  sulphide  of  tin  converted  into  oxide  by  ignition. 
If  arsenic  or  antimony  is  present,  it  may  be  determined  as  directed  in  the 
following  exercise. 

164.  Separation  of  Copper  and  Cadmium. — To  dissolve  out  any  copper 
which  may  be  present  with  the  cadmium  sulphide,  a  few  drops  of  potassium 
cyanide  should  be  poured  over  the  precipitate.     It  should  be  thoroughly 
stirred  up  with  water  and  washed  a  few  times.     If  a  considerable  amount 
of  copper  is  present,  the  bulk  of  the  precipitate  should  be  transferred  to  a 
beaker  by  washing  out  the  paper  while  still  in  the  funnel  with  a  stream  cf 
water.     The  remainder  of  the  precipitate  on  the  paper  is  dissolved  by 
washing  with  a  little  warm  dilute  nitric  acid.     The  paper  is  then  thor- 
oughly washed  with  small  portions  of  hot  water.     The  washings  are  allowed 
to  flow  into  the  beaker  containing  the  main  portion  of  the  precipitate.     The 
beaker  is  warmed  and  more  nitric  acid  is  added  if  necessary  to  dissolve 
the  precipitate.     The  solution  is  neutralized  with  sodium  carbonate  and  a 
slight  excess  of  potassium  cyanide  added.     A  small  white  precipitate  at  this 
point  may  be  lead  or  bismuth  carbonates,  which  should  be  filtered  off 
and  determined.     On  passing  hydrogen  sulphide  through  the  filtrate,  the 
cadmium  is  precipitated  as  sulphide  and  may  be  filtered  off  on  a  Gooch 
crucible  and  washed  with  water  containing  a  little  hydrogen  sulphide.     It 
is  finally  washed  with  pure  water  and  the  free  sulphur  extracted  by  washing 
with  alcohol  and  then  with  carbon  disulphide.     The  precipitate  is  dried  at 
100°  and  weighed. 

165.  Copper. — The  filtrate  from  the  cadmium  sulphide  contains  the  copper 
and  is  acidified*  with  sulphuric  acid  and  a  little  nitric  acid  and  evaporated 
to  fumes.    The  residue  is  dissolved  in  water,  filtered  if  necessary,  and  the 
copper  precipitated  as  sulphide.  If  it  is  small  in  amount  it  may  be  ignited 
and  weighed  as  oxide.  If  considerable  copper  is  present,  it  must  be  ignited 
with  sulphur  in  a  stream  of  hydrogen  and  weighed  as  cuprous  sulphide, 
Cu-jS.     When  much  copper  is  present,  it  is  better  to  determine  it  electro- 
lytically. 

166.  Separation  of  Iron  and  Zinc. — The  filtrate  from  the  first  precipita- 
tion with  hydrogen  sulphide  (§  162)  contains  any  zinc  or  iron  which  may 
have  been  present.    These  metals  may  be  determined  as  described  for  the 

*  This  should  be  done  under  a  hood  with  good  draught  to  avoid  any 
bility  of  inhaling  the  very  poisonous  hydrocyanic-acid  fumes. 


ANALYSIS  OF  WOOD'S  METAL.  133 

similar  filtrate  in  Exercise  26.    They  may  also  be  separated  in  the  follow- 
ing manner : 

The  solution  is  boiled  to  expel  hydrogen  sulphide,  neutralized  with  ammo- 
nia, and  acidified  with  acetic  acid.  Hydrogen  sulphide  is  passed  for  some 
time  and  the  solution  allowed  to  stand  for  several  hours.  The  clear  liquid 
is  carefully  decanted  through  a  filter-paper,  and  after  replacing  the  beaker 
containing  the  clear  filtrate  with  another  beaker,  the  sulphide  of  zinc  is 
brought  on  the  paper  and  washed  with  water  containing  ammonium  acetate 
and  acetic  acid.  The  precipitate  is  dissolved  in  a  little  dilute  nitric  acid 
and  the  paper  washed  with  hot  water.  The  solution  of  the  zinc  is  evaporated 
to  dryness  in  a  weighed  porcelain  crucible,  ignited  finally  over  the  blast-lamp 
to  decompose  any  zinc  sulphate  which  may  have  been  formed,  and  weighed 
as  oxide.  The  filtrate  is  boiled  to  expel  the  hydrogen  sulphide.  A  little 
nitric  acid  is  then  added  to  oxidize  the  iron,  which  is  precipitated  with 
ammoni#  and  weighed  as  oxide.  A  very  convenient  method  of  oxidizing  the 
iron  and  removing  the  hydrogen  sulphide  is  by  the  use  of  bromine  water. 
The  bromine  should  be  added  until  the  solution  is  colored,  indicating  com- 
plete oxidation  of  the  iron  and  the  presence  of  an  excess  of  bromine.  If  a 
solution  of  bromine  in  concentrated  hydrochloric  acid  is  used  a  few  drops 
will  suffice  and  the  solution  will  not  be  diluted  to  any  extent.  If  manganese 
is  to  be  removed  together  with  iron,  the  presence  of  an  excess  of  bromine 
is  advantageous ;  otherwise  it  must  be  boiled  out.  As  the  bromine  oxidizes 
hydrogen  sulphide  in  the  cold,  the  excess  of  the  latter  need  not  be  boiled 
out. 


CHAPTER  XI. 

ANALYSIS  OF  ALLOYS  CONTAINING  ARSENIC, 
ANTIMONY,   AND  TIN. 

SEPARATION  OF  ARSENIC,   ANTIMONY,   AND  TIN. 

167.  F.  W.   Clarke's  Method. — A    large  number  of  methods 
have  been  proposed  for  the  separation  of  arsenic  from  antimony 
and  tin.     One  of  the  best  is  that  of  F.  W.  Clarke.*    It  is  based 
on  the  fact  that  stannic  sulphide  is  not  precipitated  by  hydrogen  sul- 
phide from  a  boiling  solution  of  oxalic  acid.    The  sulphides  of 
arsenic  dissolve  slightly,  and  the  sulphide  of  antimony  still  more 
so,  in  boiling  oxalic-acid  solutions,  but  both  of  these  elements  are 
completely  reprecipitated  by  hydrogen  sulphide.     Free  mineral 
acids  must  be  absent  from  the  solution.     At  least  20  parts  of 
crystallized  oxalic  acid  must  be  present  for  1  part  of  tin,  and 
the  solution  should  be  diluted  to  125  c.c.  for  each  0.1  gram  of  anti- 
mony present.     Hydrogen  sulphide  is  passed  through  the  boiling 
solution  for  at  least   one-half  hour.     The  precipitate   must  be 
filtered  off  immediately,   as  stannic  sulphide  separates  out  on 
standing.     The    precipitate    generally    contains    a    little    stannic 
sulphide.     After  filtering  off  and  washing  two  or  three  times  it  is 
dissolved  in  a  little  ammonium  sulphide  and  the   solution  poured 
into  excess  of  hot  strong  solution  of  oxalic  acid.     Hydrogen  sul- 
phide  is  passed   through   the  boiling  solution  for  ten  minutes. 
The  precipitate  is  now  free  from  tin,  and  is  filtered  off  and  washed 
with  hot  water.     The  oxalic-acid  solution  of  the  tin  is  evaporated 
down  with  the  addition  of  sulphuric  acid  until  the  oxalic  acid  is 
decomposed.     The  tin  may  then  be  precipitated  with  hydrogen 
sulphide  and  weighed  as  oxide. 

1 68.  Separation  of  Arsenic   and  Antimony. — The  sulphides  of 
arsenic  and  antimony  may  be  separated  by  treatment  with  a 
saturated  ammonium-carbonate  solution  which  dissolves  the  arsenic 

*Chem.  News,  21,  124. 

134 


SEPARATION  OF  ARSENIC,  ANTIMONY,  AND   TIN.        135 

easily  and  the  antimony  only  slightly.  No  more  ammonium- 
carbonate  solution  than  necessary  to  dissolve  the  arsenic  sulphide 
should  be  used.  The  precipitate,  which  must  first  be  washed 
entirely  free  from  hydrogen  sulphide,  should  be  treated  with  small 
portions  of  the  carbonate  solution  until  no  more  arsenic  is  dis- 
solved, as  indicated  by  the  absence  of  a  precipitate  on  acidifying 
the  filtrate.  The  arsenic  is  completely  precipitated  by  acidifying 
with  hydrochloric  acid,  warming,  and  passing  hydrogen  sulphide. 
This  method  of  separating  arsenic  and  antimony  is  especially 
applicable  if  the  amount  of  arsenic  present  is  small. 

A  better  separation  of  the  arsenic  and  antimony  may  be  effected 
by  dissolving  the  sulphides  in  concentrated  hydrochloric  acid  with 
the  addition  of  small  amounts  of  potassium  chlorate.  The  solution 
is  warmed  to  expel  the  chlorine,  but  not  to  boiling,  so  as  not  to 
melt  the  sulphur  which  is  generally  liberated.  It  is  filtered  through 
asbestos,  which  is  washed  with  concentrated  hydrochloric  acid. 
It  is  cooled  in  ice-water  and  hydrogen  sulphide  passed  for  one  hour. 
The  arsenic  is  precipitated  as  As2S5  and  is  washed  with  strong 
hydrochloric  acid  and  then  with  hot  water.  Antimony  remains 
in  solution  as  trichloride.  This  method  also  serves  for  the  SEPA- 
RATION of  ARSENIC  and  TIN,  the  latter  behaving  like  antimony. 
According  to  Neher,*  for  this  separation  the  solution  should 
contain  1  volume  of  water  to  2  volumes  of  concentrated  hydro- 
chloric acid  (sp.  gr.  1.20).  If  a  stronger  acid  is  used,  the  precipi- 
tation of  the  arsenic  is  hindered. 

The  arsenic  is  best  dissolved  and  reprecipitated  as  magnesium 
ammonium  arsenate.  If  hydrogen  peroxide  is  at  hand  which  is  free 
from  phosphoric  acid,  aluminium,  etc.,  the  arsenic  sulphide  may  be 
dissolved  in  a  warm  mixture  of  ammonia  and  hydrogen  peroxide. 
A  very  excellent  solvent  is  a  solution  of  sodium  peroxide  in  water. 
The  arsenic  acid  may  then  be  precipitated  by  means  of  magnesia 
mixture.  The  arsenic  sulphide  may  also  be  dissolved  in  strong 
hydrochloric  acid  and  potassium  chlorate  or  in  fuming  nitric 
acid.  These  oxidizing  solutions  should  be  warmed,  but  not  high 
enough  to  melt  the  sulphur  which  usually  separates  out.  The 
latter  becomes  very  nearly  white  when  all  of  the  arsenic  has  been 
dissolved  out.  If  the  sulphur  is  large  in  amount  or  otherwise 

*  Zeit.  f.  Anal.  Chem.,  1893,  p.  45. 


136  ANALYSIS  OF  ALLOYS. 

troublesome,  liquid  bromine  may  be  added  in  small  portions  to 
dissolve  it.  This  method  of  precipitation  also  serves  to  separate 
ARSENIC  from  ANTIMONY.  Tartaric  acid  is  added  to  prevent  the 
precipitation  of  the  antimony. 

169.  The  Separation  of  the  Antimony  and  Tin  in  the  hydro- 
chloric-acid filtrate  from  the  arsenic  is  effected  as  follows:    The 
hydrogen  sulphide  is  decomposed  by  the  addition  of  a  little  potas- 
sium  chlorate   or   bromine.     The    antimony   is   precipitated   by 
digesting  the  somewhat  diluted  solution  on  the  water-bath  with 
the  addition  of  pure  iron.     Iron  produced  by  reduction  in  hydro- 
gen is  best,  and  in  any  event  it  must  be  free  from  phosphorus. 
The  precipitation  of  the  antimony  is  complete  in  about  one-half 
hour.     It  forms  a  black  powder.    The  tin  is  reduced  to  the  stan- 
nous-  condition.     The  solution  is  heated  until  the  iron  is  nearly 
dissolved.    The  antimony  is  then  filtered  off  on  an  asbestos  filter 
over  which  a  little  iron  has  been  sprinkled,  and  is  washed  with 
boiled  water  containing  considerable  hydrochloric  acid. 

The  TIN  is  precipitated  by  passing  hydrogen  sulphide  through 
the  filtrate,  which  for  this  purpose  is  diluted,  and  the  bulk  of  the 
acid  neutralized  with  ammonia.  The  stannous  sulphide  is  washed 
with  water  containing  hydrogen  sulphide  and  a  few  grams  of 
ammonium  sulphate.  It  is  dried,  transferred  to  a  weighed  cruci- 
ble, ignited,  and  weighed  as  stannic  oxide. 

The  ANTIMONY  is  dissolved  in  hydrochloric  acid  with  the  addi- 
tion of  a  little  potassium  chlorate.  The  solution  is  diluted  after 
the  addition  of  tartaric  acid  and  the  antimony  precipitated  as 
sulphide  and  weighed  as  oxide. 

170.  H.  Rose's  Separation  of  Antimony  from  Arsenic  and  Tin. — 
This  method  of  separation  is  based  on  the  insolubility  of  sodium 
metantimonate.    If  the   substance  is  metallic,  it  may  be  con- 
veniently oxidized  by  treatment  with  concentrated  nitric  acid 
in  a  porcelain   evaporat ing-dish  or   crucible.      The  material  is 
dried  on  the  water-bath  and  transferred  to  a  silver  crucible,  the 
porcelain  dish  being  rinsed  with  caustic-soda  solution.    The  con- 
tents of  the  crucible  are  evaporated  to  dryness,  eight  times  the 
bulk  of  the  mass  of  solid  caustic  soda  added,  heated   cautiously  to 
avoid  spattering  until  all  water  is  expelled  and  then  fused  for  some 
time.    The  mass  is  allowed  to  cool,  and  is  treated  with  hot  water 
until  the  fused  material  is  entirely  disintegrated.    One-third  the 


SEPARATION  OF  ARSENIC,  ANTIMONY,  AND   TIN.         137 

volume  of  alcohol  is  added  and  the  solution  allowed  to  stand  for 
twenty-four  hours  with  frequent  stirring. 

If  the  arsenic,  antimony,  and  tin  are  obtained  by  separation 
as  sulphides,  they  may  be  dissolved  in  solution  of  sodium  sulphide 
and  the  cold-saturated  solution  treated  with  small  portions  of 
sodium  peroxide  until  the  solution  is  colorless  and  oxygen  is 
copiously  evolved  on  the  further  addition  of  the  peroxide.  The 
solution  is  boiled  and  after  cooling  one-third  the  volume  of  alco- 
hol is  added.  The  solution  is  then  allowed  to  stand  for  twenty- 
four  hours. 

The  precipitate  obtained  by  either  method  is  washed,  first 
with  dilute  alcohol  composed  of  equal  volumes  of  alcohol  and 
water  and  then  with  a  mixture  of  one  volume  of  water  and  three 
volumes  of  alcohol.  A  few  drops  of  sodium  carbonate  solution  are 
added  to  each  of  the  washing  fluids.  If  much  tin  is  present,  it  is 
advisable  to  fuse  the  metantimonate  a  second  time  with  soda. 
The  antimony  is  dissolved  in  hydrochloric  acid  containing  a  little 
tartaric  acid,  precipitated  with  hydrogen  sulphide,  and  weighed 
as  oxide. 

The  filtrate  containing  the  arsenic  and  tin  is  acidified  with 
hydrochloric  acid  and  evaporated  until  all  of  the  alcohol  is  ex- 
pelled. The  solution  is  warmed  and  hydrogen  sulphide  passed 
until  precipitation  is  complete.  The  arsenic  and  tin  may  then  be 
separated  by  one  of  the  methods  already  given. 

171.  Electrolytic  Separation  of  Antimony  from  Arsenic  and 
Tin. — Antimony  may  also  be  separated  electrolytically  from 
arsenic  and  tin,  according  to  Classen.*  The  arsenic  must  be 
present  as  arsenic  acid,  otherwise  a  portion  of  it  will  be  precipi- 
tated with  the  antimony,  while  the  remainder  will  be  oxidized  to 
arsenic  acid.  A  saturated  solution  of  sodium  sulphide  must  first 
be  prepared.  If  the  commercial  sodium  sulphide  is  used  it  must 
be  dissolved  in  water  and  the  solution  saturated  with  hydrogen 
sulphide  which  has  been  washed  with  water  and  then  passed 
through  several  tubes  filled  with  cotton  wool.  While  passing 
hydrogen  sulphide  the  air  should  be  excluded.  The  solution  is 
filtered  and  rapidly  evaporated  in  a  large  platinum  or  porcelain 

*  Ber.,  17,  2245;   13,  1110;  23,  2160. 


138  ANALYSIS  OF  ALLOYS. 

dish  over  the  free  flame  until  a  thin  skin  begins  to  form  on  top. 
The  hot  solution  is  poured  into  glass-stoppered  bottles,  which 
must  be  completely  filled  and  immediately  well  stoppered.  The 
sodium  sulphide  may  also  be  prepared  by  completely  saturating 
a  solution  of  pure  caustic  soda  with  hydrogen  sulphide,  filtering, 
and  evaporating  down,  as  already  directed. 

The  sulphides  of  arsenic,  antimony,  and  tin  are  transferred  to 
a  weighed  platinum  dish  of  about  150  c.c.  capacity  and  dissolved 
in  80  c.c.  of  the  sodium-sulphide  solution.  Add  enough  concen- 
trated solution  of  pure  caustic  soda  to  furnish  1  to  2  grams  of 
NaOH.  The  caustic-soda  solution  must  be  clear  and  give  no 
precipitate  with  the  sodium-sulphide  solution.  The  dish  is  gently 
warmed  to  aid  solution.  It  is  electrolyzed  at  a  temperature  of 
50°  to  60°  with  a  current  of  0.5  ampere.  Precipitation  of  the 
antimony  is  complete  in  two  hours.  The  watch-crystal  is  washed 
with  a  little  water,  which  is  allowed  to  run  down  the  positive 
electrode.  The  solution  is  removed  without  interrupting  the 
current,  the  antimony  washed  with  water  and  then  with  alcohol, 
dried  at  80°  to  90°,  and  weighed. 

The  TIN  may  be  determined  by  acidifying  the  filtrate  with 
hydrochloric  acid,  concentrating  to  a  bulk  of  200  or  300  c.c., 
filtering,  if  ne'cessary,  and  passing  hydrogen  sulphide.  The  pre- 
cipitate is  washed  with  water  containing  hydrogen  sulphide  and  a 
few  grams  of  ammonium  sulphate.  It  is  weighed  as  stannic  oxide. 

172.  Separation  of  Arsenic. — Arsenic  is  very  readily  separated 
from  other  metals  as  arsenious  chloride,  AsCl3.  This  compound  is 
very  volatile.  When  pure  it  boils  at  130°.  It  is  rapidly  volatilized 
when  its  solution  in  hydrochloric  acid  is  boiled.  Even  at  tem- 
peratures considerably  below  the  boiling-point  the  arsenic  slowly 
volatilizes.  When  it  is  present  in  its  higher  state  of  oxidation, 
the  arsenic  is  not  volatilized  on  boiling  its  hydrochloric-acid 
solution.  Various  reducing  agents  have  been  used  to  convert  the 
arsenic  into  the  arsenious  chloride.  The  most  common  among 
these  are  ferrous  an.d  cuprous  salts  and  hydrogen  sulphide.  If 
small  amounts  of  arsenic  are  present  the  solution  may  be  distilled 
after  the  addition  of  concentrated  hydrochloric  acid.  On  repeat- 
ing the  distillation  once  or  twice  after  the  addition  of  concentrated 
hydrochloric  acid  all  of  the  arsenic  will  be  carried  over.  If  much 


SEPARATION  OF  ARSENIC,  ANTIMONY,  AND   TIN.        139 


arsenic  is  present,  the  solution  must  be  saturated  with  hydrochloric- 
acid  gas  and  the  distillation  carried  out  in  a  current  of  this  gas. 

173.  Dissolving  Arsenic  Compounds.  —  Various  methods  of  de- 
composing  the  ore  or  other  material  have  been  proposed,  the 
objects  sought  being  to  prevent  the  loss  of  arsenic  as  trichloride 
and  to  leave  the  hydrochloric-acid  solution  as  free  as  possible  from 
oxidizing  material.     If   the  material   is  soluble  in  hydrochloric 
acid,  it  may  be  digested  cold  with  concentrated  hydrochloric  acid 
for  some  time.     Liquid  bromine  may  then  be  added  and  the  solu- 
tion warmed,  at  first  gently  and  then  more  strongly,  until  the  excess 
of  bromine  is  expelled.     The  material  may  also  be  brought  into 
solution  by  treatment  with  concentrated  nitric  acid,  the  excess 
being  expelled  by  evaporation  after  the  addition  of  a  little  sulphuric 
acid.      The  residue  may  then  be  dis- 

solved   in    concentrated   hydrochloric 
acid. 

174.  Distillation  of  the  Arsenic.  —  If 
the  amount  of  arsenic  present  is  small, 
the  solution  may  be  decanted  into  the 
distilling-fl  ask  of  the  apparatus  shown  in 
Fig.  20  and  the  insoluble  residue  washed 
with  concentrated   hydrochloric    acid. 
About  5  grams  of  ferrous  sulphate,  or, 
still  better,  ferrous  chloride  or  cuprous 
chloride,  are  added  and  50  to  75  c.c. 
concentrated  hydrochloric  acid.     The 
solution  is  heated  to  boiling,  and  when 
the  liquid  has  boiled  down  to  a  volume 
of  about  40  c.c.,  50  c.c.  concentrated 
hydrochloric   acid   is   poured   through 
the    funnel    and    the    solution    again 
boiled  down  to  40  c.c.     The  distillate 
is  received  in  a  beaker  or  flask  con- 
taining  about   250  c.c.  of   hydrogen- 
sulphide  water.    After  the  second  dis- 

tillation this  beaker  is  removed  and  replaced  by  another  contain- 
ing hydrogen-sulphide  water.  The  distillation  is  repeated  after 
the  addition  of  50  c.c.  of  concentrated  hydrochloric  acid  to  the 


FIG.  20. 


140  ANALYSIS  OF  ALLOYS. 

flask.     A  trace  of  arsenic  is  sometimes  obtained   by  a  fourth 
distillation. 

When  LARGE  AMOUNTS  of  ARSENIC  must  be  distilled,  the  drop- 
ping-funnel  is  replaced  by  a  glass  tube  leading  to  a  flask  in  which 
hydrochloric-acid  gas  is  generated.  This  gas  is  very  readily 
obtained  by  allowing  concentrated  sulphuric  acid  to  drop  into 
concentrated  hydrochloric  acid.  After  transferring  the  arsenic 
solution  to  the  distillation  flask  it  is  saturated  with  hydrochloric 
acid  by  passing  the  gas  into  the  solution,  which  has  been  cooled  to 
zero  by  immersing  the  flask  in  a  vessel  filled  with  cracked  ice. 
When  the  gas  is  no  longer  absorbed,  but  bubbles  through  the 
solution,  the  ice  is  removed  and  the  solution  brought  to  a  boil. 
The  stream  of  hydrochloric-acid  gas  is  allowed  to  flow  and  the 
distillation  is  continued  until  the  volume  of  the  solution  is  reduced 
to  about  40  c.c.  The  arsenic  may  be  absorbed  in  hydrogen-sul- 
phide water  and  the  precipitate  of  arsenious  sulphide  filtered  off 
and  weighed  as  such. 

EXERCISE  29. 
Analysis  of  Type  Metal. 

Alloy  of  Copper,  Lead,  Antimony,  Tin,  with  Small  Amounts  of  Iron  and 

Arsenic. 

175.  Solution  of  the  Alloy.— To  1  gram  of  the  alloy,  which  has  been 
cut  into  small  shavings  with  a  clean  knife,  or  sampled  by  means  of  a 
clean  hack  saw  producing  fine  "sawings,"  is  added  15  c.c.  concentrated 
hydrochloric  acid.  The  solution  is  gently  warmed  on  the  water-bath  and 
a  drop  or  two  of  concentrated  nitric  acid  is  added  occasionally  until 
solution  is  affected.  All  of  the  metals  will  be  converted  into  chlorides 
which  will  remain  in  solution  with  the  possible  exception  of  lead  chloride. 
An  excess  of  nitric  acid  is  to  be  avoided  as  it  tends  to  form  insoluble  metas- 
tannic  acid,  which  can  be  readily  distinguished  from  the  crystalline  lead 
chloride.  If  metastannic  acid  forms,  the  operation  must  be  repeated,  using 
less  nitric  acid  or  adding  it  less  frequently.  After  a  few  trials  the  correct 
method  of  adding  the  nitric  acid  is  soon  acquired. 

176.  Lead. — The  solution  is  allowed  to  cool  and  then  stand  at  least  £ 
hour  or  better  over  night  to  allow  the  lead  chloride  to  crystallize  out. 
Ten  times  the  volume  of  absolute  alcohol  is  then  added  in  several  por- 
tions. After  standing  for  about  half  an  hour,  the  lead  chloride  is 
filtered  off  on  a  Gooch  crucible,  washed  with  a  mixture  of  4  parts  of  95% 
alcohol  and  1  part  of  concentrated  hydrochloric  acid,  and  finally  with 
pure  alcohol.  It  is  dried  for  three  hours  at  150°  and  weighed.  The  great 
advantage  of  this  method  of  separating  the  lead  is  that  the  very  trouble- 
some treatment  of  the  sulphides  of  the  metals  present  with  sodium  or 
potassium  sulphide  is  avoided.  The  most  difficult  part  of  the  operation 
is  the  solution  of  the  alloy. 


ANALYSIS  OF  TYPE  METAL.  141 

177.  Copper  and    Iron — The  filtrate  from  the  lead  chloride  is  heated 
until  the  alcohol  is  expelled.     Two  grams  of  tartaric  acid  and  an  excess 
of  ammonia  are  added  and  the  solution  warmed  until  the  precipitate 
dissolves.     By  the  addition  of  five  c.c.  of  saturated  hydrogen-sulphide 
water,   the  copper  and  the  small   amount   of  lead  still  unprecipitated 
as  well  as  a  trace  of  iron  which  may  be  present  may  be  precipitated  with- 
out bringing   down   any   of  the  tin    and   antimony.      The   solution   is 
warmed  and  when  the  dark  colored  precipitate  has  settled,  one  c.c.  of 
the  hydrogen-sulphide  water  is  added  to  the  clear  supernatant  liquid. 
If  no  further  precipitate  is  produced,  the  solution  is  filtered  and  the 
precipitate  washed  with  water  containing  hydrogen  sulphide. 

The  precipitate  is  dissolved  in  a  little  warm  dilute  nitric  acid  and  the 
lead  separated  as  sulphate,  the  nitric  acid  being  expelled  by  evaporation- 
after  the  addition  of  sulphuric  acid.  The  copper  is  precipitated  from  the 
filtrate  as  sulphide  and  if  small  in  amount  may  be  ignited  and  weighed 
as  oxide.  If  considerable  copper  is  present  it  must  be  weighed  as 
sulphide  or  without  precipitation  as  sulphide  may  be  separated  electro- 
lytically  from  the  iron.  One  or  two  c.c.  concentrated  nitric  acid  are  added 
and  a  current  of  one-half  ampere  passed  until  all  the  copper  is  precipitated. 
The  iron  may  then  be  precipitated  with  ammonia  and  weighed  as  oxide. 

178.  Separation   of  Antimony  and  Tin. — The  solution   of  antimony  and 
tin  is  acidified  with  hydrochloric  acid,  hydrogen  sulphide  passed,  and  the 
precipitate  filtered  off  and  washed  two  or  three  times.     A  hole  is  made  in 
the  point  of  the  filter-paper  by  means  of  a  glass  rod  and  the  bulk  of  the 
precipitate  washed  into  a  beaker  with  a  little  water.     Warm  dilute  hydro- 
chloric acid  is  poured  over  the  paper  to  dissolve  the  portion  of  the  precipi- 
tate still  adhering  to  the  paper.     The  precipitate  in  the  beaker  is  dissolved 
by  warming  and  adding  concentrated  hydrochloric  acid.     The  hydrogen 
sulphide  is  decomposed  by  the  addition  of  a  crystal  of  potassium  chlorate 
and  warming.     Some  pure  metallic  iron  is  added  and  the  solution  heated 
on  the  water-bath  for  about  one-half  hour  or  until  the  iron  is  nearly  dissolved. 
The  precipitated  antimony  is  filtered  off  on  a  Gooch  crucible,  a  little  iron 
having  been  sprinkled  on  the  asbestos.     The  precipitate  is  washed  with 
boiled  water  to  which  considerable  hydrochloric  acid  has  been  added. 

179.  The  antimony  is  dissolved  in  hydrochloric  acid  to  which  a  little  potas- 
sium chlorate  has  been  added.    The  solution  is  warmed  to  expel  chlorine  and, 
after  the  addition  of  tartaric  acid  and  water,  hydrogen  sulphide  is  passed. 
The  antimony  sulphide  is  filtered  off  and  washed  with  water  containing  a  little 
hydrogen  sulphide.     The  moist  precipitate  is  rinsed  into  a  capacious  porce- 
lain crucible  with  water.     The  small  portion  still  adhering  to  the  paper 
is  dissolved  in  a  little  warm  ammonium  sulphide  and  the  solution  allowed 
to  flow  into  the  crucible.     The  solution  is  evaporated  on  the  water-bath 
after  the  addition  of  a  few  cubic  centimeters  of  concentrated  nitric  acid.    If 
sulphur  separates,  a  little  liquid  bromine  is  added  when  the  solution  has 
become  quite  concentrated.    When  the  globule  of  sulphur  has  disappeared, 
expel  the  excess  of  nitric  acid  by  heating  on  the  hot  plate  or  with  the  Bunsen 


142  ANALYSIS  OF  ALLOYS. 

burner,  finally  heating  to  full  redness.  Cool  a  little,  sprinkle  some  ammo- 
nium carbonate  over  the  precipitate,  and  ignite  again  to  completely  expel 
sulphuric  acid  and  weigh  as  antimony  tetroxide,  Sb2O4. 

1 80.  Tin. — To  precipitate  the  tin  in  the  filtrate  from  the  antimony  the 
excess  of  hydrochloric  acid  is  neutralized  with  ammonia,  the  solution  diluted 
somewhat,  warmed,  and  hydrogen  sulphide  passed  until  the  tin  is  entirely 
precipitated.     The  stannous  sulphide  is  washed  with  water  containing 
hydrogen  sulphide  and  a  few  grams  of  ammonium  sulphate.    It  is  dried 
and  detached  from  the  paper  which  is  burned.     The  precipitate  and  the 
ash  are  placed  in  a  weighed  porcelain  crucible  and  heated  very  gently 
with  free  access  of  air  until  sulphur  dioxide  ceases  to  be  given  off.     The 
oxidation  may  be  assisted  by  the  addition  of  a  few  drops  of  nitric  acid. 
Finally  the  precipitate  is  strongly  heated  to  expel  sulphuric  acid,  which 
is  completely  removed  by  the  addition  of  a  little  ammonium  carbonate 
and  again  igniting.    It  is  weighed  as  stannic  oxide  SnO2. 

181.  Arsenic. — As  only  a  trace  of  arsenic  is  present,  a  5-  or  10-gram  portion 
of  the  alloy  should  be  taken  for  its  determination.     Dissolve  in  hydrochloric 
acid  and  potassium  chlorate  and  warm  to  expel  the  chlorine.     Filter  off  the 
lead  chloride  on  asbestos  and  wash  a  few  times  with  dilute  hydrochloric  acid. 
Add  one-third  the  volume  of  concentrated  hydrochloric  acid  and  pass  hydrogen 
sulphide.     Filter  off  the  precipitate  consisting  of  the  sulphides  of  copper  and 
arsenic  on  asbestos,  wash  with  hot  water  containing  hydrogen  sulphide  and 
a  little  hydrochloric  acid.     Dissolve  the  arsenic  sulphide  by  washing  the  pre- 
cipitate with  a  little  warm  dilute  ammonia.    Evaporate  the  solution  nearly  to 
dryness  in  a  porcelain  dish.     Oxidize  the  arsenic  by  warming  with  concen> 
trated  nitric  acid,  dilute  the  solution  somewhat,  neutralize  with  filtered 
ammonia,  and  add  magnesia  mixture.     After  standing  twenty-four  hours 
filter,  wash,  ignite,  and  weigh  as  magnesium  pyroarsenate  according  to  the 
directions  given  in  Chapter  VII,  page  88. 


EXERCISE  30. 
Analysis  of  Britannia  Metal. 

Alloy  of  Tin,  Antimony,  and  Copper,  with  Small  Amounts  of  Bismuth, 
Lead,  and  Iron. 

182.  Decomposition  of  the  Alloy  by  Means  of  Chlorine. — Alloys  containing 
a  large  percentage  of  tin  are  best  decomposed  by  a  stream  of  chlorine. 
The  method  is  applicable  to  alloys  containing  less  than  15%  of  lead  and 
copper. 

A  hard-glass  combustion-tube  70  cm.  long  is  taken  and  one  end  drawn 
out,  making  a  small  tube  20  cm.  long,  which  is  bent  at  right  angles. 
This  small  tube  is  connected  by  means  of  a  cork  stopper  with  a  Peligot 
tube  the  bulbs  of  which  are  nearly  filled  with  dilute  hydrochloric  acid  (1  :  3) 


ANALYSIS  OF  BRITANNIA  METAL.  143 

containing  about  1  gram  of  tartaric  acid.  A  second  Peligot  tube  is  con- 
nected with  the  first  and  contains  a  solution  of  caustic  soda  (1  :  3).  The 
chlorine  is  evolved  in  a  2-liter  flask  containing  pieces  of  pyrolusite,  over 
which  concentrated  hydrochloric  acid  is  poured.  The  flask  is  heated  on  a 
water-bath.  The  chlorine  is  passed  through  a  wash-bottle  containing 
water  and  then  through  two  wash-bottles  containing  sulphuric  acid.  It 
is  then  passed  into  the  combustion-tube,  connection  being  made  by  means 
of  a  cork  stopper.  Wherever  rubber  is  used  for  making  connections,  it 
must  be  well  coated  with  paraffine.  This  is  also  advisable  for  the  cork 
stoppers.  The  chlorine  is  not  allowed  to  pass  into  the  combustion-tube 
until  all  of  the  air  has  been  displaced  from  the  flask  and  the  wash-bottles. 
All  escaping  chlorine  should  be  absorbed  in  caustic-soda  solution. 

One  gram  of  the  alloy  in  fine  turnings  is  weighed  out  and  placed  in  a 
porcelain  boat  which  is  placed  in  the  middle  of  the  combustion-tube.  The 
chlorine  is  first  allowed  to  act  on  the  alloy  in  the  cold.  When  no  further 
action  is  observed,  the  part  of  the  tube  in  which  the  boat  is  situated  is 
heated  gently  with  the  Bunsen  burner,  and  then  more  strongly  until  the 
contents  of  the  boat  fuse.  The  CHLORIDES  of  MERCURY,  BISMUTH,  ARSENIC, 
ANTIMONY,  and  TIN  VOLATILIZE  and  are  driven  out  of  the  tube  by  heating 
it  gently  from  the  boat  to  the  end  which  is  drawn  out.  These  chlorides 
are  absorbed  in  the  hydrochloric  acid  contained  in  the  first  Peligot  tube, 
while  the  excess  of  chlorine  is  absorbed  in  the  caustic-soda  solution  con- 
tained in  the  second  Peligot  tube. 

The  chlorine  in  the  apparatus  is  then  displaced  by  means  of  a  stream 
of  dry  air  or  carbon  dioxide,  the  chlorine  generator  having  been  removed. 
The  apparatus  is  disconnected,  the  boat  containing  the  CHLORIDES  of 
COPPER,  LEAD,  and  IRON  is  placed  in  a  porcelain  dish,  and  the  tube  washed 
out  with  hot  water  which  is  allowed  to  flow  into  the  dish  containing  the  boat. 
Hydrochloric  acid  is  added  and  the  dish  warmed  until  the  contents  of  the 
bout  are  dissolved.  The  latter  is  removed  and  washed. 

183.  Lead,  Copper,  and  Iron. — The  lead  is  precipitated  by  evaporation 
with  sulphuric  acid  and  diluting  and  is  filtered  off  and  weighed  as  sulphate. 
The  copper  is  precipitated  by  means  of  hydrogen  sulphide  and  weighed  as 
sulphide  or  determined  electrolytically  from  a  nitric  acid  solution.     The 
iron  is  precipitated  by  means  of  ammonia  and  weighed  as  oxide. 

The  contents  of  the  first  Peligot  tube  are  poured  into  a  beaker  and  the 
Peligot  tube  well  washed  out  with  water  to  which  hydrochloric  acid  is 
added  if  necessary.  The  solution  is  warmed  and  hydrogen  sulphide  passed 
until  precipitation  is  complete.  The  filtrate  should  be  heated  to  boiling, 
strong  hydrochloric  acid  added,  and  hydrogen  sulphide  passed  again  to 
insure  complete  precipitation  of  the  arsenic. 

184.  Bismuth. — If   the    sulphide  precipitate  is  dark  colored,  bismuth  is 
present.     The  precipitate  is  washed  into  a  beaker,  ammonium  sulphide 
added,  and  the  solution  warmed.     The  solution  is  filtered  through  the  same 
paper  and  the  precipitate  washed  with  warm  water  containing  a  little 


144  ANALYSIS  OF  ALLOYS. 

ammonium  sulphide.  The  bismuth  sulphide  is  dissolved  in  a  little  warm 
dilute  nitric  acid  and  the  paper  washed.  The  bismuth  is  precipitated  with 
ammonia  and  ammonium  carbonate,  ignited,  and  weighed  as  oxide,  Bi203. 

185.  Separation  of  Tin  from  Arsenic  and  Antimony.  —  The  ammonium- 
sulphide  solution  of  arsenic,  antimony,  and  tin  is  poured  with  vigorous 
stirring  into  a  hot  solution  of  25  grams  of  oxalic  acid  in  200  c.c.  of  water. 
The  solution  is  heated  to  boiling  and  hydrogen  sulphide  passed  for  about 
fifteen  minutes.    The  precipitate  is  filtered  off  immediately  and  washed 
with  hot  water  containing  hydrogen  sulphide.     It  is  dissolved  in  ammo- 
nium sulphide  and  the  treatment  with  hot  oxalic  acid  and  hydrogen  sulphide 
repeated. 

186.  Tin. — The  oxalic-acid  solution  of  tin  is  evaporated  down,  with  the 
addition  of  5  c.c.  concentrated  sulphuric  acid,  to  fumes.     The  solution 
is  cooled,  cautiously  diluted  with  water,  and  hydrogen  sulphide  passed  to 
insure  complete  precipitation  of  the  tin.    Wash  the  precipitate  with  water 
containing  ammonium  acetate  and  a  little  acetic  acid,  dry,  ignite,  and 
weigh  as  stannic  oxide  SnO9. 

187.  Arsenic  and  Antimony. — The  precipitate  of  arsenic  and  antimony 
sulphides  is  treated  with  a  little  concentrated  ammonium-carbonate  solu- 
tion and  washed  to  remove  arsenic.    The  antimony  is  then  weighed  as 
oxide  according  to  the  directions  given  in  Exercise  29,  page  141.     The 
arsenic  is  determined  according  to  the  directions  given  in  the  same 
exercise. 


CHAPTER  XII. 

ANALYSIS  OF  ALLOYS  CONTAINING  IRON, 
NICKEL,   AND  ZINC. 

188.  Separation  of  Zinc  as  Sulphide. — Zinc  may  be  separated 
very  completely  from  most  metals  as  sulphide.     Hydrogen  sul- 
phide  passed   through   the   moderately   acid   solution   separates 
silver,  lead,  mercury,  bismuth,  copper,  cadmium,  arsenic,  antimony, 
and  tin  from  zinc.     On  neutralizing  the  solution  almost  com- 
pletely, or  making  it  acid  with  acetic  or  citric  acid,  or  a  drop  or 
two  of  a  mineral  acid,  and  passing  hydrogen  sulphide,  zinc  is  pre- 
cipitated as  sulphide,  while  cobalt,  nickel,  manganese,  aluminium, 
and  iron  remain  in  solution  as  well  as  the  alkaline-earth  metals  and 
the  alkalies.    Zinc  may  be  separated  from  chromium  and  aluminium 
by  adding  sodium  sulphide  to  a  solution  of  these  metals  containing 
tartaric  acid  made  alkaline  with  ammonia. 

Although  the  separation  of  zinc  as  sulphide  from  other  metals 
is  very  perfect  and  the  metal  can  be  weighed  with  the  greatest 
accuracy  as  sulphide,  the  method  is  not  very  largely  used.  This 
arises  from  the  fact  that  zinc  sulphide  is  very  difficult  to  filter 
and  wash  because  of  its  tendency  to  pass  through  the  filter-paper. 
Ignition  of  a  precipitate  with  sulphur  in  a  stream  of  hydrogen  is 
also  somewhat  troublesome.  The  latter  difficulty  may  be  obviated 
by  dissolving  the  sulphide  in  hydrochloric  or  nitric  acid  and  pre- 
cipitating the  zinc  as  carbonate  and  weighing  as  oxide  or  precipi- 
tating as  zinc-ammonium  phosphate  and  weighing  as  zinc  pyrophos- 
phate.  The  zinc  sulphide  is  washed  more  readily  if  hydrogen 
sulphide  and  ammonium  sulphate  or  acetate  are  present  in  the 
wash-water. 

189.  Saparation  of  Zinc  as  Zincate. — Zinc  may  also  be  sepa- 
rated from  cobalt,  nickel,  iron,  and  manganese  by  pouring  the  acid 
solution  of  these  metals  into  excess  of  caustic-soda  solution.     The 
zinc  hydroxide  first  precipitated  redissolves  in  the  excess  of  alkali, 

145 


146  ANALYSIS  OF  ALLOYS. 

forming  sodium  zincate,  while  the  hydroxides  of  the  other  metals 
are  insoluble  in  the  caustic  alkali  and  may  be  filtered  off.  Before 
filtering,  the  solution  should  be  diluted  to  precipitate  a  small 
amount  of  cobalt,  which  dissolves  in  concentrated  caustic-soda 
solution.  The  concentrated  alkali  is  also  liable  to  ruin  the 
filter-paper. 

The  zinc  may  be  precipitated  as  sulphide  by  passing  hydrogen 
sulphide  through  the  filtrate.  ,  The  sulphide  obtained  in  this 
manner  from  an  alkaline  solution  coagulates  and  settles  readily 
and  does  not  tend  to  pass  through  the  filter-paper.  If  the  hydro- 
gen sulphide  is  passed  too  long  the  zinc  sulphide  redissolves.* 
The  presence  of  sodium  or  ammonium  chloride  tends  to  prevent 
this  action.  The  danger  may  be  avoided  by  adding  hydrogen 
sulphide  water  until  no  more  precipitate  forms.  The  alkali  is 
washed  out  of  the  precipitate  with  considerable  difficulty,  how- 
ever, and  the  precipitate  also  carries  down  silica.  After  igniting 
with  sulphur  in  a  stream  of  hydrogen  the  alkali  may  be  extracted 
with  hot  water  and  the  zinc  sulphide  again  ignited  and  weighed 
as  sulphide.  For  determining  silica,  the  zinc  sulphide  is  dissolved 
in  hydrochloric  acid  and  the  solution  evaporated  to  dry  ness.  The 
zinc  chloride  is  dissolved  hi  water  and  the  silica  filtered  off,  ignited, 
and  weighed. 

EXERCISE  31. 
Analysis  of  Brass  or  Bronze. 

Attoy  of  Lead,  Copper,  Tin,  and  Zinc,  Small  Amounts  of  Arsenic  and  Iron. 

190.  Solution  of  the  Alloy. — Weigh  out  1  gram  of  the  alloy  and  place  in  a 
300-c.c.  beaker,  add  10  c.c.  concentrated  nitric  acid  and  5  c.c.  water.    Cover 
the  beaker  with  a  watch-crystal  and  place  in  a  dish  of  cold  water.     After 
one-half  hour  place  the  beaker  on  the  water-bath  and  evaporate  the  solu- 
tion to  dryness.     100  c.c.  of  boiling  water  and  a  few  drops  of  nitric  acid  are 
added  and  the  solution  boiled  for  five  minutes. 

191.  Tin. — The  stannic  oxide  is  filtered  off  and  washed  with  hot  water. 
The  moist  precipitate  is  introduced  into  a  weighed  porcelain  crucible  and 
the  paper  burned  in  the  usual  manner.     If  the  amount  of  tin  is  small  (less 
than  l%)it  is  weighed  at  this  point,  otherwise  it  is  fused  with  six  times  its 
weight  of  a  mixture  of  equal  parts  of  sulphur  and  sodium  carbonate.     The 
fused  mass  is  dissolved  in  hot  water  and  the  solution  filtered.     The  copper, 
lead,  and  iron  which  were  carried  down  with  the  stannic  oxide  will  remain 
on  the  paper  as  sulphides,  while  the  filtrate  will  contain  all  of  the  tin  and 

*  L.  W.  McCay,  Jour.  Am.  Chem.  Soc.,  30.  376. 


ANALYSIS  OF  BRASS  OR  BRONZE.  147 

any  arsenic  or  antimony  which  may  have  been  present.  The  insoluble 
sulphides  are  dissolved  in  a  little  nitric  acid,  the  paper  washed,  and  the 
solution  added  to  the  nitrate  from  the  stannic  oxide. 

If  arsenic  and  antimony  are  absent,  the  tin  may  be  precipitated  out  of 
the  sodium  sulphide  solution  and  weighed.  The  excess  of  sulphur  should 
first  be  removed  frcm  the  solution  by  heating  to  boiling  after  the  addition  of 
caustic  soda  and  then  adding  hydrogen  peroxide  in  small  quantities  until 
the  solution  is  nearly  decolorized.  It  is  then  acidified  with  hydrochloric 
acid  while  stirring  constantly,  heated,  and  hydrogen  sulphide  passed.  The 
stannic  sulphide  is  washed  with  hot  water  containing  ammonium  acetate 
and  a  little  acetic  acid.  It  is  ignited  and  weighed  as  stannic  oxide  in  the 
usual  manner. 

192.  Arsenic  and  Antimony. — If  arsenic  is  present  in  the  alloy,  a  small 
amount  of  this  element  will  be  present  in  the  sodium  sulphide  solution  of 
the  tin  and  will  be  precipitated  with  the  stannic  sulphide.     It  may  be 
removed  by  treating  the  precipitate  with  a  little   concentrated  solution 
of  ammonium  carbonate  and  washing.     The  solution  of  arsenic  should  be 
added  to  the  nitric  acid  solution  of  the  alloy. 

If  antimony  is  also  present  in  the  alloy,  the  sulphides  of  arsenic,  anti- 
mony, and  tin  must  be  separated  by  one  of  the  methods  given  in 
Chapter  XI,  page  141. 

193.  Lead. — To  the  filtrate  from  the  stannic  oxide  5  c.c.  concentrated 
sulphuric  acid  are  added  and  the  solution  evaporated  in  a  porcelain  dish 
until  the  nitric  acid  is  entirely  expelled  and  white  fumes  of  sulphuric  acid 
are  given  off.     The  solution  is  cooled  by  floating  the  dish  on  cold  water 
and  diluted  with  75  c.c.  of  water.     After  stirring  thoroughly,  25  c.c.  alcohol 
are  added  and  the  solution  allowed  to  stand  for  at  least  one  hour.     The  lead 
sulphate  is  filtered  off  on  a  Gooch  crucible,  washed  with  water  containing 
about  1%  of  sulphuric  acid  and  25%  of  alcohol  and  then  with  pure  alcohol 
until  free  from  acid.     It  is  dried  on  the  hot  plate  and  weighed. 

194.  The  copper  is  best  determined  electrolytically.     The  filtrate  from 
the  lead  sulphate  is  heated  on  the  hot  plate  until  most  of  the  alcohol  is 
expelled.     Two  c.c.  concentrated  nitric  acid  are  added  and  the  warm  solu- 
tion (about  60°)  electrolyzed  with  a  current  of  one-half  to  one  ampere  for 
about  six  hours. 

Hydrogen  sulphide  is  passed  through  the  acid  filtrate  from  the  copper 
to  precipitate  traces  of  arsenic,  antimony,  or  unseparated  tin  which  may  be 
present.  If  more  than  traces  are  found,  the  metals  must  be  separated  and 
determined  by  the  methods  given  in  the  preceding  exercises.  When  the 
amount  of  copper  is  large,  as  is  generally  the  case,  it  is  advisable  to  divide 
the  solution  into  two  portions  for  the  electrolysis,  as  about  300  mg.  of 
copper  is  generally  sufficient  for  a  good  determination.  The  solution  may 
be  divided  by  weighing  it  and  then  pouring  out  about  half  of  it  and  again 
weighing  or  the  solution  may  be  diluted  to  a  known  volume  as  250  or  500  c.c. 
and  a  portion  measured  out.  The  copper  may  be  determined  in  each 
portion  and  the  filtrates  combined  fcr  the  zinc  determination.  For  the 


148  ANALYSIS  OF  ALLOYS. 

duplicate  zinc  determination  the  copper  may  be  precipitated  as  sulphide, 
which  is  filtered  off,  well  washed,  and  discarded. 

195.  Iron. — The  filtrate  from  the  copper  is  boiled  to  expel  hydrogen 
sulphide  and  a  little  nitric  acid  added  to  oxidize  the  iron,  which  is  precipi- 
tated with  ammonia  and  weighed  as  oxide.     If  more  than  a  small  amount 
of  iron  is  present,  the  precipitate  must  be  redissolved  and  reprecipitated  to 
completely  separate  it  from  the  zinc. 

196.  Zinc. — The  filtrate  from  the  ircn  is  evaporated  to  small  bulk  and 
the  zinc  precipitated  and  weighed  as  pyrophosphate.    The  zinc  may  also  be 
precipitated  and  weighed  as  sulphide. 

EXERCISE  32. 

Analysis  of  German  Silver. 

AUoy  of  Copper,  Zinc,  and  Nickel,  with  Small  Amounts  of  Lead,  Iron,  and  Tin 

One  gram  of  the  alloy  is  weighed  out  and  dissolved  in  nitric  acid  as 
directed  in  the  preceding  exercise.  The  tin,  lead,  and  copper  are  deter- 
mined as  directed  in  the  same  exercise. 

Hydrogen  sulphide  is  passed  through  the  acid  filtrate  from  the  copper 
to  precipitate  traces  of  arsenic,  antimony,  tin,  or  unseparated  copper  which 
rr  ay  be  present.  If  more  than  traces  are  found,  the  metals  must  be  sepa- 
rated and  determined  by  the  methods  given  in  the  preceding  exercises. 

197.  Zinc. — The  filtrate  is  boiled  until  the  hydrogen  sulphide  is  expelled 
and  the  solution  concentrated  to  a  small  bulk  and  the  acid  nearly  neutral- 
ized with  caustic  soda.  Five  to  ten  grams  of  caustic  soda  are  dissolved  in 
about  50  c.c.  of  water  and  the  solution  of  zinc  and  nickel  added  slowly 
with  constant  stirring.  The  solution  is  diluted  with  an  equal  bulk  of  water 
and  the  precipitate  filtered  off  and  washed.  The  zinc  in  the  filtrate  is  pre- 
cipitated with  hydrogen  sulphide,  filtered  off,  and  washed  free  from  alkali. 
The  zinc  sulphide  is  dried  and  detached  from  the  paper  as  completely  as 
rozsible. 

The  portion  still  adhering  to  the  paper  is  dissolved  in  nitric  acid  and  the 
solution   evaporated   to   dryness  in  a  porcelain  crucible.     The  remainder 
of  the  precipitate  is  added  and  the  whole  ignited  with  sulphur  in  a  stream 
of  hydrogen.     If  the  precipitate  is  small  it  need  not  be  dried,  but  is  imme- 
diately dissolved  in  nitric  acid  and  after  evaporation  converted  into  sul- 
phide.    The  sulphide  is  tested  for  alkali  by  digestion  with  hot  water.     If 
alkali  is  found  it  must  be  completely  extracted  and  the  sulphide  again 
weighed  after  ignition  with  sulphur  in  hydrogen.     The  precipitate  is  then 
dissolved  in   nitric  acid  and   the  solution   evaporated  to    dryness.       The 
zinc  nitrate  is  dissolved  in  water  and  the  silica  filtered  off,  washed,  ignited, 
and  weighed.      The  zinc  sulphide  may  also  be  dissolved   in   hydrochloric 
acid,  the  zinc  precipitated  as  zinc  ammonium  phosphate  and  weighed  as 
pyrophosphate. 

198.  Iron  and  Nickel. — If  iron  is  absent,  the  nickel  hydroxide  may  be 
washed  and  after  transferring  the  precipitate  to  a  weighed  porcelain  crucible 


ANALYSIS  OF  GERMAN  SILVER.  149 

and  burning  the  paper  it  may  be  reduced  to  metallic  nickel  by  heating  in  a 
stream  of  hydrogen  and  weighed.  If  iron  is  present,  the  precipitate  is 
dissolved  in  hydrochloric  acid  and  the  iron  precipitated  with  ammonia. 
Unless  a  very  small  amount  is  present  it  must  be  redissolved  and  repre- 
cipitated,  and  after  washing  is  ignited  and  weighed  as  oxide.  The  nickel 
is  then  reprecipitated  as  hydroxide  by  means  of  an  excess  of  caustic  soda, 
reduced  in  a  stream  of  hydrogen  and  weighed  as  the  metal. 

EXERCISE  33. 
Analysis  of  Manganese-Phosphorus-Bronze. 

Alloy  of  Copper,  Lead,  Tin,  Zinc,  Manganese,  Phosphorus  (less  than  1%), 

Traces  of  Iron. 

199.  Solution. — One  gram  of  the  alloy  is  weighed  out  and  dissolved  in 
nitric  acid  as  directed  in  Exercise  31,  page  146.    Nearly  all  of  the  phosphorus 
remains  with  the  stannic  oxide  as  a  phosphate.      After  fusing  the  impure 
precipitate  and  separating  the  impurities  as  given  in  Exercise  31,  and  pre- 
cipitating the  tin  as  sulphide,  the  solution  containing  only  the  phosphorus 
as  phosphoric  acid  is  discarded,  as  this  element  is  determined  in  a  separate 
portion  of  the  alloy. 

200.  Lead,   Copper,   and   Zinc  are  determined   as   given  in  Exercise  31. 
The  phosphoric  acid  which  did  not  rcirain  with  the  stannic  oxide  will  be 
present  in  the  alkaline  solution  of  the  zinc.     This  element  should  therefore 
be  precipitated  and  weighed  as  pyrophosphate. 

201.  Iron. — In  order  to  separate  manganese  and  iron  from  zinc,  bromine 
or  hydrogen  peroxide  is  added  to  the  nitrate  from  the  copper.    The  solu- 
tion is  boiled  and  excess  of  ammonium  added  to  redissolve  any  zinc  phos- 
phate which  may  be  precipitated.     The  precipitate  consisting  of  ferric 
hydroxide  and  manganese  dioxide  is  filtered  off  and  washed.     It  is  dis- 
solved in  a  little  hydrochloric  acid  and  the  paper  well  washed.    The  solu- 
tion is  boiled  until  the  chlorine  is  completely  expelled,  then  neutralized 
with  ammonia,  warmed,  and  the  trace  of  iron  filtered  off  immediately. 
Unless  the  precipitate  is  very  small  it  is  redissolved  in  hydrochloric  acid 
and  again  precipitated  with  ammonia  and  quickly  filtered  off  and  washed. 
It  is  ignited  and  weighed  as  oxide. 

202.  Manganese. — The  combined  filtrates   from  the  iron   contain  all  of 
the  manganese  unless  the  amount  of  iron  present  is  considerable.    The 
solution  should  be  evaporated  to  dryness  in  a  porcelain  dish  and  the  ammo- 
nium chloride  volatilized  by  gently  heating  with  the  Bunsen  burner.     The 
residue  is  dissolved  in  a  few  c.c.  water  and  a  few  drops  of  hydrochloric 
acid  and  the  manganese  precipitated  and  weighed  as  sulphide  as  directed 
in  Exercise  22. 

203.  Volumetric  Determination  of  Iron  and  Manganese. — If  considerable 
iron  is  present,  the  method  of  separation  given  is  not  applicable.     In  this 
case  the  simplest  methods  of  determining  the  two  metals  are  volumetric. 
The  ammonium  precipitate  should  be  dissolved  in  sulphuric  acid  with 


150  ANALYSIS  OF  ALLOYS.    ,^ 

expelled  by  boiling.  The  solution  must  be  made  up  to  a  definite  vol- 
ume and  divided  into  two  equal  portions.  For  this  purpose  a  100-c.c. 
flask  should  be  used  which  has  been  calibrated  with  a  50-c.c.  pipette 
by  emptying  the  pipette  twice  into  the  dry  flask  and  making  a  mark  on 
the  stem.  The  solution  of  iron  and  manganese  is  evaporated  to  small 
bulk,  transferred  to  the  flask,  made  up  to  the  mark  and  thoroughly 
mixed.  50  c.c.  are  withdrawn  with  the  dry  pipette.  The  solution 
adhering  to  the  walls  of  the  pipette  is  rinsed  out  with  distilled  water  and 
added  to  the  portion  remaining  in  the  flask.  One  of  these  portions  is  re- 
duced with  zinc  and  the  iron  titrated  with  standard  permanganate.  (See 
p.  316.)  The  other  portion  is  shaken  up  with  zinc  oxide  until  the  free 
acid  is  neutralized.  One  gram  of  zinc  sulphate  and  a  drop  or  two  of 
dilute  nitric  acid  are  added  and  the  solution  diluted  to  several  hundred 
cubic  centimeters.  The  manganese  is  titrated  with  standard  potassium 
permanganate  according  to  Volhard.  (See  p.  327.) 

204.  Phosphorus. — For  the  determination  of  phosphorus  a  5-gram  por- 
tion of  the  alloy  is  taken,  as  the  percentage  of  this  element  is  usually  small 
(seldom  more  than  0.2%).  The  material  is  placed  in  a  200-c.c.  beaker  and 
20  to  30  c.c.  concentrated  nitric  acid  added.  The  beaker  is  covered  with  a 
watch-crystal  and  after  the  first  violent  action  of  the  acid  has  ceased  it  is 
placed  on  the  water-bath  and  heated  until  the  alloy  is  completely  decom- 
posed and  the  residue  is  pure  white.  All  of  the  phosphoric  acid  will  remain 
with  the  tin  provided  a  sufficient  amount  of  the  latter  is  present  in  the  alloy. 
From  six  to  eight  times  as  much  tin  as  P205  must  be  present.  Unless  at  least 
5%  of  tin  has  been  found,  a  preliminary  test  should  be  made  by  dissolving 
about  a  gram  of  the  alloy  in  concentrated  nitric  acid,  filtering,  and  testing 
the  filtrate  for  phosphoric  acid  with  molybdate  mixture.  If  phosphoric 
acid  is  found  in  the  filtrate,  metallic  tin  must  be  added  before  dissolving 
the  alloy  in  nitric  acid.  From  one-half  to  one  gram  will  usually  be  found 
sufficient. 

The  nitric  acid  solution  of  the  alloy  is  diluted  and  the  stannic  oxide 
containing  the  phosphoric  acid  is  filtered  off  and  washed  a  few  times.  After 
drying,  the  precipitate  is  transferred  to  a  porcelain  crucible,  the  paper  is 
burned,  and  the  ash  added.  After  adding  three  times  its  weight  of  potassium 
cyanide,  cover  the  crucible  and  fuse  for  a  few  minutes  at  a  red  heat.  The 
stannic  oxide  is  reduced  to  metallic  tin  and  the  phosphoric  acid  forms  potas- 
sium phosphate.  After  cooling,  extract  the  fused  mass  with  hot  water,  filter, 
and  wash  the  paper  with  hot  water. 

Expel  the  hydrocyanic  and  cyanic  acids  by  boiling  with  concentrated 
hydrochloric  acid.  This  operation  must  be  conducted  under  a  hood  with 
good  draught.  Evaporate  to  dryness  to  dehydrate  the  silicic  acid  which 
has  been  dissolved  from  the  porcelain  by  the  action  of  the  potassium  cyanide. 
Dissolve  the  dry  residue  in  a  little  hydrochloric  acid  and  pass  hydrogen 
sulphide  to  precipitate  a  small  amount  of  tin  and  copper  which  is  present. 
Filter,  wash  the  precipitate,  and  destroy  the  hydrogen  sulphide  in  the  fil- 


ANALYSIS  OF  MANGANESE-PHOSPHORUS-BRONZE.         151 

trate  by  adding  bromine  water  and  boiling.  If  the  volume  of  the  solution 
exceeds  50  c.c.,  concentrate  by  boiling.  Cool  and  precipitate  the  phos- 
phoric acid  by  adding  about  one-half  gram  of  crystallized  magnesium 
chloride  or  sulphate  dissolved  in  a  little  water  and  then  neutralizing  the 
solution  with  filtered  ammonia  while  stirring  vigorously.  Add  a  small 
excess  of  ammonia.  Assure  yourself  that  the  phosphoric  acid  is  all  pre- 
cipitated by  adding  a  little  magnesia  mixture  to  the  clear  supernatant 
liquid.  After  standing  several  hours,  filter,  wash  with  dilute  ammonia, 
ignite  in  a  porcelain  crucible,  and  weigh  as  magnesium  pyrophosphate. 

The  precipitation  of  the  metals  present  with  hydrogen  sulphide  may 
be  omitted  and  the  separation  effected  by  precipitating  the  phosphoric 
acid  as  molybdate.  The  dry  residue  should  then  be  dissolved  in  nitric 
acid,  and  after  filtering  off  the  silica,  the  phosphoric  acid  is  precipitated  as 
directed  in  Chapter  IX,  page  118. 


ANALYSIS   OF  MINERALS. 

CHAPTER  XIII. 

MINERALS  CONTAINING  IRON,   ALUMINIUM, 
AND  CHROMIUM. 

SELECTION   AND  PREPARATION   OF   SAMPLE. 

205.  Taking  the  Sample  for  Analysis. — One  of  the  most  impor- 
tant operations  preliminary  to  many  chemical  analyses  is  the 
selection  of  the  sample  to  be  analyzed.  When  the  material  to  be 
analyzed  is  a  chemical,  such  as  the  various  salts  made  for  commercial 
purposes,  the  task  is  comparatively  simple  since  the  product  is 
usually  quite  uniform  and  small  grained.  Samples  are  taken 
from  a  number  of  the  barrels  or  other  receptacles.  In  many  cases 
it  is  advisable  to  take  samples  from  various  parts  of  the  barrel, 
such  as  the  end,  middle,  etc.  This  is  most  readily  accomplished 
by  means  of  a  "  butter-tryer  "  or  "  thief  "  or  some  similar  apparatus. 
These  various  samples  are  brought  together,  thoroughly  mixed, 
and  a  smaller  sample  taken  out.  This  may  be  done  by  taking 
small  portions  from  various  parts  of  the  pile  or  by  dividing  it 
into  quarters.  One  quarter  or  two  diagonal  quarters  are  taken, 
well  mixed,  and  the  sampling  repeated  in  the  same  manner  until 
a  convenient  quantity  for  analysis  is  obtained. 

Care  in  sampling  dry  and  apparently  uniform  products  is 
necessary  for  a  good  many  reasons.  Many  substances  when 
exposed  to  the  air,  as  in  the  outer  portions  of  a  barrel,  absorb  cr 
lose  moisture,  oxidize,  lose  volatile  constituents,  or  in  other  ways 
change  in  composition.  Even  in  the  case  of  a  mixture  of  two 
solid  substances  of  different  specific  gravity  the  heavier  constituent 
tends  to  settle  to  the  bottom,  so  that  after  the  most  thorough 
mixing  the  material  will  not  be  uniform  after  standing  for  some 
time. 

If  LIQUIDS  are  to  be  sampled  they  must  be  thoroughly  shaken 
or  parts  of  the  first,  middle,  and  last  runnings  of  a  cask  or  drum 

152 


SELECTION  AND  PREPARATION  OF  SAMPLE.  153 

must  be  taken.  These  portions  should  be  thoroughly  mixed  before 
taking  the  sample  for  analysis. 

METALS  and  ALLOYS  are  by  no  means  uniform,  since,  on  solidi- 
fying from  the  molten  state,  the  heavier  constituents  are  apt  to 
settle  to  the  bottom,  as  do  also  the  constituents  which  crystallize 
out  or  solidify  first.  In  the  case  of  a  bar  the  outer  portions  are 
not  always  of  the  same  composition  as  the  interior  of  the  metal. 
Drilling  through  a  piece  of  metal  with  a  clean  tool  is  therefore 
apt  to  give  the  best  sample.  If  the  metal  is  in  the  form  of  a  sheet, 
portions  should  be  punched  or  cut  from  the  outer  edges  as  well  as 
from  the  centre.  If  car-toads  of  metals  &s"pigs"  are  to  be  sam- 
pled, one  or  more  "pigs"  in  each  car  should  be  punched  or  drilled 
and  the  samples  combined  by  melting  or  mixing  before  the  labora- 
tory sample  is  taken.  The  turnings  of  the  more  brittle  metals 
may  be  pulverized  by  grinding  in  a  mortar  or  a  coffee-mill. 

If  a  ROCK  is  to  be  analyzed,  the  sample  is  usually  selected 
because  of  its  apparent  homogeneity.  The  physical  properties 
usually  give  sufficient  evidence  of  the  presence  of  foreign  material. 
If  the  rock  or  mineral  is  to  be  analyzed  with  a  view  to  its  use  as 
the  source  of  a  given  metal  or  other  constituent,  the  sample  must 
include  all  foreign  material  which  would  be  taken  out  in  mining 
operations. 

The  cars  or  shiploads  of  ORE  are  usually  sampled  by  taking 
out  shovelfuls  of  the  material  from  various  parts  of  the  load. 
These  portions  are  thrown  together  on  a  floor  or  other  convenient 
place.  The  larger  pieces  are  broken  with  a  hammer  and  the  material 
well  mixed.  ;JThe  pile  is  quartered  and  the  quarter  taken  is  still 
further  powdered  and  quartered.  This  process  is  continued  until 
a  sample  of  convenient  size  is  obtained.  About  25  grams  will 
usually  be  found  sufficient,  though  in  some  cases  as  much  as  a 
kilogram  will  be  required  for  the  determination  of  elements  present 
in  very  small  amount. 

206.  Pulverizing. — The  tools  used  in  breaking  off  and  pow- 
dering ores  or  rocks  include,  in  the  first  place,  HAMMERS  with 
hardened  surfaces.  These  are  made  in  various  shapes  and  sizes 
to  suit  the  geologist  prospecting  in  the  field  or  the  laboratory 
assistant  in  breaking  up  the  large  specimens  of  rock.  A  chilled 
IRON  PLATE  with  a  hardened  steel  MULLER  to  which  a  rocking 


154 


ANALYSIS  OF  MINERALS. 


motion  may  be  given  by  means  of  a  long  handle  may  be  used  to 
reduce  many  ores  and  rocks  to  powder  fine  enough  for  analysis. 


FIG.  21. 


Hardened  STEEL  MORTARS  are  also  useful  in  crushing  moderately 
small  pieces  of  ore  or  rock.     The  most  convenient  method  of 


^ J 


FIG.  22. 


crushing  large  samples  of  rock  or  ore  is  by  the  use  of  ORE-CRUSHERS. 
The  jaws  between  which  the  ore  is  placed  are  actuated  by  means 


FIG.  23. 


of  a  long  handle  so  connected  with  levers  that  a  very  great  crush- 
ing pressure  may  be  brought  to  bear  on  the  material.  It  is  cus- 
tomary to  remove  by  means  of  a  magnet  particles  of  iron  which 


SELECTION  AND  PREPARATION  OF  SAMPLE.  155 

have  been  detached  from  the  tools  during  the  process  of  pulverizing. 
Very  few  rocks  or  ores  are  entirely  free  from  magnetic  particles,  so 
that  this  method  of  procedure  is  open  to  objection. 

Hard  rocks  and  ores,  as  well  as  those  known  to  contain  iron, 
should  be  pulverized  as  far  as  possible  in  AGATE  MORTAES.  These, 
as  well  as  the  iron  mortars,  are  frequently  arranged  to  be  oper- 
ated by  mechanical  means.  Many  chemists,  however,  still  prefer 
the  hand  grinding. 

207.  Sifting. — The  requisite  degree  of  fineness  is  frequently 
secured  by  passing  the  powdered  material  through  sieves  of  the 
requisite  size  of  mesh.  Eighty-mesh  BRASS  SIEVES  are  suitable  for 
most  ores.  In  special  cases  where  the  sample  is  very  irregular, 
sieves  of  100  to  120  mesh  to  the  square  inch  should  be  used.  Even 
when  kept  scrupulously  clean  a  small  amount  of  the  metal  of  the 
sieve  is  apt  to  contaminate  the  sifted  material.  This  error  may 
be  entirely  disregarded  if  a  constituent  present  in  very  small 
amount  is  to  be  determined.  When  a  complete  analysis  is  to  be 
made,  many  chemists  prefer  to  use  BOLTING-CLOTH  or  FINE  LINEN. 
Where  the  state  of  oxidation  of  the  iron  is  to  be  determined,  the 
shreds  of  the  cloth,  which  almost  necessarily  contaminate  the 
sifted  material,  undoubtedly  influence  the  result  to  an  appreciable 
degree.  For  this  reason  some  chemists  advocate  grinding  the 
material  without  sifting  at  all. 

In  sifting  through  cloth  large  particles  are  readily  forced 
through  if  the  powder  is  rubbed  over  the  cloth.  A  wide-mouthed 
bottle  should  be  selected,  thoroughly  cleaned  and  dried.  The 
cloth  should  be  stretched  over  the  mouth  of  the  bottle  and 
fastened  securely  by  winding  a  string  around  the  neck.  The 
material  to  be  sifted,  after  having  been  ground  in  small  portions 
in  an  agate  mortar,  is  placed  on  the  cloth  and  a  piece  of  sheet 
rubber  or  leather  stretched  over  the  mouth  of  the  bottle  and 
held  securely  with  the  hand  or  by  a  rubber  band  placed  around 
the  neck.  The  rubber  or  leather  is  now  lightly  tapped  with 
a  spatula  until  no  more  fine  material  sifts  into  the  bottle. 
The  coarser  material  is  again  ground  hi  the  mortar  and  sifted. 
In  no  case  should  the  coarse  material  be  discarded.  Frequently 
the  more  friable  portions  of  a  rock  or  ore  are  different  in  compo- 
sition from  the  particles  which  resist  grinding  longest.  In  pul- 


156  ANALYSIS  OF  MINERALS. 

verizing  a  sample  the  rule  must  be  strictly  adhered  to  that  the 
whole  of  a  portion  taken  for  powdering  must  be  ground  and  passed 
through  the  sieve.  In  sampling,  the  large  pieces  must  be  broken 
completely  so  as  to  mingle  with  the  finer  material  and  constitute  a 
fair  portion  of  the  sample  finally  taken. 

208.  Drying. — After  being   reduced   to   a  sufficient   state   of 
fineness,  the  material  to  be  analyzed  is  frequently  dried  at  a  tem- 
perature of  about  100°  to  remove  hygroscopic  moisture  before 
being  weighed  out.     Other  workers  weigh  out  the  air-dried  mate- 
rial and  determine  the  percentage  of  water  lost  at  100°.      It  is 
held  that  the  amount  of  hygroscopic  water  present  is  a  very  char- 
acteristic property  of  some  rocks  and  minerals,   and  serves  to 
distinguish  minerals  which  are  in  other  respects  identical.     Hygro- 
scopic material  can  also  be  weighed  out  much  more  accurately 
when  air-dried  than  when  entirely  desiccated.     In  the  latter  case 
it  gradually  absorbs  moisture,  as  it  is  exposed  to  the  air  from  time 
to  time  in  the  course  of  the  analysis,  so  that  the  material  weighed 
out  the  first  day  is  not  identical  in  composition  with  that  weighed 
out  later* 

SEPARATION  OF  IRON,  ALUMINIUM,  AND  CHROMIUM. 

Iron  and  aluminium  occur  very  frequently  together,  and  are 
generally  precipitated  together  from  solutions  containing  man- 
ganese, cobalt,  nickel,  and  zinc.  The  methods  suitable  for  pre- 
cipitating these  two  metals  are  not  generally  applicable  to  the 
complete  precipitation  of  chromium.  The  separation  of  iron  and 
aluminium  will  therefore  be  first  considered,  and  then  that  of 
chromium. 

209.  Separation  as  Hydroxide. — In  the  presence  of  excess  of 
ammonium  chloride  and  ammonia,  nickel,  cobalt,  and  zinc  are 
not  at  all,  while  manganese  is  only  slowly  precipitated  by  being 
oxidized  to  the  hydrated  sesquioxide,  while  ferric  iron,  aluminium 
and  chromium  are  very  nearly  completely  precipitated.     If  all 
of  these  metals  are  present  in  solution,  the  iron,  aluminium,  and 
chromium  may  be  completely  precipitated  by  means  of  ammonia 
and  ammonium  chloride,  but  the  precipitate  will  be  contaminated 
with  considerable  amounts  of  manganese  and  varying   amounts 
of   zinc,  nickel,  and   cobalt.     In   the   absence   of   chromium,  by 


SEPARATION  OF  IRON,  ALUMINIUM,  AJfD  CHROMIUM.     157 

redissolving  and  reprecipitating  the  iron  and  aluminium  once  or 
twice,  the  separation  from  nickel  and  cobalt  may  be  made  com- 
plete and  very  nearly  so  from  zin3. 

Only  a  small  part  of  the  other  metals  present  in  the  solution 
will  be  carried  down  each  time  with  the  iron  and  aluminium  pre- 
cipitate. On  dissolving  the  first  precipitate  only  this  small  part 
of  the  contaminating  metals  will  be  present  in  the  solution;  the 
bulk  of  these  metals  will  be  in  the  filtrate.  On  making  a  second 
precipitation  the  fraction  carried  down  will  be  very  small  indeed, 
so  that  on  a  third  precipitation  the  amount  carried  down  may 
be  wholly  neglected.  This  process  will  not  free  the  precipitate 
from  manganese,  because  this  metal  is  oxidized  very  readily 
hi  alkaline  solution  to  the  insoluble  dioxide. 

The  solution  should  be  heated,  excess  of  ammonia  added,  and 
if  aluminium  is  present  ammonium  carbonate  added.  After 
digesting  hot  for  some  time  the  precipitate  is  filtered  off,  washed 
a  few  times,  redissolved  in  hydrochloric  acid,  and  the  operation 
repeated.  If  the  amount  of  nickel  and  cobalt  is  considerable,  a 
third  precipitation  is  necessary.  Manganese  must  be  absent. 
The  precipitate  may  be  ignited  and  weighed,  and  one  of  the  con- 
stituents determined,  the  other  one  being  obtained  by  differ- 
ence. 

210.  Separation  as  Basic  Acetates. — If  manganese  and  zinc 
are  present,  the  aluminium  and  iron  must  be  precipitated  as  basic 
acetates.  This  method  of  separation  is  based  on  the  fact  that 
iron  and  aluminium  may  be  precipitated  as  basic  acetates  by 
boiling  a  solution  of  these  metals  which  contains  ammonium  or 
sodium  acetate  and  free  acetic  acid.  The  acid  present  prevents 
the  precipitation  of  manganese,  zinc,  cobalt,  and  nickel.  The 
separation  is  therefore  much  sharper  than  when  the  iron  and 
aluminium  are  precipitated  from  alkaline  solution.  Oxidizing 
agents  must  be  absent  or  manganese  will  be  precipitated,  but  the 
iron  must  be  in  the  ferric  condition. 

The  proportion  of  free  acetic  acid  to  ammonium  acetate  must 
be  carefully  regulated.  They  should  be  present  in  gram-molecular 
proportion.  As  the  commercial  ammonium  acetate  is  not  always 
neutral  it  should  be  tested  before  use  and  neutralized.  The  sim- 
plest way  of  securing  the  proper  proportion  of  free  acid  and  neu- 


158  ANALYSIS  OF  MINERALS. 

tral  salt  is  to  make  the  ammonium  acetate  by  neutralization  of 
acetic  acid.  If  the  same  volume  of  acetic  acid  is  neutralized  as 
has  already  been  added  to  the  solution  the  correct  amount  of 
ammonium  acetate  will  be  secured.  The  volume  of  the  solution 
must  also  be  carefully  regulated  with  reference  to  the  amount  of 
iron  present. 

The  perfectly  cold  hydrochloric  acid  solution  must  be  made 
very  nearly  neutral  by  the  addition  of  concentrated  and  then 
dilute  ammonium  carbonate  solution  until  the  slightest  turbidity 
is  noticed.  If  a  precipitate  forms  it  must  be  dissolved  in  hydro- 
chloric acid,  and  the  solution  again  cautiously  neutralized.  One  c.c. 
of  5  N  acetic  acid  is  added  for  each  0.1  gram  of  iron.  Water  is 
added  until  at  least  150  c.c.  is  present  for  each  0.1  gram  of  iron. 
Heat  to  boiling  hi  a  porcelain  dish,  and  add  1  c.c.  of  5  N  acetic 
acid  which  has  been  exactly  neutralized  with  ammonia  for  each 
0.1  gram  of  iron  present.  Continue  the  boiling  for  two  or  three 
minutes.  After  allowing  the  precipitate  to  settle,  decant  the 
liquid  and  wash  by  decantation  a  few  times,  bringing  the  wash- 
water  to  a  boil  each  time  before  decantation.  The  precipitate  is 
free  from  zinc  and  all  but  a  trace  of  manganese,  but  if  nickel  and 
cobalt  are  present  it  is  apt  to  contain  a  little  of  these  metals, 
more  of  the  former  than  of  the  latter.  It  may  be  freed  from 
these  metals  by  redissolving  and  reprecipitating.  If  the  filtrate 
has  a  slight  yellow  color  the  iron  has  not  been  completely  precipi- 
tated because  of  the  presence  of  too  much  acetic  acid.  A  few 
drops  of  ammonia  may  be  added  and  the  solution  warmed  until 
the  precipitate  collects.  It  is  washed  with  hot  water  containing 
a  little  acetic  acid  and  ammonium  acetate. 

This  method  gives  better  results  with  iron  than  with  alu- 
minium, unless  iron  is  also  present  in  considerable  amount.  Small 
amounts  of  chromium  may  also  be  precipitated  by  this  method 
if  considerable  iron  is  present.  The  precipitate  cannot  be  ignited 
and  weighed .  if  alkalies  were  present  in  the  solution  because  it 
carries  down  alkalies  which  cannot  be  washed  out.  It  must  be 
dissolved  in  hydrochloric  acid  and  reprecipitated  with  ammonia. 

The  method  has  the  disadvantage  that  the  precipitate  is  very 
bulky,  and  is  frequently  slimy  and  difficult  to  wash.  For  this 
reason  it  is  not  used  when  a  large  amount  of  iron  must  be  handled. 


SEPARATION  OF  IRON,  ALUMINIUM,  AND  CHROMIUM.     159 

211.  Separation  of  Iron  and  Aluminium  as  Basic  Carbonates.^ 
Iron  and  aluminium  may  also  be  separated  from  manganese, 
cobalt,  nickel,  and  zinc,  by  precipitation  of  the  former  as  basic 
carbonates.    The  hydrochloric  acid  solution  is  carefully  neutral- 
ized with  ammonium  carbonate,  until  a  slight  turbidity  appears. 
A  drop  or  two  of  acetic  acid  is  added.    Ammonium  chloride  must 
be  present  to  the  extent  of  about  1  gram  for  each  0.1  gram  of  iron. 
The  solution  is  gradually  brought  to  a  boil  and  the  heating  con- 
tinued until  the  carbon  dioxide  is  completely  expelled.     The  pre- 
cipitate is  washed  by  decantation,  a  few  drops  of  ammonia  being 
added  to  the  wash-water.     A  slight  yellow  color  in  the  filtrate 
indicates  incomplete  precipitation  of  iron.     A  few  drops  of  ammo- 
nia may  be  added,  but  care  must  be  taken  not  to  make  the  solution 
alkaline  or  manganese  will  be  precipitated. 

212.  Separation  of    Chromium  as  Chromic  Acid. — Chromium 
is  best  separated  from  other  metals  by  methods  which  are  based 
on  the  acid  properties  of  chromium  trioxide.     It  is  converted  into 
the  acid  oxide  by  treatment  with  an  alkaline  oxidizing  agent.     A 
mixture  of  sodium  carbonate  and  potassium  nitrate  has  been  largely 
used  for  this  purpose.     On  extracting  the  fused  mass  with  water 
the  oxides  of  zinc,  cobalt,  nickel,  and  iron  remain  undissolved. 
Part  of  the  manganese  remains  undissolved  while  part  goes  into 
solution  as  manganate.     On  boiling  the  solution  after  the  addi- 
tion of  a  little  alcohol  the  manganese  is  completely  precipitated 
as  the  dioxide.     Part  of  the  aluminium  remains  with  the  insoluble 
portion,  while  part  passes  into  solution  with  the  sodium  chromate 
as  sodium  alumina te.      The  aluminium  may  be  separated  from 
the  chromium  by  precipitation  as  aluminium  hydroxide.     The 
solution  must  be  acidified  with  hydrochloric  acid  to  decompose 
the    sodium    aluminate.     On    adding    ammonium    chloride    and 
ammonia  until  the  solution  is  faintly  alkaline  and  warming,  the 
aluminium  is  completely  precipitated  as  hydroxide.     The  chro- 
mium in  the  condition  of  chromic  acid  is  not  precipitated  by  the 
ammonia. 

The  potassium  nitrate  used  to  oxidize  the  chromium  is  reduced 
during  the  fusion  to  nitrite  which  is  present  in  the  water  solution 
of  the  melt.  On  acidifying  this  solution,  nitrous  acid  is  liberated 
which  tends  to  reduce  the  chromic  acid.  To  destroy  the  nitrous 


160  ANALYSIS  OF  MINERALS. 

acid  potassium  chlorate  or  still  better  liquid  bromine  is  added  to 
the  alkaline  solution.  On  acidifying  and  warming  gently,  the 
nitrous  acid  is  oxidized  and  the  excess  of  bromine  or  chlorine 
expelled.  The  chromium  in  the  filtrate  from  the  aluminium  may 
be  precipitated  and  weighed  as  lead  or  barium  chromate.  It 
may  also  be  reduced  by  sulphurous  acid,  precipitated  with  ammo- 
nia and  weighed  as  oxide,  or  it  may  be  determined  volumetrically 
by  means  of  ferrous  sulphate  and  a  standard  chromate  solution. 

Recently  fusion  with  SODIUM  PEROXIDE  has  come  into  exten- 
sive use.  This  flux  seems  to  be  especially  adapted  to  decompose 
the  refractory  chrome  iron  ore.  A  silver,  nickel,  or  copper  cruci- 
ble must  be  used,  as  platinum  is  strongly  attacked  by  the  alkaline 
flux.  A  mixture  of  5  to  6  parts  of  caustic  soda  with  3  to  4  parts 
of  sodium  peroxide  has  also  been  considerably  used. 

If  the  chromium  is  in  solution  with  other  metals  the  separation 
is  not  so  simple.  The  acid  solution  is  nearly  neutralized  with 
sodium  carbonate,  sodium  acetate  added,  the  solution  heated, 
and  chlorine  passed  or  bromine  added.  The  chromium  is  oxidized 
to  chromic  acid  and  remains  in  solution.  Iron  and  aluminium 
are  precipitated  during  the  boiling  by  the  sodium  acetate.  Any 
manganese  present  is  precipitated  as  the  peroxide  together  with 
part  of  the  cobalt,  nickel,  and  zinc. 

213.  Rothe's  Ether  Separation  of  Iron. — The  method  pro- 
posed by  J.  W.  Rothe  for  the  separation  of  iron  from  chromium, 
aluminium,  manganese,  cobalt,  nickel,  and  copper  has  been  much 
used,  and  overcomes  many  difficulties  met  with  in  analyzing 
compounds  containing  much  iron.  It  depends  on  the  fact  that 
ferric  chloride  may  be  almost  completely  extracted  by  means  of 
ether  from  a  hydrochloric  acid  solution  of  sp.  gr.  1.100  to  1.105 
containing  the  metals  mentioned.  Nitric  acid,  chlorine,  and 
more  than  small  amounts  of  sulphuric  acid  must  be  absent,  and 
the  solution  must  be  kept  absolutely  cold,  as  in  a  warm  solution 
the  ferric  chloride  is  reduced  by  the  ether.  All  suspended  matter 
such  as  silica,  carbon,  etc.,  must  be  removed  by  filtration  The 
apparatus  designed  for  this  separation  is  shown  in  Fig  24.  The 
two  bulbs  A  and  B  are  connected  by  a  three-way  stop-cock  at  E^ 
so  that  liquid  may  flow  from  one  bulb  to  the  other  or  from  either 
bulb  through  the  exit-tube  to  a  receptacle  placed  below  II  the 


SEPARATION  OF  IRON,  ALUMINIUM,  AND  CHROMIUM.     161 

hydrochloric  acid  solution  of  the  iron  is  placed  hi  A,  about  an 
equal  volume  of  ether  is  placed  hi  B.  A  slight  pressure  is  pro- 
duced over  the  ether  by  means  of  a  rubber  bulb.  By  cautiously 
turning  the  stop-cock  E  into  the  position  2,  the  ether  is  allowed  to 
flow  into  the  bulb  A.  The  liquids  are  well  mixed  by  vigorous 


FIG.  24. 

shaking.  During  this  operation  it  is  well  to  keep  the  bulb  A  well 
cooled  by  holding  it  in  a  stream  of  cold  water.  After  allowing  a 
few  minutes  for  the  liquids  to  separate,  the  stop-cock  E  is  turned  so 
that  the  lower  heavy  liquid  may  flow  into  B.  Leave  just  enough  of 


162  ANALYSIS  OF  MINERALS. 

this  liquid  to  fill  the  capillary  tube  (7,  shake  down  any  heavy  liquid 
adhering  to  the  sides  of  A,  and  allow  to  stand  until  the  ethereal 
layer  is  clear.  Again  connect  A  and  B  by  turning  the  stop-cock  C, 
and  transfer  the  remainder  of  the  heavy  liquid  to  B  together 
with  a  little  of  the  ether  solution.  Turn  the  stop-cock  E  into  the 
position  3,  and  allow  the  ether  solution  to  flow  out  into  a  beaker. 
Rinse  the  walls  of  the  bulb  A  with  a  little  ether,  and  after  stand- 
ing a  few  minutes  observe  if  any  of  the  heavy  solution  collects  in 
C.  If  so,  transfer  as  before  to  B.  Repeat  the  extraction  with 
50  c.c.  of  ether.  (See  Exercise  35,  page  170.) 

214.  Volumetric    Separation    of   Iron    and   Aluminium.  —  The 
iron   and   aluminium  precipitate   obtained   in   separations  from 
other  metals  is  best  ignited  and  weighed  as  Al203+Fe203  and  one 
of  the  metals   determined,  the  other  being  obtained  by  differ- 
ence.   For  this  purpose  it  is  simplest  to  determine  the  iron  volu- 
me trically.    The  precipitate  is  fused  with  acid  potassium  sul- 
phate until  dissolved,  the  fused  mass  dissolved  in  water,  and  the 
solution  filtered  to  separate  a  small  amount  of  silica  which  may 
be  present.    This  is  ignited  and  weighed,  and  the  amount  deducted 
from  the  weight  of  the  oxides.     The  solution  of  the  iron  and  alu- 
minium is  treated  with  zinc  to  reduce  the  iron,  which  is  titrated 
with  standard  potassium  permanganate  solution  as  directed  in 
Chapter  XXIV,  page  316.    If  the  precipitate  is  large  it  may  be 
dissolved  in  hydrochloric  acid  before  weighing  and  divided  into 
two  portions.    The  iron  is  determined  in  one  portion  while  in  the 
other  both  metals  are  precipitated  as  hydroxides,  ignited,  and 
weighed  as  oxides. 

215.  Separation  of  Iron  from  Aluminium  as  Aluminate. — The 
two  metals  may  also  be  separated  by  means  of  boiling  caustic 
potash  or  soda  solution.     The  hydrochloric  acid  or  potassium 
bisulphate  solution  of  the  two  metals  is  nearly  neutralized,  and 
poured  slowly  with  constant  stirring  into  excess  of  caustic  potash  or 
soda  solution,  heated  nearly  to  boiling  in  a  platinum  or  silver  dish. 
The  iron  is  precipitated  as  hydroxide,  while  the  aluminium  remains 
in  solution.    The  iron  precipitate  is  washed  more  readily  if  part 
of  it  is  reduced  by  adding  a  little  sodium  sulphite  and  heating 
before  pouring  into  the  boiling  alkaline  solution.    Before  weighing, 
the  iron  precipitate  must  be  dissolved  in  hydrochloric  acid  and  re- 


ANALYSIS  OF  CHROMITE.  163 

precipitated  with  ammonia  to  free  it  from  the  alkali  which  is 
always  present. 

The  aluminium  in  the  filtrate  is  precipitated  by  acidifying 
with  hydrochloric  acid  and  just  neutralizing  with  ammonia  and 
warming.  The  caustic  potash  or  soda  must  be  free  from  impuri- 
ties which  are  precipitated  by  ammonia  and  ammonium  chloride. 
If  pure  caustic  cannot  be  obtained  by  solution  of  the  metal  in 
water  in  a  platinum  or  silver  dish  or  other  method,  a  blank  deter- 
mination of  the  amount  of  impurities  present  must  be  made. 

EXERCISE  34. 

Analysis  of  Chromite  (FeO,MgO)(Cr2O3,Al2O8). 
Silica  and  Small  Amounts  of  Manganese  are  Generally  Present. 

This  ore  is  chiefly  of  value  for  its  content  of  chromium.  The  deter- 
mination of  the  amount  of  this  element  is  the  most  important  and  frequently 
the  only  determination  required.  For  this  reason,  and  also  for  convenience 
in  determining  the  amount  of  the  other  elements  present,  a  separate  portion 
is  taken  for  the  determination  of  chromium. 

216.  Solution  of  Ore. — Weigh  one-half  gram  of  the  finely  pulverized  ore 
and  mix  in  a  small  copper  or  nickel  crucible  with  2  to  5  grams  of  powdered 
sodium  peroxide.     Brush  off  carefully  the  platinum  or  glass  rod  used  for 
mixing  the  material.     If  the  sodium  peroxide  is  not  pure  yellow  and  fresh, 
the  larger  amount  should  be  taken,  as  it  decomposes  rapidly,  forming  sodium 
carbonate.      Heat  the  crucible  with  a  small  flame  of  the  Bunsen  burner, 
so  that  the  contents  are  completely  fused  only  after  several  minutes.     Keep 
the  material  fused  for  about  ten  minutes. 

When  cold,  place  the  crucible  in  a  porcelain  dish  and  dissolve  the  con- 
tents in  50  to  100  c.c.  of  hot  water.  Boil  for  fifteen  to  twenty  minutes  to 
decompose  the  sodium  peroxide. 

217.  Chromium. — Filter  off  and  wash  the  precipitate  consisting  of  iron, 
magnesium,  manganese  and  some  of  the  silica,  as  well  as  a  little  nickel  or 
copper  from  the  crucible.     The  filtrate  will  contain  all  of  the  chromium 
and  aluminium  and  part  of  the  silica.     Acidify  the  filtrate  with  acetic  acid, 
warm  and  filter  off  any  alumina  and  silica  which  separates  out.     Precipi- 
tate the  chromic  acid  by  adding  lead  acetate  or  barium  chloride  and  weigh  as 
lead  or  barium  chromate.     The  chromium  may  also  be  determined  volu- 
metrically  as  directed  in  Exercise  63,  page  333.    This  volumetric  method 
will  usually  be  found  superior  to  the  gravimetric  method. 

218.  Iron. — The  insoluble  material  filtered  off  from  the  solution  of  the 
sodium  peroxide  fusion  may  be  used  for  the  determination  of  iron.     It  is 
dissolved  in  a  little  sulphuric  acid,  the  material  adhering  to  the  crucible 


164  ANALYSIS  OF  MINERALS. 

being  dissolved  in  the  same  manner.  Particles  of  undecomposed  chromite 
may  be  distinguished  from  the  gelatinous  silicic  acid  as  a  fine  heavy  powder 
which  quickly  settles  to  the  bottom  of  the  beaker  en  stirring  the  solution. 
If  material  of  this  kind  is  present  it  must  be  filtered  off  and  again  fused  with 
sodium  peroxide.  The  solution  of  the  melt  must  be  filtered  and  the  filtrate 
added  to  the  bulk  of  the  chromium  solution.  The  insoluble  residue  is 
dissolved  in  hydrochloric  acid  and  added  to  the  solution  of  the  insoluble 
material  from  the  first  fusion.  The  iron  may  then  be  titrated  as  directed  in 
Exercise  57,  page  320. 

219.  Silica. — For  the  determination  of  the  elements  present  other  than 
chromium,  a  separate  portion  of  one-half  to  one  gram  is  fused  with  sodium 
peroxide,  in  the  same  manner  as  directed  for  the  determination  of  chromium. 
The  fused  mass  is  dissolved  in  water,  boiled  to  decompose  the  sodium  per- 
oxide and  the  insoluble  matter  filtered  off  and  washed  free  from  chromium. 
The  filtrate  is  acidified  with  hydrochloric  acid,  evaporated  to  dryness  and 
heated  one-half  hour  on  the  water-bath  to  dehydrate  the  silicic  acid,  which 
is  filtrated  off  and  washed. 

220.  Aluminium.  —  The   filtrate   will   contain   the   aluminium    and   the 
chromium.     It  is  neutralized  with  ammonium  carbonate  and  a  little  hydro- 
gen peroxide  or  bromine  added,  and  the  solution  boiled  to  reoxidize  the 
chromium  which  has  been  reduced  during  the  evaporation  with  hydro- 
chloric acid.     The  solution  should  be  boiled  in  a  porcelain  or  platinum 
dish  to  avoid  solution  of  the  glass  of  the  beaker  in  the  alkaline  liquid.     The 
aluminium  hydroxide  is  filtered  off,  washed  free  from  chromium,  ignited, 
and  weighed  as  oxide. 

The  chromium  and  aluminium  may  also  be  precipitated  together  and 
weighed  as  oxides,  the  amount  of  aluminium  being  found  by  deducting 
the  weight  of  chromium  oxide  as  obtained  by  the  previous  determination 
of  this  element.  The  filtrate  from  the  silica  is  warmed  after  the  addition 
of  alcohol  or  sulphurous  acid  to  completely  reduce  the  chromium.  The 
aluminium  and  chromium  are  then  precipitated  with  ammonia  and  weighed 
as  oxides. 

221.  Silica. — The  material  insoluble   in  the  sodium  peroxide  solution, 
which  contains  the  manganese,  magnesium,  and  part  of  the  silica,  is  dis- 
solved in  warm  hydrochloric  acid  and  any  undecomposed  chromite  filtered 
off   and    again    fused    with    sodium  peroxide.     The  acid  solution  of  the 
insoluble  matter  is  evaporated  to  dryness  on  the  water-bath  to  dehydrate 
the  silica  which  is  filtered  off,  washed,  and  weighed,  the  weight  being  added 
to  that  of  the  silica  already  found,  unless  the  same  filter-paper  was  used 
for  both  precipitates. 

222.  Manganese. — If  a  copper  crucible  was  used,  hydrogen  sulphide  is 
passed  through  the  acid  filtrate  from  the  silica  to  precipitate  the  copper  dis- 
solved, which  is  filtered  off  and  washed.     Bromine  water  or  liquid  bromine 
dissolved  in  hydrochloric  acid  is  added  to  the  filtrate  to  oxidize  the  iron  and 
to  decompose  the  excess  of  hydrogen  sulphide.     Sufficient  bromine  is  added 


ANALYSIS  OF  CHROMITE.  165 

to  color  the  solution  distinctly.  Ammonia  is  added  until  the  reaction  is 
distinctly  alkaline.  The  solution  is  warmed  and  the  precipitate  consisting 
of  the  iron  and  any  manganese  which  may  be  present  is  filtered  off  and 
washed.  It  may  be  ignited  and  weighed,  the  manganese  being  obtained  by 
difference,  the  amount  of  iron  being  known.  The  manganese  will  be  present 
as  Mn3O4.  If  the  amount  of  manganese  i^  small  it  may  be  determined 
more  accurately  by  Ford's  method.  The  precipitate  is  dissolved  in  a 
little  nitric  acid  and  the  determination  carried  out  as  directed  in  Chapter 
XXVIII,  page  385. 

223.  The  magnesium  will  be  found  in  the  filtrate  from  the  iron  precipi- 
tate. It  is  evaporated  to  small  bulk,  precipitated  as  magnesium  ammo- 
nium phosphate,  and  weighed  as  pyrophosphate. 


CHAPTER  XIV. 

ANALYSIS   OF   SULPHIDES   CONTAINING  MAN- 
GANESE, NICKEL,  COBALT,  AND  MERCURY. 

SEPARATION  OF  MANGANESE,  NICKEL,  AND  COBALT. 

224.  Separation  of  Nickel  and  Cobalt  as  Sulphides. — One  of 
the  best  methods  of  separating  manganese  from  nickel  and  cobalt 
depends  on  the  properties  of  the  sulphides  of  these  metals.  The 
sulphide  of  manganese  is  readily  soluble  in  dilute  acids,  including 
acetic  acid.  The  sulphides  of  nickel  and  cobalt  after  precipitation 
are  almost  insoluble  in  all  dilute  acids.  These  two  metals  are 
not  precipitated,  however,  from  solutions  acid  with  mineral  acids, 
but  do  come  down  from  warm  acetic-acid  solution. 

The  acid  solution  of  manganese,  cobalt,  and  nickel  from  which 
zinc,  iron,  aluminium,  etc.,  have  been  separated  by  methods 
already  given,  is  concentrated  to  a  small  bulk  (30  to  50  c.c.). 
The  solution  is  made  alkaline  with  sodium  carbonate,  acidified 
with  acetic  acid  and  3  to  4  grams  of  sodium  acetate  dissolved  in 
a  little  water  are  added.  The  solution  is  heated  to  70°,  and  hydro- 
gen sulphide  passed  until  saturated.  The  sulphides  of  nickel  and 
cobalt  are  filtered  off  and  washed.  The  filtrate  and  washings 
are  concentrated,  a  little  ammonium  sulphide  and  then  acetic 
acid  are  added,  and  hydrogen  sulphide  again  passed.  A  little 
more  of  the  cobalt  and  nickel  sulphides  is  frequently  obtained. 
A  second  test  of  the  filtrate  in  this  manner  sometimes  yields  a 
small  precipitate. 

Another  method  of  carrying  out  this  separation  which  requires 
but  one  precipitation  is  as  follows:  The  sulphate  solution  of  man- 
ganese, cobalt,  and  nickel  is  transferred  to  a  pressure-bottle  of 
about  one-half  liter  capacity.  The  solution  is  neutralized  with 
ammonia,  30  c.c.  ammonium-acetate  solution  (1  to  10),  and  20  c.c. 
50%  acetic  acid  added.  The  solution  is  diluted  to  300  to  400  c.c., 
and  hydrogen  sulphide  passed  for  one  to  two  hours.  The  bottle 

is  closed,  placed  hi  a  water-bath  containing  cold  water  which  is 

166 


SEPARATION  OF  MANGANESE,  NICKEL,  AND  COBALT       167 

brought  to  a  boil  in  one  hour.  The  bottle  is  allowed  to  cool  in 
the  water-bath  to  50°.  The  precipitated  sulphides  are  filtered 
off  and  washed  with  water  containing  acetic  acid  and  hydrogen 
sulphide. 

225.  Electrolytic  Separation   of   Manganese   from   Cobalt   and 
Nickel. — Small  amounts  of  manganese  can  readily  be  separated 
from  cobalt  and  nickel  electrolytically.     The  sulphate  solution  of 
the  three  metals  should  be  treated  with  30  c.c.  of  a  cold-saturated 
ammonium-sulphate   solution,    and    30   to   50   c.c.   of   ammonia 
(sp.  gr.  0.96).     The  solution  is  diluted  to  about  150  c.c.,  and  elec- 
trolyzed  with  a  current  of  about  0.7  ampere  at  the  ordinary  tem- 
perature.   The  manganese  separates  as  dioxide  on  the  anode. 
This  precipitate  carries  down  small  amounts  of  nickel  and  cobalt 
unless  these  metals  are  deposited  before  the  manganese  dioxide 
forms.    Small  amounts  of  nitrates  and  chlorides  hinder  the  pre- 
cipitation of  nickel  and  cobalt.     The  deposited  metal  is  washed 
with  water  without  interrupting  the  current.    Finally  it  is  washed 
with  alcohol,  dried  in  the  air-bath,  and  weighed. 

SEPARATION  OF   NICKEL  AND   COBALT. 

226.  Precipitation  of  Cobalt  as  Tripotassium  Cobaltic  Nitrite, 
Co(N02)3.3KN02,  separates  it  from  nickel  as  well  as  from  man- 
ganese and  zinc.     The  alkaline  earth-metals  and  iron  must  be 
absent.     The  method  is  satisfactory  for  mixtures  of  nickel  and 
cobalt  in  all  proportions,  but  is  especially  suited  for  the  separation 
of  a  little  cobalt  from  much  nickel.    The  solution  of  the  metals  is 
evaporated  to  small  bulk,  and  neutralized  with  potassium  hydrox- 
ide.   About  5  grams  of  potassium  nitrite  dissolved  hi  water  are 
added.     Acetic  acid  is  added  until  the  solution  is  acid  and  nitrous 
fumes  are  evolved.     It  is  allowed  to  stand  for  at  least  twenty- 
four  hours.    A  portion  of  the  supernatant  liquid  is  taken  out  and 
tested  for  cobalt  by  adding  more  potassium  nitrite.     If  a  precipi- 
tate forms  after  long  standing,  more  potassium  nitrite  must  be 
added  to  the  mam  solution.    The  precipitate  is  filtered  off  and 
washed  with  a  solution  of  potassium  acetate  (1  to  10)  containing 
a  little  potassium  nitrite.    The  precipitate  may  be  dissolved  in 
hydrochloric  acid  and  the  cobalt  reprecipitated  with  caustic  soda 


168  ANALYSIS  OF  MINERALS. 

and  weighed  as  oxide.  It  may  also  be  dissolved  in  dilute  sul- 
phuric acid  and  the  solution  evaporated  to  dryness  in  a  weighed 
platinum  dish  or  crucible.  The  residue  consisting  of  the  sulphates 
of  cobalt  and  potassium  is  heated  to  dull  redness  and  weighed. 
If  weighed  in  this  manner  the  potassium  acetate  and  nitrite  must 
finally  be  washed  out  with  a  little  distilled  water.  The  precipi- 
tate may  also  be  dissolved  in  hot  dilute  sulphuric  acid,  the  solution 
concentrated,  treated  with  excess  of  ammonia,  ammonium  sulphate 
added,  and  the  cobalt  determined  electrolytically. 

If  the  NICKEL  has  previously  been  weighed  with  the  cobalt,  it 
may  be  obtained  by  difference.  Otherwise  the  nickel  may  be 
determined  in  the  filtrate  from  the  cobalt.  For  this  purpose  the 
solution  is  treated  with  hydrochloric  acid  and  the  nickel  precipi- 
tated with  caustic  soda  and  bromine  water.  The  nickelic  hydrox- 
ide may  be  filtered  off,  washed,  and  reduced  to  metallic  nickel  by 
heating  in  a  stream  of  hydrogen,  or  it  may  be  dissolved  in  sulphuric 
acid  and  a  little  sulphurous  acid  and  determined  electrolytically. 

227.  Separation  of  Cobalt  and  Nickel  by  Means  of  Nitroso-/?- 
Naphthol. — The  cobalt  compound  Co(C10H60(NO))3  is  insoluble 
in  hydrochloric  acid,  while  the  corresponding  nickel  compound  is 
soluble.  Nitric  acid  is  expelled  by  evaporation  with  sulphuric 
acid.  The  solution  is  diluted,  5  c.c.  hydrochloric  acid  and 
a  freshly  prepared  hot  solution  of  nitroso-/?-naphthol  in  50% 
acetic  acid  added,  until  further  addition  produces  no  precipitate. 
The  solution  is  digested  in  a  warm  place  for  a  few  hours. 
The  precipitate,  consisting  of  cobalt  nitroso-/?-naphthol  as  well  as 
considerable  of  the  reagent,  is  filtered  off  and  washed,  first  with 
cold  and  then  with  warm  12%  hydrochloric  acid  until  all  the 
nickel  has  been  washed  out.  It  is  then  washed  with  hot  water 
until  free  from  acid.  The  precipitate  is  dried  and  placed  in  a 
weighed  Rose  crucible.  A  little  pure  recrystallized  oxalic  acid  is 
added  and  the  paper  burned,  at  first  with  low  heat  and  then  with 
the  full  flame  of  the  Bunsen  burner.  When  the  carbon  is  com- 
pletely burned  the  precipitate  is  reduced  to  metallic  cobalt  by 
heating  in  a  stream  of  hydrogen. 

The  filtrate  is  evaporated  after  the  addition  of  sulphuric  acid. 
The  NICKEL  may  then  be  precipitated  as  hydroxide  and  weighed 
as  oxide,  or  it  may  be  determined  electrolytically. 


ANALYSIS  OF  SMALTITB.  169 

EXERCISE  35. 

Analysis  of  Smaltite  (Co,Ni,Fe)(S,As)?. 
Silica  and  Small  Amounts  of  Manganese  Are  Usually  Present. 

228.  Solution. — One   gram  of   the  finely  powdered  material  is  weighed 
out  and  transferred  to  an  Erlenmeyer  flask  of  about  250  c.c.  capacity. 
This  is  best  done  by  means  of  a  small  tube  sealed  at  one  end.     The  tube 
containing  about  one  gram  of  the  smaltite  is  carefully  weighed.     It  is  then 
inserted  into  the  neck  of  the  flask  which  is  tilted.     On  raising  the  flask  up- 
right and  tapping  the  tube  gently  the  smaltite  is  deposited  on  the  bottom 
of  the  flask.     The  tube  is  carefully  withdrawn  and  weighed.     Twenty  c.c.  of 
aqua  regia,  made  by  adding  to  3  volumes  of  concentrated  nitric  acid  1  volume 
of  concentrated  hydrochloric  acid,  are  added.     A  small  funnel  is  placed  in 
the  neck  of  the  flask  and,  after  being  shaken  and  standing  until  the  violent 
action  has  ceased,  the  flask  is  warmed  gently  on  the  water-bath  until  the 
mineral  is  entirely  decomposed.     If  sulphur  floats  on  top  of  the  solution, 
liquid  bromine  should  be  added,  a  few  drops  at  a  time,  and  the  solution 
warmed  until  the  sulphur  is  entirely  oxidized. 

229.  Silica. — The  solution  is  poured  into  a  porcelain  dish,  the  flask  and 
funnel  rinsed  with  water  and  after  the  addition  of  concentrated  sulphuric 
acid,  evaporated  on  the  hot  plate  until  fumes  of  sulphuric  acid  are  copiously 
evolved.     The  solution  is  cooled  and  diluted  with  water.      The  siliceous 
residue  is  immediately  filtered  off,  washed,  and  weighed. 

230.  Arsenic,  Antimony,   and  Tin. — A  little    sulphur    dioxide  is    added 
to  the  filtrate,  which  is  warmed  to  reduce  arsenic  to  arsenous  acid.     The 
excess  of  sulphur  dioxide  is   expelled   by  boiling.     Hydrogen  sulphide  is 
passed  through  the  warm  solution  until  no  more  precipitate  forms.    The 
arsenious  sulphide  is  filtered   off   on   a   Gooch  crucible   and   the   filtrate 
again  treated  with  sulphur  dioxide  and  hydrogen  sulphide  to  insure  com- 
plete precipitation  of  the  arsenic.      After  washing  the  arsenious  sulphide 
the  sulphur  is  extracted  by  treatment  with  alcohol  and  carbon  disulphide. 
The  precipitate  is  then  dried  and  weighed.     The  color  of  the  precipitate 
indicates  the  absence  of  lead,  bismuth,  and  copper  and  all  but  a  trace  of 
antimony.     The  absence  of  the  latter  element  as  well  as  tin  may  be  shown 
by  treating  the  weighed  precipitate  with  a  concentrated  solution  of  ammo- 
nium  carbonate.     The   sulphides    of   antimony    and   tin   remain    on    the 
filter   and   may  be   dissolved  in  hydrochloric  acid  and  separated  by  the 
methods  given  in  Chapter  XI,  page  134.     The  arsenic  must  also  be  repre- 
cipitated  and  weighed. 

231.  Iron.  —  The    filtrate    from    the    hydrogen    sulphide    precipitate    is 
evaporated  with  the  addition  of  nitric  acid  to  oxidize  the  iron,     The  solution 
is  made  alkaline  with  ammonia,  warmed,  and  the  precipitate  filtered  off 


170 


ANALYSIS  OF   MINERALS. 


and  washed  two  or  three  times.  The  filtrate  will  contain  most  of  the  cobalt 
and  nickel,  while  the  precipitate  will  contain  all  of  the  iron  and  a  small 
amount  of  the  nickel  and  cobalt.  It  is  dissolved  in  dilute 
hydrochloric  acid  and  the  paper  well  washed.  The  solu- 
tion is  evaporated  to  a  syrupy  consistency  and  transferred 
with  hydrochloric  acid  of  sp.  gr.  1.124  to  the  bulb  A  of  the 
apparatus  shown  in  Fig.  24,  on  p.  161,  and  the  extraction 
with  ether  carried  out  as  there  described.  The  apparatus 
shown  in  Fig.  25  may  also  be  used.  Before  transferring 
the  solution  to  the  bulb  D,  a  few  cubic  centimeters  of  ether 
are  poured  into  the  bulb  and  allowed  to  flow  into  B.  By 
warming  the  bulb  B  with  the  hand  and  opening  and  closing 
the  stop-cock  C,  a  partial  vacuum  is  created  in  B.  A  and 
C  are  closed  and  the  solution  transferred  to  D,  rinsing  the 
beaker  with  hydrochloric  acid  of  sp.  gr.  1.124  until  the  bulb 
is  filled  to  the  mark  on  D,  making  the  volume  60  c.c.  Place 
the  bulb  B  in  cold  water,  open  the  stop-cock  A,  and  allow  the 
liquid  in  D  to  flow  into  B.  Close  A  and  fill  D  with  ether 
(100  c.c.)  and  allow  it  to  flow  slowly  into  B.  Mix  the 
liquids  gradually,  placing  the  bulb  in  cold  water  from  time 
to  time  to  prevent  a  large  rise  in  temperature.  Finally 
close  A.  Shake  well,  opening  A  occasionally  to  relieve 
the  pressure,  then  place  the  apparatus  in  cold  water  for  about 
five  minutes  to  allow  the  liquids  to  separate. 
Allow  the  lower  liquid,  which  is  very  nearly  free  from  iron,  to  flow  into 
a  beaker.  Introduce  about  10  c.c.  of  hydrochloric  acid  (sp.  gr.  1.1)  into 
the  bulb  D.  Allow  it  to  flow  into  B,  shake,  and  after  standing  a  few  minutes 
allow  the  lower  liquid  to  flow  into  the  beaker.  This  liquid,  together  with 
the  filtrate  from  the  ferric  hydroxide,  will  contain  all  of  the  cobalt,  nickel, 
manganese,  and  aluminium  and  a  little  of  the  iron.  Draw  off  the  ether 
solution  of  the  iron  into  another  beaker  and  repeat  the  ether  extraction 
of  the  iron,  the  nickel-cobalt  solution  being  poured  back  again  into  the 
bulb  D.  Rinsing  the  beaker  will  be  unnecessary. 

The  ether  solution  of  the  ferric  chloride  should  be  poured  into  a  Florence 
or  distilling  flask  connected  with  a  condenser  by  means  of  cork  stoppers 
and  the  beaker  rinsed  with  water.  The  ether  is  distilled  off  on  the  water- 
bath,  care  being  taken  that  the  ether  is  not  placed  near  a  Bunsen  burner 
or  other  flame.  The  ferric  chloride  is  transferred  to  a  beaker  and  the  iron 
precipitated  with  ammonia  and  weighed  as  oxide,  or  reduced  with  zinc  and 
determined  volumetrically. 

232.  Aluminium  and  Chromium.  —  The  combined  cobalt  nickel  solu- 
tion is  warmed  to  expel  the  ether  and  a  little  bromine  or  hydrogen 
peroxide  added.  The  solution  is  neutralized  with  ammonia,  a  little 
ammonium  carbonate  added,  and  boiled.  A  small  precipitate  of  iron  is 
usually  obtained  and  any  aluminium  or  chromium  present  is  precipitated. 


FIG.  25. 


ANALYSIS  OF  SMALTITE.  171 

The  precipitate,  which  is  usually  quite  small,  is  filtered  off,  redissolved, 
and  reprecipitated.  In  the  absence  of  aluminium  and  chromium,  it  is 
added  to  the  main  portion  of  the  iron  and  weighed.  If  aluminium  and 
chromium  are  present,  the  metals  must  be  separated  by  methods  already 
given. 

233.  Manganese,  Cobalt,  and  Nickel. — About  10  c.c.  of  concentrated  sul- 
phuric acid  are  added  to  the  filtrate,  which  is  evaporated  on  the  hot 
plate  until  the  hydrochloric  acid  is  entirely  expelled,  as  indicated  by  the 
evolution  of  dense  fumes  of  sulphuric  acid.  After  cooling,  water  is  added 
and  the  solution  is  neutralized  with  ammonia.  30  to  50  c.c.  of  ammonia 
(sp.  gr.  0.96)  is  then  added  and  the  nickel  and  cobalt  determined  electro- 
lytically.  The  trace  of  manganese  which  may  be  present  will  be  con- 
verted into  the  dioxide  and  deposited  on  the  anode.  It  may  be  ignited 
and  weighed  as  Mn3O4. 

The  deposit  of  nickel  and  cobalt  after  being  weighed  is  dissolved  hi 
warm  dilute  nitric  acid  and  the  solution  evaporated  to  about  50  c.c. 
It  is  neutralized  with  caustic  potash  and  5  grams  of  potassium  nitrite  dis- 
solved in  water  are  added.  Acetic  acid  is  added  until  the  solution  is  acid 
and  the  slight  precipitate  produced  by  the  caustic  potash  dissolved.  After 
standing  for  twenty-four  hours,  the  clear  liquid  is  tested  for  cobalt  by 
adding  more  potassium  nitrite.  If  cobalt  is  present,  the  entire  solution 
must  be  allowed  to  stand  for  some  time  after  the  addition  of  potassium 
nitrite.  The  precipitate  is  filtered  off  and  washed  with  a  10%  solution  of 
potassium  acetate  containing  a  little  potassium  nitrite.  It  is  then  dis- 
solved in  hot  dilute  sulphuric  acid,  excess  of  ammonia  added,  and  the  cobalt 
determined  electrolytically. 

334.  Sulphur  is  determined  in  a  separate  portion  of  0.5  gram,  which 
is  dissolved  in  aqua  regia  as  already  directed,  or  in  fuming  nitric  acid. 
The  reagents  must  be  absolutely  free  from  sulphur.  If  necessary,  a 
blank  determination  should  be  made  side  by  side  with  the  determination 
of  the  mineral.  Instead  of  the  smaltite,  a  little  pure  sodium  or  potassium 
chloride  should  be  added  to  the  blank. 

After  the  expulsion  of  the  nitric  acid  with  hydrochloric  acid  and  filter- 
ing off  the  silica,  the  solution  is  ready  for  the  precipitation  of  the  sulphuric 
acid  by  means  of  barium  chloride.  If  much  sulphur  is  present,  it  is  well 
to  add  a  little  sodium  or  potassium  chloride  to  the  nitric  acid  solution 
before  evaporation  to  prevent  loss  of  sulphuric  acid.  The  evaporation 
should  be  conducted  on  a  hot  plate  or  water-bath  which  is  wide  enough 
to  keep  the  products  of  combustion  of  the  illuminating-gas  away  from 
the  solution,  otherwise  sulphur  dioxide  may  be  absorbed.  Sulphuretted 
hydrogen  or  ammonium  sulphide  must  also  be  absent  from  the  atmosphere. 

The  solution  is  diluted  to  300  or  400  c.c.,  heated  to  boiling,  and  barium 
chloride  solution  added  slowly  with  constant  and  vigorous  stirring  until 
no  more  precipitate  forms  on  adding  another  drop  of  the  reagent.  After 
digesting  the  precipitate  on  the  hot  plate  until  the  solution  is  clear,  it  is 


172  ANALYSIS  OF  MINERALS. 

filtered  cff,  washed  with  hot  water,  ignited,  and  weighed.  If  the  color  of 
the  ignited  precipitate  is  not  pure  white,  but  reddish,  iron  has  been  carried 
down.  Several  grams  of  sodium  carbonate  are  added  to  the  precipitate 
and  after  fusion  the  melt  is  dissolved  in  water  and  the  barium  carbonate 
filtered  off  and  washed.  The  sulphuric  acid  in  the  filtrate  is  precipitated' 
by  barium  chloride  after  acidifying  with  hydrochloric  acid.  The  error 
due  to  a  slight  reddish  color  of  the  precipitate  may  generally  be  neglected. 

EXERCISE  36. 
Analysis  of  Pyrite,  FeS2,  Arsenopyrite,  Fe(S,As)2,  or  Chalcopyrite,  CuFeS2. 

Small  Amounts  of  Silica,  Arsenic,  and  Antimony  May  be  Present. 

235.  Sulphur  Available  for  Sulphuric-acid  Manufacture. — As  the  natural 
sulphides  are  frequently  used  as  a  source  of  sulphur,  especially  in  the 
manufacture  of  sulphuric  acid,  the  determination  of  this  element  is 
frequently  carried  out  for  the  purpose  of  ascertaining  the  amount  of 
sulphur  available  under  the  conditions  of  manufacture  of  sulphuric  acid. 
For  this  purpose  the  determination  is  carried  out  as  follows: 

One-half  gram  of  the  powdered  sulphide  is  treated  in  an  Erlenmeyer 
flask  of  about  250  c.c.  capacity  with  10  c.c.  of  a  mixture  of  3  volumes 
of  concentrated  nitric  acid  and  1  volume  of  concentrated  hydrochloric 
acid.  The  mouth  of  the  flask  is  closed  with  a  small  funnel  and  the  solution 
is  gently  heated  on  the  water-bath  until  the  material  is  decomposed.  If 
unoxidized  sulphur  separates,  a  few  crystals  of  potassium  chlorate  are  added 
from  time  to  time  and  the  heating  continued  until  the  globules  of  sulphur 
have  disappeared.  Liquid  bromine  may  be  added  for  the  same  purpose. 

The  solution  is  transferred  to  a  porcelain  dish  and  evaporated  to  dry- 
ness  after  the  addition  of  one-half  gram  of  pure  sodium  or  potassium  chloride. 
If  the  evaporation  is  conducted  on  the  water-bath  only,  the  addition  of 
the  alkali  chlorides  may  be  omitted.  Five  c.c.  of  concentrated  hydro- 
chloric acid  are  added  and  the  solution  again  evaporated  to  dryness.  The 
residue  is  dissolved  in  a  few  cubic  centimeters  of  dilute  hydrochloric  acid 
and  100  c.c.  of  hot  water.  The  insoluble  residue  is  filtered  off  on  a  small 
paper  and  well  washed  with  hot  water.  This  residue  consists  of  silica, 
undecomposed  silicates,  and  the  sulphates  of  lead,  barium,  and  calcium,  if 
these  elements  are  present.  This  material  is  reserved  for  analysis  to  obtain 
the  percentage  of  total  sulphur  present. 

The  filtrate  is  neutralized  with  ammonia  and  a  considerable  excess  added. 
It  is  warmed  for  ten  to  fifteen  minutes  at  60°-7Q°,  but  not  to  boiling.  I/ 
the  solution  does  not  smell  strongly  of  ammonia  more  must  be  added. 
The  FERRIC  HYDROXIDE  is  filtered  off  and  washed  twice  by  decantation  with 
very  hot  water.  The  precipitate  is  transferred  to  the  filter-paper,  which 
should  not  be  completely  filled,  so  that  it  may  be  well  stirred  up  with  the 
stream  of  hot  water.  The  precipitate  must  be  washed  until  the  wash- 


ANALYSIS  OF  PYRITE.  173 

water  no  longer  gives  a  precipitate  with  barium  chloride.  As  basic  sulphates 
of  iron  may  be  present  in  the  ferric  hydroxide,  the  precipitate  is  dried  and 
fused  with  sodium  carbonate.  The  melt  is  dissolved  in  water  and  the  solu- 
tion tested  for  sulphuric  acid. 

The  nitrate  from  the  iron  is  acidified  with  hydrochloric  acid,  evaporated 
to  a  bulk  of  about  300  c.c.,  heated  to  boiling,  and  barium  chloride  solution 
added  slowly  with  constant  stirring  until  a  considerable  excess  is  present. 
Twenty  c.c.  of  a  10%  solution  is  usually  more  than  sufficient  for  0.5  gram 
of  pyrite.  After  digesting  hot  until  the  solution  is  clear,  the  barium  sul- 
phate is  filtered  off,  ignited,  and  weighed. 

236.  Total  Sulphur.  —  To  obtain  the  total  percentage  of    sulphur  the 
material  insoluble  in  aqua  regia  is  fused  with  sodium  carbonate.     If  lead  is 
absent,  the  fusion  may  be  conducted  in  a  platinum  crucible,  otherwise  a 
porcelain  crucible  must  be  used.     The  melt  is  dissolved  in  hot  water,  fil- 
tered, the  residue  well  washed,  and  the  sulphuric  acid  in  the  filtrate  pre- 
cipitated with  barium  chloride  and  weighed. 

The  total  percentage  of  sulphur  may  also  be  determined  as  follows: 
Fuse  0.5  gram  of  the  sulphide  with  10  grams  of  a  mixture  of  two  parts  sodium 
carbonate  and  one  part  potassium  nitrate.  The  material  should  be  well 
mixed  by  stirring  with  a  platinum  or  glass  rod,  which  is  then  cleaned  with 
a  camel's-hair  brush.  The  material  is  gradually  brought  to  fusion  and 
maintained  in  the  fluid  state  for  some  time.  The  fused  mass  is  extracted 
with  hot  water.  If  lead  is  present  it  is  precipitated  by  passing  carbon 
dioxide  through  the  solution.  The  insoluble  material  is  filtered  off  and 
washed  with  hot  water  containing  a  little  sodium  carbonate,  first  by  decanta- 
tion  and  then  on  the  filter-paper. 

The  filtrate  is  acidified  with  hydrochloric  acid  and  evaporated  to  dry- 
ness  to  remove  nitric  acid.  The  residue  is  dissolved  in  hot  water  to  which 
a  little  hydrochloric  acid  has  been  added.  Any  insoluble  material  present 
is  filtered  off  and  well  washed.  The  sulphuric  acid  in  the  filtrate  is  deter- 
mined by  precipitation  with  barium  chloride. 

237.  Arsenic. — For  the  determination  of  ARSENIC,  ANTIMONY,  and  COPPER, 
a   portion   weighing   from    0.5   to    2   grams  is  taken.      If  the  percentage 
of  these  elements  is  small,  the  larger  amount  is  taken.     The  material  is 
treated  cold  in  a  beaker  with  20  to  30  c.c.  concentrated  hydrochloric  acid. 
The  solution  is  agitated  from  time  to  time.      One  or  two  c.c.  of  liquid 
bromine  or  concentrated  nitric  acid  are  then  added   and  the   solution  is 
heated,  gently  at  first  and  then  more  strongly,  until  the  bromine  or  nitrous 
fumes  are  expelled.    The  arsenic  is  then  distilled  and  determined  as  directed 
in  Chapter  XI,  p.  133.     Ferrous  sulphate  or  chloride  is  used  as  the  reducing- 
agent. 

238.  Antimony. — The  contents  of    the  flask  are  rinsed  into  a  beaker, 
the  solution  diluted  to  200  to  300  c.c.,  warmed,  and  hydrogen  sulphide 
passed.     The  precipitate,  consisting  of  the  sulphides  of  copper  and  antimony, 
is  filtered  off  and  washed  with  water  containing  hydrogen  sulphide.     The 


174  ANALYSIS  OF  MINERALS. 

antimony  sulphide  is  dissolved  in  a  little  sodium-sulphide  solution  and  the 
residue  washed  with  water  containing  a  little  sodium  sulphide.  The  sodium 
sulphide  may  be  made  by  saturating  with  hydrogen  sulphide  one-half  of  a 
caustic  soda  solution  and  then  adding  the  other  half.  The  antimony  is 
precipitated  by  neutralizing  the  sodium  sulphide  solution  with  hydrochloric 
acid  and  passing  hydrogen  sulphide.  It  may  be  ignited  and  weighed  as  the 
tetroxide  after  treatment  with  nitric  acid. 

239.  Copper. — The  copper  sulphide  is  dissolved  in  a  little   nitric  acid 
and  determined  electrolytically. 

240.  Iron. — Iron  is   usually  determined  volumetrically.     The  ammonia 
precipitate  from  the   aqua-regia  solution  made  for  determining  sulphur 
contains  nearly  all  of  the  iron.     A  small  amount  is  frequently  left  in  the 
insoluble  residue  as  ferrous  silicate.     On  fusing  this  residue  with  sodium 
carbonate  the  ferrous  silicate  is  decomposed  and  the  insoluble  ferric  hydrate 
may  be  filtered  off.     These  two  precipitates  may  be  dissolved  in  hydrochloric 
acid  and  the  iron  titrated,  as  given  in  Chapter  XXIV,  page  320. 

Both  iron  and  silica  may  be  determined  in  the  portion  fused  with  sodium 
carbonate  and  potassium  nitrate  for  the  determination  of  total  sulphur. 
All  of  the  iron  will  be  present  in  the  precipitate  filtered  off  from  the  water 
solution  of  the  fused  material.  It  may  be  dissolved  in  hydrochloric  acid, 
precipitated  with  ammonia  and  weighed  as  oxide,  or  it  may  be  determined 
volumetrically. 

241.  Silica. — The  silica   constitutes   the  insoluble  material  filtered   off 
after  acidifying   the  sodium   carbonate  solution,  evaporating  to  dryness 
and  dissolving  in  hydrochloric  acid  and  water.     It  is  ignited  and  weighed. 

EXERCISE  37. 
Analysis  of  Cinnabar,  HgS. 

Iron,  Manganese,  Copper,  Aluminium,  and  Calcium  may  also  be  Present. 
Sulphur  is  determined  by  the  method  given  on  p.  167. 

242.  Mercury. — One  gram  of  the  mineral  is  treated  with  20  to  30  c.c. 
concentrated  hydrochloric  acid  to  which  40  c.c.  water  is  added.     One  to 
two  grams  of  potassium  chlorate  are  added  in  small  portions  and  the  solu- 
tion warmed  until  the  cinnabar  is  entirely  decomposed.     The  solution  is 
evaporated  to  dryness  on  the  water-bath  and  the  residue  dissolved  in  water 
and  hydrochloric  acid.     Any  insoluble  material  is  filtered  off,  ignited,  and 
weighed.     A  little  sulphurous  acid  is  added  to  reduce  any  iron  present  to 
the  ferrous  condition.     The  solution  is  warmed   to   expel   the   excess  of 
sulphurous  acid  and  hydrogen  sulphide  passed  until  no  more  precipitate  is 
formed.     If  copper  is   absent,  the  mercuric  sulphide  is  filtered  off  on  a 
Gooch  crucible  which   has  been  dried   at   100°.     Sulphur  is  extracted  by 
washing  the  precipitate  with  alcohol  and  carbon  disulphide.     The  precipi- 
tate is  then  dried  at  100°  and  weighed. 

If  copper  is  present  the  precipitate,  after  being  washed,  is  dissolved  in 
aqua  regia  and  the  chlorine  expelled  by  boiling  after  the  addition  of  dilute 


ANALYSIS  OF  CINNABAR.  175 

hydrochloric  acid.  When  entirely  free  from  chlorine  or  nitric  acid,  phos- 
phorous acid  is  ad  led  and  the  solution  is  allowed  to  stand  in  a  warm  place 
for  at  least  twelve  hours.  The  precipitated  mercurous  chloride  is  filtered 
off  on  a  Gooch  crucible,  washed  with  hot  water,  dried  at  100°,  and  weighed. 

243.  Copper. — The  nitrate  is  evaporated  to  small  bulk  after  the  addition 
of  a  few  cubic  centimeters  of  dilute  sulphuric  acid  to  expel  the  hydrochloric 
acid,  diluted  with  water,  and  the  copper  determined  electrolytically.     The 
copper  may  also  be  precipitated  as  sulphide  which  is  dissolved  in  2  or  3  c.c. 
of  concentrated  nitric  acid,  the  solution  diluted  and  electrolyzed. 

244.  Mercury  may  also  be    determined  very  accurately  in  a    separate 
portion  as  follows:  A  combustion-tube  30  to  45  cm.  long  and  10  to  15  mm. 
wide  is  taken.     The  end  is  sealed,  then  a  layer  of  magnesite  in  large  pieces, 
chalk,  or  sodium  bicarbonate  is  introduced  until  the  tube  is  filled  for  about 
10  cm.     Then  a  layer  of  recently  ignited  lime  is  introduced,  occupying 
about  5  cm.,  then  an  intimate  mixture  of  the  ore  with  lime  for  about  6  cm. 
This  mixture  should  be  made  by  grinding  the  constituents  together  in  a 
mortar.     The  mortar  should  be  rinsed  with  lime,  which  is  next  introduced, 
occupying  about  5  cm.     Finally  a  layer  of  about  5  cm.  of  lime  is  introduced, 
and  a  loose  plug  of  asbestos  inserted.     The  tube  is  then  drawn  out  and 
bent  at  right  angles.     It  is  then  tapped   gently  so  as   to  produce  a  free 
passage  for  the  gases  the  entire  length  of  the  tube,  and  is  then  placed  in  a 
combustion-furnace  with  the  drawn-out  end  dipping  into  a  flask. 

The  burners  of  the  furnace  are  lighted  so  that  the  lime  near  the  drawn-out 
end  of  the  tube  is  first  heated  to  redness.  The  burners  are  then  lighted  in 
order  until  the  magnesite  in  the  closed  end  of  the  tube  evolves  carbon  dioxide, 
which  sweeps  out  the  last  traces  of  mercury  vapor.  While  the  tube  is  still 
hot  it  is  cut  off  at  the  point  where  it  was  drawn  out  and  bent  and  the  globules 
of  mercury  washed  out  of  the  portion  cut  off  by  means  of  a  jet  of  water. 
By  shaking  the  flask  the  mercury  is  brought  together  into  a  single  drop. 
The  water  is  decanted  and  the  mercury  transferred  to  a  weighed  porcelain 
crucible.  The  water  is  absorbed  in  filter-paper  and  the  mercury  dried  in  a 
desiccator  and  weighed. 

245.  The  Iron,  Aluminium,  Manganese,  and  Calcium  will  be    present  in 
the  filtrate  from  the  mercuric  sulphide.    If  the  amount  of  iron  and  alaminium 
is  small,  these  metals  may  be  precipitated  as  hydroxides  and  weighed  as 
oxides.     The  separation  from  manganese  is  effected  in  the  manner  described 
in  Exercise  33,  p.  149,  except  that  an  excess  of  ammonia  must  be  avoided 
or  the  aluminium  will  be   redissolved.     The  manganese  is  determined  as 
directed  in  the  same  exercise.     The  filtrate  from  the  manganese  is  acidified 
with  hydrochloric  acid  and  evaporated  to  a  bulk  of  200  or  300  c.c.     It  is 
neutralized  with  ammonia,  warmed  nearly  to  boiling,  and  a  little  ammonium 
oxalate   added.     After  digesting  for  some  time,  the   calcium   oxalate   is 
filtered  off  and  washed  with  hot  water.     The  moist  precipitate  is  placed 
in  the  platinum  crucible,  the  paper  burned  in  the  usual  manner,  and  the 
precipitate  ignited,  finally,  over  the  blast-lamp  and  weighed  as  oxide. 


CHAPTER  XV. 

ANALYSIS  OF  CARBONATES  CONTAINING  CAL- 
CIUM, BARIUM,  STRONTIUM,  AND  MAGNESIUM. 

SEPARATION  OF  CALCIUM,  BARIUM,  AND  STRONTIUM 
FROM  THE  ALKALI  METALS  AND  FROM  EACH  OTHER. 

246.  Calcium,  Barium,  and  Strontium  may  be  separated  from 
magnesium  and  the  alkali  metals  by  precipitation  as  carbonates, 
by  ammonia  and  ammonium  carbonate.     A  sufficient  amount  of 
ammonium  salts  must  be  present  to  prevent  the  precipitation  of 
magnesium.     It  is  very  seldom  that  this  method  of  separation  is 
used  because  the  occurrence  of  these  three  elements  together  is 
very  rare,  although  recent  work  has  shown  that  traces  of  barium 
and  strontium  occur  in  rocks  far  more  frequently  than  was  for- 
merly supposed.     Excellent  methods  of  separation  of  each  of  the 
alkaline-earth  metals  from  magnesium  and  the  alkalies  are  avail- 
able, by  which  the  metal  is  left  in  the  best  condition  for  weighing. 

SEPARATION  OF  CALCIUM. 

247.  Separation   of  Calcium  as   Oxalate. — Calcium  is  usually 
separated  from  magnesium  and  the  alkali  metals  by  means  of 
ammonium  oxalate.     The  precipitation  is  carried  out  in  a  hot 
solution.     Sufficient  ammonium  oxalate  must  be  present  to  con- 
vert both  calcium  and  magnesium  into  oxalates,  since  calcium 
oxalate    is    slightly    soluble    in    magnesium    chloride.      Enough 
ammonium  chloride  must  be  present  to  prevent  the  precipitation 
of  magnesium  hydroxide  when  the  solution  is  made  alkaline.     A 
sufficient  amount  is  usually  present  from  the  precipitation  of  iron 
and  aluminium.     If  an  excessive  amount  has  been  introduced,  it 
must  be  removed  by  evaporating  the  solution  to  dryness  and  vola- 
tilizing the  ammonium  salts.     The  solution  may  also  be  boiled 
down  to  small  bulk  and  the  ammonium  salts  decomposed  by  the 
addition  of  nitric  acid.     It  is  generally  sufficient  to  treat  in  this 

176 


SEPARATION  OF  CALCIUM,  BARIUM,  AND  STRONTIUM.      Ill 

manner  the  filtrate  from  the  second,  third,  and  subsequent  precipi- 
tations of  the  iron  and  aluminium.  If  the  ammonium  salts  from 
the  entire  solution  have  been  expelled,  the  residue  is  dissolved  in 
hydrochloric  acid  and  the  solution  neutralized  with  ammonia.  If 
a  precipitate  forms  it  is  dissolved  in  acid,  and  the  solution  again 
neutralized.  This  is  repeated  until  a  precipitate  of  magnesium 
hydroxide  is  no  longer  formed  on  making  the  solution  alkaline. 

The  first  precipitate  of  calcium  oxalate  will  invariably  contain 
some  magnesium  and  also  some  sodium,  which  cannot  be  washed 
out.  It  must  therefore  be  redissolved  in  hydrochloric  acid,  and 
reprecipitated  by  neutralizing  with  ammonia  and  adding  a  little 
ammonium  oxalate.  The  second  precipitate  will  be  free  from 
magnesium  and  sodium.  If  STRONTIUM  is  present  it  will  be  almost 
completely  precipitated  with  the  calcium.  The  two  metals  may 
be  weighed  together  as  oxides  after  ignition  of  the  oxalate,  and 
separated  by  methods  given  later.  If  BARIUM  is  present  to  the 
extent  of  3  or  4  mg.,  it  will  not  contaminate  the  calcium  after 
the  second  precipitation.  If  more  than  3  or  4  mg.  of  barium  are 
present,  the  calcium  cannot  be  separated  as  oxalate. 

248.  Separation    of    Barium    and    Strontium    as   Sulphates. — 
Barium  and  strontium  may  be  separated  from  magnesium  and 
the  alkali  metals  by  precipitation  as  sulphate.     If  strontium  is 
present  an  equal  volume  of  alcohol  must  be  added  to  insure  com- 
plete precipitation.     The  precipitate  must  also  be  washed  with 
alcohol.     Barium  sulphate  frequently  carries  down  a  small  amount 
of  the  alkalies  which  cannot  be  washed  out.     It  must,  therefore, 
in  very  exact  analyses,  be  redissolved  in  a  little  warm  concen- 
trated sulphuric  acid  and  reprecipitated  by  diluting  with  water. 

249.  Separation   of  Calcium   as   Sulphate. — Calcium  may  also 
be  separated  from  magnesium  and  the  alkalies  by  precipitation  as 
the  sulphate.     The  solution  must  be  evaporated  until  free  hydro- 
chloric acid  and  water  are  expelled.     The  residue  is  dissolved  in 
alcohol,  and  a  slight  excess  of  concentrated  sulphuric  acid  added. 
After  digesting  for  some  time  the  solution  is  filtered  and  the  pre- 
cipitate is  washed  with  absolute  alcohol  until  free  from  acid.     A 
small  amount  of  magnesium  sulphate  is  then  washed  out  by  means 
of  35  to  40%  alcohol.     The  alcohol  may  be  expelled  from  the 
filtrate  and  the  magnesium  precipitated  as  phosphate.     If  barium 


178  ANALYSIS  OF  MINERALS. 

and  strontium   are   present  they  will  be  precipitated  with  the 
calcium. 


SEPARATION  OF  BARIUM,  STRONTIUM,  AND  CALCIUM. 

250.  Precipitation  of  Barium  as  Chromate. — Two  absolutely 
reliable  methods  for  the  separation  of  barium,  strontium,  and  cal- 
cium have  been  devised  by  Fresenius.  The  metals  are  precipi- 
tated as  carbonates  with  ammonia  and  ammonium  carbonate. 
If  the  separation  from  the  magnesium  has  been  effected  by  sulphuric 
acid  the  precipitate  is  fused  with  sodium  carbonate,  the  melt 
treated  with  hot  water,  and  the  carbonates  of  the  alkaline-earth 
metals  filtered  off  and  well  washed.  The  precipitate  is  dissolved 
in  the  least  amount  of  dilute  hydrochloric  acid,  and  the  excess 
removed  by  evaporation.  The  chlorides  are  dissolved  in  300  c.c. 
of  water,  6  drops  of  acetic  acid  (sp.  gr.  1.065)  and  a  few  drops  of 
ammonium  acetate  added.  The  solution  is  heated,  and  10  c.c. 
of  a  neutral  10%  ammonium  chromate  solution  added.  After 
allowing  the  precipitate  to  settle  and  the  solution  to  cool  for  one 
hour,  decant  on  a  small  filter  and  wash  by  decantation  two  to 
three  times  with  water  containing  ammonium  chromate.  Finally 
transfer  the  precipitate  to  the  paper  and  wash  until  the  filtrate 
gives  no  precipitate  with  ammonia  and  ammonium  carbonate. 
About  100  c.c.  of  wash-water  should  be  used.  The  washing  is 
continued  with  pure  warm  water,  until  the  filtrate  gives  only  a 
slight  reddish  coloration  with  silver  nitrate.  About  110  c.c.  will 
be  required. 

The  precipitate  which  contains  all  of  the  barium  and  a  little  of 
the  strontium  is  washed  into  the  precipitating-dish,  the  portion 
adhering  to  the  filter-paper  dissolved  in  a  little  warm  dilute  nitric 
acid,  and  the  paper  well  washed.  About  2  c.c.  nitric  acid  (sp.  gr. 
1.20)  should  be  used.  The  solution  is  diluted  to  200  c.c.  and 
heated.  5  c.c.  of  a  30%  ammonium  acetate  solution  is  added 
very  gradually.  Ammonium  chromate  solution  (10%)  is  then 
added  until  the  odor  of  acetic  acid  has  entirely  disappeared. 
After  standing  one  hour,  the  supernatant  liquid  is  decanted 
through  the  filter  or  on  a  Gooch  crucible,  and  the  precipitate  is 
digested  with  hot  water  which  is  allowed  to  cool.  The  precipitate 


SEPARATION  OF  CALCIUM,  BARIUM,  AND  STRONTIU&.      179 

is  then  brought  on  the  filter-paper  or  crucible  and  washed  with 
cold  water  until  the  filtrate  gives  a  scarcely  perceptible  test  for 
chromic  acid  with  silver  nitrate. 

251.  Weighing  Barium  as  Chromate. — The  now  pure  barium 
chromate  is  removed  from  the  paper,  the  latter  burned,  and  the 
whole  gently  ignited  in  a  platinum  or  porcelain  crucible.      If  the 
heat  is  too  great  the  chromic  acid  will  be  reduced  and  the  precipitate 
will  become  green.      The  portions  of  the  precipitate  adhering  to 
the  filter-paper  will  also  be  reduced  during  the  incineration  of 
the  latter.     On  gently  heating  the  precipitate,  the  chromium  will 
be  reoxidized.     If  a  Gooch  crucible  has  been  used  the  precipitate 
may  finally  be  washed  with  alcohol,  dried  on  the  hot  plate,  and 
weighed. 

252.  Separation    of    Calcium    and     Strontium    as    Nitrates. — 
One  c.c.  of  nitric  acid  is  added  to  the  filtrate  from  the  barium, 
which  is  concentrated  and  the  calcium  and  strontium  precipitated 
by  ammonia  and  ammonium  carbonate.     If  only  strontium  is 
present  the  precipitate  of  strontium  carbonate  may  be  weighed 
or  it  may  be  dissolved  ha  hydrochloric  acid,  precipitated  by  the 
addition  of  sulphuric  acid  and  alcohol,  and  weighed  as  sulphate. 
If  only  calcium  is  present  the  precipitation  as  carbonate  may 
be  omitted  and  the  calcium  immediately  precipitated  as  oxalate 
and  weighed  as  oxide. 

If  both  strontium  and  calcium  are  present  the  mixed  carbo- 
nates are  dissolved  in  nitric  acid  and  the  solution  evaporated  to  dry- 
ness  in  a  small  porcelain  dish  and  heated  for  some  time  in  an  air- 
bath  at  130°.  The  temperature  may  rise  to  180°  without  decom- 
posing the  nitrates.  The  dried  material  is  pulverized  and  treated 
five  times  with  5-c.c.  portions  of  a  mixture  of  equal  parts  of  abso- 
lute alcohol  and  ether  and  the  solution  decanted  each  time  into  a 
small  flask.  The  calcium  nitrate  is  dissolved  by  the  alcohol  and 
ether  in  which  the  strontium  nitrate  is  insoluble.  After  expelling 
the  ether  and  alcohol  from  the  residue,  it  is  dissolved  in  water, 
evaporated  to  dryness,  and  again  heated  for  some  time  at  130°  hi 
the  air-bath.  The  mass  is  pulverized  and  transferred  to  the  flask 
with  about  15  c.c.  of  the  ether-alcohol.  The  flask  is  corked  and 
allowed  to  stand  with  occasional  shaking  for  twenty-four  hours. 
The  solution  is  then  filtered  through  a  small  filter,  the  strontium 


180  ANALYSIS  OF  MINERALS. 

nitrate  washed  by  decantation  and  then  on  the  filter  with  5-c.c. 
portions  of  the  ether-alcohol  about  ten  or  twelve  times.  The  stron- 
tium nitrate  in  the  flask,  on  the  paper,  and  in  the  dish,  if  it  was 
not  all  transferred,  is  then  dissolved  in  a  little  hot  water  and  the 
strontium  precipitated  as  sulphate  by  the  addition  of  sulphuric 
acid  and  alcohol.  The  calcium  in  the  ether-alcohol  solution  may 
be  precipitated  as  sulphate  and  washed  with  absolute  alcohol. 
It  is  ignited  at  a  low  red  heat  and  weighed  as  calcium  sulphate. 
The  ether  and  alcohol  may  also  be  distilled  off,  the  calcium  nitrate 
dissolved  in  water,  and  the  calcium  precipitated  as  oxalate  and 
weighed  as  oxide. 

As  BARIUM  as  well  as  STRONTIUM  nitrate  is  insoluble  in  the 
ether-alcohol  mixture,  the  carbonates  of  the  three  metals  may  be 
converted  into  nitrates  and  treated  in  the  manner  directed  for 
the  separation  of  calcium  and  strontium.  The  nitrates  of  barium 
and  strontium  may  then  be  dissolved  in  water  and  the  barium 
separated  from  the  strontium  by  two  precipitations  of  the  barium 
as  chromate.  Gooch  and  Browning  have  suggested  the  use  of 
amyl  alcohol  in  place  of  the  mixture  of  absolute  alcohol  and 
ether.  As  the  amyl  alcohol  boils  at  138°,  it  may  be  rendered 
anhydrous  by  boiling  out  the  water.  The  preparation  of  abso- 
lute alcohol  or  ether  is  quite  laborious. 


EXERCISE  38. 
Analysis  of  Dolomite,  (CaO,MgO)C02. 

Small  Amounts  of  Silica,  Iron,  Aluminium,  and  Manganese  Are 
Usually  Present. 

253.  Solution  of  the  Dolomite. — One  gram  of  the  powdered  material  is 
weighed  out,  placed  in  a  beaker  (about  100  c.c.),  and  dissolved  in  about 
15  c.c.  dilute  hydrochloric  acid.  The  beaker  should  be  covered  with  a 
watch-crystal  and  the  acid  poured  slowly  down  the  lip  to  prevent  loss 
by  spattering.  The  beaker  is  warmed  on  the  hot-plate  or  water-bath 
until  carbon  dioxide  ceases  to  be  evolved.  The  watch-crystal  is  rinsed  with 
a  little  water  and  the  open  beaker  heated  on  the  water-bath  until  the  solu- 
tion is  evaporated  to  a  syrupy  consistency.  It  is  then  stirred  with  a  glass 
rod  until  dry.  The  residue  is  heated  at  least  one-half  hour  on  the  water- 
bath  and  then  dissolved  in  water  and  a  little  hydrochloric  acid  and  the 
solution  heated  for  a  few  minutes. 


ANALYSIS  OF  DOLOMITE.  181 

254.  Silica. — The  siliceous  residue  is   filtered  off  on  a  small  paper  and 
washed  with  hot  water  until  free  from  chlorides.     The  moist  paper  is  placed 
in  a  weighed  platinum  crucible,  burned  in  the  usual  manner,  and  the  siliceous 
residue  finally  ignited  for  a  few  minutes  over  the  blast-lamp  and  weighed. 
The  silica  is  then  volatilized  by  moistening  the  weighed  precipitate  with 
water,  adding  2  or  3  c.c.  of  hydrofluoric  acid  and  a  drop  of  sulphuric  acid, 
warming  until  the  precipitate  is  dissolved,  and  then  evaporating  to  dryness 
on  the  hot-plate  and  igniting.     The  hydrofluoric  acid  must  leave  no  residue 
on  evaporation.     Finally  the  residue  in  the  crucible  is  ignited  over  the  blast- 
lamp  and  brought  to  constant  weight.     The  weight  of  the  residue  must  be 
subtracted  from  the  weight  of  the  siliceous  residue  to  obtain  the  weight  of 
silica.     The  residue  is  dissolved  in  hydrochloric  acid  and  added  to  the 
filtrate  from  the  silica. 

255.  Iron  and  Aluminium. — Ten  c.c.  of  concentrated  hydrochloric  acid 
are  added  to  the  acid  solution,  which  should  not  exceed  200  c.c.     It  is 
neutralized   with   filtered   ammonia  wrhile   stirring  vigorously  and   heated 
nearly  to  boiling  on  the  hot-plate.     If  an  excess  of  ammonia  is   present, 
very  dilute  hydrochloric  acid  is  added  until  the  faintest  odor  of  ammonia 
can  be  detected  over  the  hot  solution  or  a  moist  piece  of  red  litmus-paper 
held  over  the  solution  is  slowly  turned  blue.     The  neutralization  and  diges- 
tion should  not  occupy  more  than  one-half  hour  and  a  Jena  beaker  should 
be  used.    The  precipitate  of  iron  and  aluminium  is  filtered  off  and  if  more 
than  a  few  flakes  are  present,  it  is  dissolved  in  a  little  warm  dilute  hydro- 
chloric acid,  the  paper  washed  a  few  times  with  hot  water,  and  the  solution 
again  neutralized  with  ammonia,  digested  on  the  hot-plate  for  a  short  time, 
and  filtered.     The  precipitate  is  washed  free  from  chlorides  with  hot  water, 
the  moist  paper  transferred  to  the  platinum  crucible  and  burned.     The 
precipitate  is  heated  with  the  blast-lamp  for  a  few  minutes  and  weighed. 
As  this  precipitate  seldom  weighs  more  than  a  few  milligrams,  the  separa- 
tion of  the  iron  and  aluminium  is  not  necessary. 

256.  Manganese. — If  the  combined  filtrates  from  the  iron  and  aluminium 
exceed  300  c.c.,  the  solution  should  be  acidified  with  hydrochloric  acid 
and  concentrated  on  the  hot-plate.     To  precipitate  the  manganese  the  solu- 
tion is  made  alkaline  with  ammonia  and  bromine  water  or  hydrogen  per- 
oxide is  added.    A  few  cubic  centimeters  of  hydrogen  peroxide  are  sufficient, 
while  enough  bromine  water  must  be  added  to  produce  a  yellow  color  in 
the  solution.     It  is  heated  nearly  to  boiling,  ammonia  added  if  necessary 
to  make  it  alkaline,  the  precipitate  filtered  off  on  a  small  paper  and 
washed  with  hot  water.     The  moist  paper  is  burned  in  the  crucible  and 
the  manganese  weighed  as  Mn304. 

257.  Calcium. — The  filtrate  is  again  concentrated  on  the  hot-plate  to 
about  250   c.c.   after  acidifying  with  hydrochloric  acid.     The   calcium   is 
precipitated  by  making  the  solution  alkaline  with  ammonia  and  adding 
to  the  hot  solution  with  vigorous  stirring  30  to  40  c.c.  of  a  5%  solution  of 


182  ANALYSIS  OF  MINERALS. 

ammonium  oxalate.  The  solution  is  digested  on  the  water-bath  for  about 
one  hour  or  until  the  precipitate  has  settled,  leaving  a  clear  supernatant 
liquid.  This  is  decanted  through  a  filter-paper  and  the  precipitate  in  the 
beaker  is  washed  two  or  three  times  by  decantation  with  hot  water.  About 
50  c.c.  of  water  are  used  each  time.  The  precipitate  is  well  stirred  up  each 
time  and  allowed  to  settle.  A  little  warm  dilute  hydrochloric  acid  is  poured 
over  the  precipitate  on  the  paper  and  allowed  to  flow  into  the  beaker  con- 
taining the  main  portion  of  the  precipitate.  The  calcium  is  reprecipitated 
by  neutralizing  the  hot  solution  with  ammonia  while  stirring  vigorously. 
A  few  cubic  centimeters  of  ammonium  oxalate  are  added  and  the  precipi- 
tate digested  on  the  hot-plate  as  before.  Finally  the  calcium  oxalate 
is  filtered  off  and  washed  with  hot  water  until  free  from  chlorides.  The 
moist  precipitate  may  be  placed  in  the  platinum  crucible,  gently  heated 
until  dry,  and  the  paper  burned,  then  heated  more  strongly  until  the  carbon 
is  completely  incinerated,  and  finally  with  the  blast-lamp  for  five  to  ten 
minutes.  After  weighing,  the  heating  with  the  blast-lamp  and  weighing  are 
repeated  until  the  weight  is  constant. 

258.  Magnesium. — The   filtrates   from  the   calcium   are   combined   and, 
after  acidifying  with  hydrochloric  acid,  evaporated  on  the  hot-plate  to 
200  or  300  c.c.     Excess  of  disodium   phosphate  or   sodium  ammonium 
phosphate  solution  and  a  moderate  excess  of  ammonia  are  added  with  stir- 
ring.     After  being  allowed  to  stand  for  twenty-four  hours  the  solution 
is  decanted  quite  completely  from  the  precipitate,  which  is  dissolved  in  a 
few  cubic  centimeters  of  dilute  hydrochloric  acid.     A  few  drops  of  phos- 
phate solution  are  added,  then  ammonia  with  stirring,  until  a  moderate  excess 
is  present.     The  volume  of  the  solution  should  now  not  be  greater  than  50 
to  75  c.c.     After  standing  from  twelve  to  twenty-four  hours,  the  precipitate 
is  filtered  off  and  washed  with  dilute  ammonia  until  free  from  chlorides. 
It  is  then  dried,  detached  from  the  paper,  and  the  latter  burned  on  the 
platinum  wire.     The  precipitate  may  also  be  placed  in  the  crucible  fol- 
lowed by  the  folded  paper.    The  precipitate  is  first  heated  gently,  the  lid 
being  on  the  crucible.     When  ammonia  is  no  longer  evolved,  the  full  flame 
of  the  Bunsen  burner  is  applied  and  finally  the  blast-lamp  for  a  few  minutes. 

259.  Carbon    Dioxide   by   Loss.  —  If  the  dolomite  is  free  from  organic 
matter,  carbon  dioxide  may  be  determined  by  loss  after  fusing  with  borax.* 
For  this  purpose  5  grams  of  vitrified  borax  are  fused  in  a  platinum  crucible 
with  the  Bunsen  burner.     If  considerable  water  is  present,  it  may  be  more 
quickly  expelled  by  heating  over  the  blast-lamp.     As  borax  is  slightly  vola- 
tile at  this  temperature,  the  material  must  be  brought  to  constant  weight 
by  heating  to  fusion  with  the  Bunsen  burner.     One  gram  of  the  dolomite  is 
placed  on  the  borax,  the  lid  is  placed  on  the  crucible,  and  the  contents  are 

*  Microcosmic  salt  may  also  be  used  for  this  determination,  as  described  on 
page  111. 


ANALYSIS  OF  DOLOMITE.  183 

brought  to  fusion  with  the  Bunsen  burner.  The  lid  must  not  be  removed 
until  the  borax  is  fused,  as  it  is  apt  to  crack  so  that  small  pieces  may  be 
thrown  out  of  the  crucible.  When  the  dolomite  has  dissolved  in  the  melted 
borax  and  bubbles  of  gas  escape  only  at  considerable  intervals,  the  crucible 
is  cooled  in  the  desiccator  and  weighed.  When  the  weight  is  constant, 
the  amount  of  carbon  dioxide  is  found  from  the  loss  in  weight  of  the  borax 
and  dolomite.  The  borax-glass  may  be  removed  from  the  crucible  by 
pressing  it  between  the  thumb  and  fingers  on  all  sides.  This  generally  loosens 
the  melt  so  that  it  drops  out  on  gently  tapping  the  crucible. 

DIRECT   WEIGHING    OF    CARBON    DIOXIDE. 

260.  Apparatus. — Carbon  dioxide  may  also  be  determined  by  absorp- 
tion in  caustic  potash  and  ascertaining  the  increase  in  weight.  For  this 
purpose  the  apparatus  shown  in  Fig.  18,  p.  113,  is  very  convenient  and 
easily  made.  The  glass  tube  is  about  70  cm.  long  and  not  less  than  12  mm. 
in  diameter.  It  is  drawn  out  so  as  to  pass  through  the  rubber  stopper  in 
the  flask.  The  upper  half  is  filled  with  granulated  calcium  chloride,  a 
plug  of  asbestos  or  glass  wool  being  first  inserted  and  pushed  to  the  middle 
of  the  tube.  A  few  lumps  of  pumice-stone  which  have  been  saturated  with 
copper  sulphate  solution  and  dried  at  about  125°  are  first  dropped  in  to  absorb 
any  hydrochloric  acid  gas  which  may  pass  over.  Large  lumps  of  calcium 
chloride  are  then  dropped  in  and  then  smaller  ones,  the  fine  material  being 
at  the  end  where  another  plug  of  cotton  or  glass  wool  is  placed.  After 
being  filled  in  this  manner,  each  end  is  connected  with  a  calcium  chloride 
tube  or  other  drying  apparatus  and  a  slow  stream  of  carbon  dioxide  or 
hydrochloric  acid  gas  passed  through  the  tube.  This  is  maintained  for  twenty- 
four  hours,  when  the  acid  gas  is  displaced  by  a  stream  of  air.  The  upper  end 
of  the  tube  must  be  carefully  protected  from  exposure  to  the  atmosphere 
or  moisture.  The  flask  may  have  a  capacity  of  100  or  200  c.c.  The  end 
of  the  dropping-funnel  should  be  drawn  out  and  the  end  of  the  small  tube 
turned  up  to  prevent  the  escape  of  carbon  dioxide  through  the  dropping- 
funnel.  A  straight  calcium  chloride  tube  filled  with  soda-lime  is  fitted 
to  the  dropping-funnel  by  means  of  a  small  perforated  rubber  stopper. 

261.  For  the  Geissler  caustic  potash  bulb  a  solution  of  caustic  potash 
is  made  by  dissolving  one  part  of  the  potash  in  two  parts  of  water.  The 
bulbs  are  filled  not  more  than  two-thirds  full  with  this  solution.  The  cal- 
cium chloride  tube  is  filled  with  soda-lime  and  calcium  chloride,  the  soda- 
lime  being  nearest  the  caustic  potash.  The  calcium  chloride  should  be 
moderately  fine,  so  that  the  gases  may  leave  the  apparatus  well  dried.  Before 
placing  the  soda-lime  in  the  tube  a  small  plug  of  glass  wool  or  cotton  should 
be  inserted.  If  a  caustic  potash  bulb  without  the  attached  calcium  chloride 
tube  is  used  a  separate  U-tube  or  straight  calcium  chloride  tube  must  be 
filled  as  directed  and  weighed  separately. 


184  ANALYSIS  OF  MINERALS. 

262.  Testing  for  Air  Leaks. — When  setting  up  the  apparatus,  it  is  well 
to  shellac  the  small  end  of  the  rubber  stoppers,  as  rubber  is  pervious  to 
carbon  dioxide.  The  small  entrance-tube  of  the  Geissler  bulb  can  generally 
be  inserted  directly  into  the  rubber  stopper  in  the  end  of  the  glass  tube, 
thus  saving  an  extra  rubber  joint.  When  the  apparatus  has  been  set  up. 
it  is  tested  for  leaks  by  closing  the  stop-cock  in  the  dropping-funnel  and  suck- 
ing out  a  little  air  through  the  caustic  potash  bulb..  Enough  air  should  be 
sucked  out  to  cause  the  solution  to  rise  an  inch  or  two  in  the  limb  of  the 
Geissler  bulb  nearest  the  long  glass  tube.  This  column  of  liquid  forms  an 
index  of  the  partial  vacuum  in  the  apparatus.  The  position  of  the  liquid 
is  carefully  noted.  If  the  liquid  slowly  falls,  a  leak  exists  which  must  be 
found  and  closed.  The  most  delicate  index  is  formed  by  the  small  drops  of 
liquid  that  generally  remain  in  one  end  of  the  small  tubes  connecting  the 
bulbs  of  the  absorption  apparatus.  If  the  apparatus  is  absolutely  gas- 
tight,  a  drop  of  this  kind  will  remain  motionless  for  ten  to  fifteen  minutes. 

263.  Determination  of  the  Carbon  Dioxide.  —  When  the  apparatus 
has  been  made  tight,  1  gram  of  the  dolomite  is  weighed  out  and  trans- 
ferred to  the  flask.  After  replacing  the  flask  the  apparatus  is  again  tested 
for  leaks.  Air  is  now  slowly  drawn  through  the  apparatus  for  about  half 
an  hour  to  remove  any  carbon  dioxide  which  may  be  present.  The  caustic 
potash  bulb  is  detached,  thoroughly  cleaned,  and  after  standing  in  the  bal- 
ance-case for  twenty  to  thirty  minutes,  is  weighed.  Whenever  this  bulb  is 
detached,  the  hole  in  the  stopper  in  the  end  of  the  long  glass  tube  should 
be  closed  with  a  glass  plug. 

After  replacing  the  weighed  caustic  potash  bulb,  introduce  from  30  to  50 
c.c.  of  dilute  hydrochloric  acid  into  the  dropping-funnel.  Open  the  stop- 
cock and  allow  the  acid  to  flow  into  the  flask  at  such  a  rate  that  not  more 
than  two  bubbles  of  air  per  second  pass  through  the  caustic  potash  solution. 
After  all  of  the  acid  has  been  introduced,  close  the  stop-cock  in  the  funnel 
and  warm  the  flask  gently,  finally  to  boiling.  A  very  small  flame  must  be 
used  at  first  to  prevent  the  gases  from  passing  too  rapidly  through  the 
caustic  potash  solution.  It  must  at  all  times  pass  slowly  enough  to  permit 
the  bubbles  to  be  readily  counted.  When  water  begins  to  condense  in 
the  lower  part  of  the  long  glass  tube,  the  heating  of  the  liquid  should  be 
stopped,  the  stop-cock  quickly  opened,  and  suction  applied  to  the  absorption- 
bulb.  When  the  first  violent  flow  of  air  into  the  flask  has  ceased,  air  is 
drawn  through  the  apparatus  at  the  rate  of  two  bubbles  per  second  for 
about  three-quarters  of  an  hour.  For  this  purpose  a  Bunsen  filter-pump 
may  be  used.  The  flow  of  air  may  be  regulated  by  a  screw  pinch-cock.  A 
better  device  consists  of  a  large  bottle  filled  with  water  which  is  led  out 
through  a  siphon  extending  to  the  bottom  of  the  bottle  and  passing  through 
a  doubly  perforated  stopper.  Through  the  second  perforation  passes  the 
tube  connected  to  the  carbon  dioxide  apparatus.  The  flow  of  water  is 
controlled  by  a  pinch-cock  placed  on  a  rubber  tube  at  the  end  of  the  siphon. 


ANALYSIS  OF  DOLOMITE.  185 

The  caustic  potash  bulb  is  disconnected,  carefully  cleaned,  placed  in 
the  balance-case,,  and  weighed.  After  replacing  the  caustic  potash  bulb, 
the  solution  of  the  dolomite  is  gradually  heated  to  boiling  again  and  air 
drawn  through  the  apparatus  in  the  same  manner  as  before,  after  which 
the  caustic-potash  bulb  is  again  disconnected  and  weighed  to  ascertain 
if  all  of  the  carbon  dioxide  has  been  swept  out  of  the  apparatus.  If  the 
operation  has  been  properly  conducted,  not  more  than  2  or  3  mg.  will  be 
obtained  on  the  second  weighing.  When  sufficient  experience  has  been 
gained  the  operation  may  be  so  conducted  that  only  one  weighing  will  be 
necessary. 


CHAPTER  XVI. 

ANALYSIS  OF  SILICATES  AND  SEPARATION  OF 
SODIUM  AND  POTASSIUM. 

SEPARATION  OF    MAGNESIUM   FROM  THE    ALKALIES. 

IN  the  course  of  an  analysis  the  other  metals  are  first  precipi- 
tated, leaving  magnesium  and  the  alkalies  in  solution.  Magnesium 
may  then  be  precipitated  as  phosphate  and  weighed.  During  the 
course  of  this  determination  alkalies  are  introduced  into  the  solu- 
tion so  that  it  is  not  available  for  the  determination  of  these  metals. 
A  different  method  of  precipitation  of  the  magnesium  must  there- 
fore be  adopted  or  a  fresh  portion  of  the  unknown  must  be  taken 
and  the  magnesium  as  well  as  the  other  heavy  metals  separated 
from  the  alkalies.  The  latter  is  by  far  the  most  satisfactory  method 
of  procedure,  and  is  invariably  adopted  when  sufficient  material 
for  analysis  is  at  hand. 

264.  Separation  of  Magnesium  by  Means  of  Mercuric  Oxide. — 
The  magnesium  may  be  removed  without  introducing  alkalies 
by  means  of  mercuric  oxide.  The  solution  of  the  chlorides 
from  which  other  acids  must  be  absent  is  treated  with  mercuric 
oxide  which  is  free  from  alkalies.  The  operation  should  be  con- 
ducted in  a  platinum  dish.  It  is  best  to  use  freshly  precipitated 
moist  mercuric  oxide.  If  much  ammonium  chloride  is  present, 
the  solution  should  first  be  evaporated  to  dryness  in  the  platinum 
dish  and  the  residue  gently  ignited  until  ammonium  salts  are 
expelled.  The  residue  is  then  dissolved  in  a  little  water  and 
excess  of  the  mercuric  oxide  added  and  the  material  well  stirred. 
The  solution  is  evaporated  to  dryness  with  frequent  stirring. 
The  residue  is  heated,  at  first  gently  and  then  more  strongly,  until 
mercuric  chloride  is  no  longer  evolved.  All  of  the  mercuric  oxide 
need  not  be  volatilized.  The  residue  is  extracted  repeatedly 
with  small  quantities  of  hot  water  and  the  solution  rapidly  fil- 

186 


SEPARATION  OF  MAGNESIUM  FROM   THE  ALKALIES.     187 

tered.     The  washing  must  be  discontinued  as  soon  as  the  residue 
is  free  from  chlorides. 

By  this  operation  the  magnesium  is  converted  into  oxide  accord- 
ing to  the  following  equation:  MgCl2+HgO=MgO  +  HgCl2.  The 
chlorides  of  the  alkalies  are  not  decomposed  in  this  manner.  As 
the  magnesium  oxide  is  slightly  soluble,  the  solution  of  the  alkalies 
will  contain  a  little  magnesium.  Unless  the  amount  of  magnesium 
present  is  less  than  1%,  the  treatment  with  mercuric  oxide  must  be 
repeated.  The  insoluble  mixture  of  magnesium  and  mercuric 
oxides  may  be  ignited,  the  mercury  volatilized,  and  the  magnesium 
weighed  as  oxide.  The  magnesium  may  also  be  dissolved  in 
hydrochloric  acid  after  volatilization  cf  the  mercury  and  deter- 
mined as  phosphate.  The  mercuric  oxide  used  must  not  leave 
a  residue  on  volatilizing  a  portion  by  heating  it  to  redness  in  a 
platinum  dish  or  crucible.  Alkalies  are  frequently  present  and 
remain  as  an  alkaline^  residue  after  volatilization  of  the  mercury. 
If  mercuric  oxide  free  from  alkalies  cannot  be  obtained  the  amount 
used  must  be  weighed  and  a  correction  made  for  the  amount  of 
alkali  present. 

265.  Separation  of  Magnesium  by  Means  of  Barium  Hydroxide. — 
The   magnesium  may  also   be   removed  by  treating  the  concen- 
trated solution  with  a  solution  of  barium  hydroxide  until  it  is 
strongly  alkaline.      After  filtering  off   and  washing  the  magne- 
sium hydroxide  the  excess  of  barium  must  be  removed  from  the 
solution  of  the  alkalies  by  precipitation  with  ammonia  and  am- 
monium carbonate. 

266.  Separation  of  the  Alkalies. — The   solution  of  potassium 
and  sodium  chlorides  is  evaporated  to  dryness  in  a  weighed  plati- 
num dish,  finally  on  the  water-bath.     The  residue  must  be  heated 
to  low  redness  to  completely  dry  the  chlorides  before  weighing. 
Two  difficulties  are  met  with  in  this  process.     The  crystals  of 
sodium  and  potassium  chloride  tend  to  enclose  water  so  that  on 
drying  the  material  decrepitates  with  considerable  violence,  tending 
to  throw  the  material  out  of  the  dish.    The  chlorides  of  sodium 
and  potassium  are  volatile  at  a  very  low  temperature.      It  is 
therefore  necessary  after  evaporating  on  the  water-bath  to  cover 
the  platinum  dish  with  a  watch-crystal  and  transfer  it  to  the  hot- 
plate and  dry  as  completely  as  possible  before  heating  with  the 


188  ANALYSIS  OF  MINERALS. 

Bunsen  burner.  The  burner  must  be  held  in  the  hand  and  the 
flame  waved  under  the  dish  and  removed  as 
soon  as  any  portion  of  the  dish  becomes  red- 
hot.  If  the  decrepitation  is  excessive,  the 
watch-crystal  on  the  dish  may  be  inverted 
and  the  platinum  dish  placed  in  another  of 
larger  size.  Any  material  which  is  thrown  out 
of  the  first  dish  is  then  deflected  downwards  into  the  second  one. 

The  weighed  chlorides  are  dissolved  in  water.  A  slight  amount 
of  insoluble  matter  which  is  usually  present  is  filtered  off  on  a 
small  filter-paper  and  well  washed.  The  paper  is  thrown  into  the 
platinum  dish  and  ignited.  The  dish  with  the  residue  is  weighed. 
This  weight  is  subtracted  from  the  weight  of  the  dish  and  the  dry 
chlorides.  To  the  solution  of  the  alkali  chlorides  enough  platinic 
chloride  is  added  to  convert  both  the  sodium  and  potassium  chlor- 
ides into  the  chlorplatinic  salts.  The  subsequent  manipulation 
is  the  same  as  that  given  for  potassium  in  Chapter  VII.  The 
sodium  is  found  by  difference. 

DECOMPOSITION   OF  SILICATES. 

The  first  step  in  the  analysis  of  a  silicate  has  for  its  object  the 
breaking  of  the  bond  between  the  silica  and  the  basic  elements 
present,  so  that  the  latter  may  be  brought  into  solution.  As  the 
silicates  are  among  the  most  stable  bodies  found  in  nature,  the 
accomplishment  of  this  object  is  not  always  easy.  When  com- 
pletely decomposed,  or  opened  up,  as  the  operation  is  sometimes 
termed,  the  analysis  of  the  silicate  involves  only  the  ordinary 
problems  of  separation  and  determination. 

267.  Fusion  with  Alkali  Carbonates. — By  far  the  most  largely 
used  and  perhaps  the  best  method  of  accomplishing  this  object 
consists  in  fusing  the  very  finely  ground  material  with  sodium 
carbonate  or  the  so-called  fusion  mixture,  which  consists  of  sodium 
and  potassium  carbonates  in  the  proportion  of  their  molecular 
weights.  This  mixture  fuses  at  a  somewhat  lower  temperature 
than  sodium  carbonate  alone.  The  result  of  this  method  of 
decomposition  is  that  the  silica  is  largely  converted  into  sodium 
silicate,  the  silicic  acid  displacing  the  volatile  carbonic  acid  in 


DECOMPOSITION  OF  SILICATES.  189 

the  alkali  carbonate.  The  bases  remain  as  carbonates,  or  if  a 
given  base  forms  a  carbonate  unstable  at  the  temperature  of 
the  fusion  it  will  remain  as  oxide.  The  bases  which  have  marked 
acid  properties,  such  as  aluminium,  will  combine  more  or  less 
completely  with  the  alkali. 

268.  Determination   of   Silica. — On   treating    the   fused   mass 
with  water  and  acidifying  with  hydrochloric  acid  the  alkali  sili- 
cate is  decomposed  with  liberation  of  silicic  acid.    The  remaining 
alkali  salts  which  are  formed  are  also  decomposed.     On  evapo- 
rating to  dryness,  to  dehydrate  the  silicic  acid,  and  again  taking  up 
with  water  and  hydrochloric  acid,  all  of  the  elements  present  go 
into  solution  with  the  exception  of  silica  and  part  of  the  TITANIUM 
which  is  frequently  present  in  silicates.     It  is  seldom  that  the 
decomposition  of  the  silicate  and  the  solution  of  the  bases  in 
hydrochloric  acid  is  so  complete  that  the  silica  is  absolutely  pure. 
On  the  other  hand,  some  of  the  silica  goes  into  solution  even  after 
very  prolonged  drying. 

To  recover  the  dissolved  silica,  the  filtrate  is  evaporated  to 
dryness  and  the  residue  again  taken  up  with  water  and  hydro- 
chloric acid.  The  most  complete  separation  of  silica  is  effected 
by  heating  the  dry  residue  on  the  water-bath  for  one-h&if  to 
one  hour.  If  the  residue  is  dried  on  the  hot-plate  or  in  tt  j  air- 
oven  at  a  temperature  above  100°,  more  of  the  silica  tends  to  go 
into  solution  again.  The  residue  after  the  first  evaporation  may 
be  dried  on  the  hot-plate.  This  is  a  more  rapid  method,  and  any 
silica  which  goes  into  solution  will  be  recovered  by  the  evaporation 
of  the  filtrate,  the  residue  of  which  must  be  dried  at  100°.  A  small 
amount  of  silica  (1  to  2  mg.)  goes  into  solution  even  after  the 
second  evaporation.  This  is  precipitated  with  the  iron  and  alu- 
minium and  may  be  recovered  by  dissolving  these  elements  in 
acid-potassium  sulphate,  dissolving  the  melt  in  sulphuric  acid, 
evaporating  till  fumes  appear,  diluting  with  water,  and  filtering. 

269.  Determination  of  Aluminium,  Iron,  and   Titanium. — The 
aluminium  and  titanium  oxides  which  are  usually  present  with  the 
silica  may  be  recovered  by  treating  the  ignite<  1  and  weighed  pre- 
cipitate with  hydrofluoric  and  sulphuric  acids  and  evaporating. 
In  this  manner  the  silica  may  be  completely  volatilized.     Unless 


190  ANALYSIS  OF  MINERALS. 

sulphuric  acid  is  present,  some  of  the  titanium  will  also  be  volatil- 
ized. After  blasting,  the  residue  is  weighed  and  the  correction 
applied  to  the  weight  of  the  silica.  As  the  bulk  of  this  residue  is 
generally  composed  of  titanium  and  aluminium  oxides,  it  is  advis- 
able to  combine  it  with  the  precipitate  of  aluminium,  iron,  and 
titanium  hydroxides.  After  igniting  and  weighing,  the  combined 
precipitate  is  dissolved  by  fusing  with  acid-potassium  sulphate. 
The  iron  is  reduced  by  passing  hydrogen  sulphide  through  the 
water  solution  of  the  fusion.  It  is  then  determined  volume trically. 
The  titanium  is  then  oxidized  by  means  of  hydrogen  peroxide 
and  the  amount  determined  by  comparing  the  intensity  of  color 
with  the  color  of  a  standard  titanium  solution.  (See  page  194.) 

The  other  metals  are  separated  and  determined  by  methods 
which  have  already  been  described. 

270.  Decomposition  of  Silicates  for  the  Determination  of  the  Al- 
kalies.— As  alkalies  have  been  introduced  into  the  fusion  mixture, 
these  metals  must  be  determined  in  a  separate  portion,  which  must 
be  decomposed  by  a  different  method.     One  of  the  best  of  these 
methods  is  that  of  J.  LAWRENCE  SMITH.     The  silicate  is  decom- 
posed by  gently  heating  an  intimate  mixture  of  the  silicate  with 

.  1  part  of  ammonium  chloride  and  8  parts  of  pure  calcium  carbonate. 
The  alkalies  as  well  as  some  of  the  calcium  are  converted  into 
chlorides.  Ammonia,  the  excess  of  the  ammonium  chloride,  and 
carbon  dioxide  are  volatilized.  On  treating  with  water,  the  chlo- 
rides of  the  alkalies  dissolve,  while  the  other  bases  together  with 
most  of  the  calcium  remain  undibsolved.  One  advantage  of  the 
method  is  found  in  the  fact  that  all  of  the  magnesium  remains 
in  the  insoluble  residue,  thus  separating  it  from  the  alkalies.  The 
calcium  in  the  nitrate  is  precipitated  with  ammonia  and  ammo- 
nium carbonate  and  filtered  off.  The  nitrate  is  evaporated  to 
small  bulk  and  a  little  more  calcium  precipitated  with  ammonium 
oxalate  and  filtered  off.  The  alkalies  in  the  filtrate,  which  now 
contains  nothing  but  these  metals  as  chlorides,  are  separated  and 
determined  by  methods  already  given. 

271.  Decomposition  of  Silicates  by  Fusion  with  Boric  Oxide. — 
Another  method  for  decomposing  silicates  by  which  a  solution  is 
ob tamed  in  which  all  of  the  bases,  including  the  alkalies,  may  be 


DECOMPOSITION  OF  SILICATES.  191 

determined,  has  been  devised  by  JANNASCH  and  HEIDENREICH.* 
The  flux  is  powdered  boric  oxide.  It  is  prepared  by  recrystallizing 
two  or  three  times  the  commercial  article  to  free  it  from  traces  of 
alkali.  The  purified  crystals  are  dried  and  fused  in  a  large  plati- 
num crucible  until  free  from  water.  By  suddenly  cooling  the 
fused  mass  it  cracks  into  pieces  of  a  convenient  size  for  powdering. 
The  powdered  material  is  kept  in  well-stoppered  bottles,  as  it  is 
hygroscopic. 

Nearly  all  silicates  are  readily  decomposed  when  mixed  with  3 
to  8  or  more  parts  of  this  flux  and  heated,  at  first  gently,  and  finally 
with  the  blast-lamp.  The  silicate  must  be  very  finely  powdered, 
from  one-half  to  one  hour  being  taken  for  the  grinding  of  0.5-  to 
1-gram  portions.  A  platinum  crucible  holding  45  to  60  c.c. 
should  be  used.  After  intimately  mixing  the  silicate  and  the 
flux,  gentle  heat  from  a  Bunsen  burner  is  applied.  Frothing 
and  rising  in  the  crucible  is  prevented  as  far  as  possible  by  stirring 
with  a  short  platinum  wire  which  does  not  reach  above  the  edge 
of  the  crucible.  When  the  material  has  been  in  quiet  fusion  for 
some  time  in  the  covered  crucible,  the  flame  of  the  blast-lamp  is 
applied.  From  twenty  to  thirty  minutes  are  generally  required  for 
the  entire  operation. 

Some  silicates  like  andolusite,  cyanite,  and  topaz  are  not  fully 
decomposed  when  heated  with  the  flux  by  means  of  the  blast- 
lamp,  For  these  minerals  more  of  the  flux  must  be  used,  as  high 
as  30  parts  to  1  of  the  mineral  being  taken.  After  the  preliminary 
heating  with  the  Bunsen  burner,  a  few  additional  grams  of  boric 
oxide  are  added  and  a  flame  which  is  fed  by  oxygen  in  place  of  air 
is  applied.  An  ordinary  blast-lamp  is  used  with  an  opening  2J  mm. 
wide.  It  is  supplied  with  gas  from  at  least  5  or  6  ordinary  gas- 
cocks  and  the  flame  is  made  broad  and  free  from  luminosity.  The 
heating  is  continued  until  the  fusion  is  as  transparent  as  glass. 

272.  Volatilization  of  the  Boric  Acid. — After  fusion,  the  hot 
covered  crucible  is  immersed  in  cold  water  until  cold.  The  con- 
tents are  then  placed  in  a  large  porcelain  or  platinum  dish  and  a 
saturated  solution  of  hydrochloric  acid  hi  methyl  alcohol  is  added 

*  Zeit.  fur  anorg.  Chem.,  Vol.  XII,  p.  208,  1896. 


192  ANALYSIS  OF  MINERALS. 

while  the  dish  is  covered  with  a  watch-crystal.  The  crucible  is 
rinsed  with  more  of  the  methyl  chloride  and  the  solution  in  the 
dish  is  gently  heated  until,  with  occasional  additions  of  the  methyl 
chloride,  solution  is  complete.  This  requires  from  ten  to  fifteen 
minutes.  The  solution  is  then  boiled  down  to  small  bulk  and 
evaporated  to  dryness  on  a  water-bath  in  which  the  water  is  not 
quite  boiling,  so  that  the  temperature  in  the  dish  is  80°  to  85°. 
More  methyl  chloride  is  added  and  the  heating  at  this  temperature 
is  continued  until  the  boric  acid  is  completely  expelled.  All  of 
the  metals  will  then  be  in  solution  as  chlorides  and  may  be  sepa- 
rated and  determined  by  methods  already  given. 

This  method  of  decomposing  silicates  has  the  advantage  that 
a  solution  of  the  mineral  is  obtained  which  is  small  in  bulk  and 
free  from  the  large  amount  of  alkali  salts  which  are  introduced 
by  the  carbonate  fusion.  The  separations  of  many  of  the  metals 
are  much  cleaner  and  sharper  in  the  absence  of  a  large  amount 
of  the  alkali  salts.  All  of  the  metals,  including  the  alkalies,  may 
be  determined  in  one  portion.  This  is  a  decided  advantage 
when  only  a  small  amount  of  the  mineral  is  at  hand.  On  the  other 
hand,  the  purification  of  the  boric  acid,  and  more  especially  the 
uncertainty  as  to  the  complete  decomposition  of  the  mineral,  are 
disadvantageous.* 

EXERCISE  39. 

Analysis  of  Feldspar,  (K20,Al203)(Si02)fl. 
Iron,  Calcium,  Magnesium,  Sodium,  and  Titanium  are  also  Usually  Present. 

273.  Decomposition  of  the  Mineral. — One  gram  of  the  finely  powdered 
material  is  placed  in  a  rather  large  platinum  crucible  and  mixed  with  5 
grams  of  a  mixture  of  5  parts  of  sodium  and  7  parts  of  potassium  carbonate. 
The  crucible  is  heated  with  the  Bunsen  burner  until  the  mass  is  in  quiet 
fusion  and  no  more  carbon  dioxide  is  evolved.  About  one-half  hour  will  be 
required.  Seize  the  crucible  with  the  tongs  and  cause  the  contents  to  solidify 
in  a  thin  film  around  the  sides  of  the  crucible  by  giving  it  a  rotary  motion 
while  the  contents  are  cooling.  Transfer  the  crucible  to  a  beaker  just  large 
enough  to  allow  the  crucible  to  be  placed  on  its  side.  Add  enough  water 
to  cover  the  crucible  and  heat  on  the  water-bath  until  the  melt  is 
disintegrated.  Take  out  the  crucible  with  a  pair  of  tongs  and  rinse  it  well. 
The  lid  must  also  be  well  washed. 

*  For  common  errors  in  the  determination  of  silica,  see  Jour.  Am.  Chem.  Soc., 
J4  (1902),  p.  262. 


ANALYSIS  OF  FELDSPAR.  193 

The  material  in  the  beaker  should  be  examined  for  undecomposed  sili- 
cate by  rubbing  the  bottom  with  a  glass  rod.  Any  gritty  substance  is 
undecomposed  feldspar.  The  main  portion  of  the  solution  may  be  decanted 
and  a  little  water  added.  When  the  material  in  the  water  is  stirred  .up, 
undecomposed  silicate  settles  quickly,  while  separated  silica  rises  readily 
and  settles  slowly.  When  strong  hydrochloric  acid  is  added  and  the  beakei 
is  heated,  undecomposed  silicate  does  not  dissolve.  Filter  it  off  on  a  small 
paper,  wash,  and  add  the  nitrate  to  the  main  solution.  Burn  the  paper 
and  fuse  the  insoluble  material  with  five  times  its  weight  of  the  mixture  of 
sodium  and  potassium  carbonate.  The  second  fusion  is  treated  exactly 
as  the  first.  Failure  to  secure  complete  decomposition  the  first  time  is 
generally  due  to  insufficient  grinding  of  the  silicate. 

273.  Silica. — Transfer  the  alkaline  solution  to  a  platinum  or  porcelain  dish, 
co^rer  with  a  watch-crystal  and  add  excess  of  hydrochloric  acid.  The  fused 
material  may  also  be  dissolved  by  the  following  method.  The  melt  is 
poured  into  the  crucible  cover.  As  soon  as  redness  has  gone,  and  while 
still  hot,  slip  the  crucible  and  cover  into  a  covered  beaker  containing  enough 
dilute  hydrochloric  acid  to  neutralize  the  sodium  carbonate.  Solution  is 
complete  in  a  few  minutes  and  most  of  the  silicic  acid  remains  in  solution. 
Decant  the  solution  if  there  is  any  undecomposed  silicate  which  is  filtered 
off  and  again  fused. 

The  solution  is  evaporated  to  dryness  on  the  water-bath  and  heated 
for  at  least  one-half  hour.  Add  20  c.c.  dilute  hydrochloric  acid  and  then 
heat  on  the  water-bath  until  nothing  more  dissolves.  About  one-half  hour 
is  generally  sufficient.  Add  50  c.c.  hot  water,  filter  off  the  silica  and  wash 
a  few  times.  Evaporate  the  filtrate  to  dryness  and  heat  one-half  hour  on 
the  water-bath.  Add  20  c.c.  dilute  hydrochloric  acid  to  the  residue  and 
heat  on  the  water-bath  until  nothing  more  dissolves.  Add  50  c.c.  hot 
water  and  filter  off  the  silica  on  the  paper  containing  the  main  portion, 
and  wash  with  hot  water  containing  hydrochloric  acid,  and  finally  with 
pure  water,  until  the  precipitate  is  free  from  chlorides.  Particular  care 
should  be  taken  to  wash  this  silica  very  thoroughly.  If  it  should  turn  black 
on  ignition  it  indicates  insufficient  washing. 

Drjr  the  precipitate,  transfer  it  to  the  weighed  platinum  crucible,  fold 
the  paper  and  place  it  on  the  precipitate.  Cover  the  crucible  and  heat 
gently  with  the  Bunsen  burner  until  volatile  matter  is  expelled,  finally 
with  the  full  flame  of  the  Bunsen  burner.  Remove  the  cover,  incline  the 
crucible,  and  continue  heating  until  the  carbon  is  completely  burned  and  the 
precipitate  is  pure  white.  Replace  the  cover  and  heat  with  the  blast-lamp 
for  fifteen  to  twenty  minutes,  cool,  and  weigh.  Moisten  the  precipitate  with 
a  little  water,  add  a  few  cubic  centimeters  of  hydrofluoric  acid  and  a  drop 
or  two  of  sulphuric  acid.  Evaporate  the  acid  and  ignite  the  residue  with 
the  blast-lamp.  The  weight  of  the  residue  must  be  subtracted  from  the 
weight  of  the  silica.  The  aluminium  iron  precipitate  is  ignited  in  this  plat- 
inum crucible  together  with  the  small  residue  from  the  silica  precipitate. 


194  ANALYSIS  OF  MINERALS. 

275.  Aluminium. —  In  the  absence  of  manganese,  the  IRON,  TITANIUM, 
and  ALUMINIUM  may  be  precipitated  with  ammonia  and  ammonium  chloride. 
The  precipitate  must  be  redissolved  and  reprecipitated.     If  MANGANESE  is 
present,  the  alkali  fusion  of  the  silicate  will  generally  have  a  bluish-green 
color.     Absence  of  this  color  is  not  proof  of  the  absence  of  manganese.     In 
the  presence  of  manganese,  the  aluminium  must  be  precipitated  as  basic 
acetate.     The  second  precipitation,  however,  may  be  made  by  ammonia 
and  ammonium  chloride.     These  precipitations  are  carried  out  as  directed 
on  p.  157,  Chapter  XIII.     On  concentrating  the  two  nitrates  separately  to 
i  bulk  of  75  or  100  c.c.,  a  small  precipitate  of  aluminium  hydroxide  some- 
times separates  out  in  one  or  both  of  the  solutions.     This  is  filtered  off  on 
a  small  filter-paper,  the  first  filtrate  being  first  passed  through  the  paper, 
the  second  one  being  used  to  wash  the  first  beaker  and  the  filter-paper. 
The  small  precipitate  is  then  redissolved  in  a  little  hydrochloric  acid  and 
reprecipitated,  filtered  off  on  the  same  paper,  washed,  and  added  to  the  main 
precipitate. 

276.  Iron. — After  igniting  and  weighing  the  aluminium-iron  precipitate, 
it  is  dissolved  in  acid-potassium  sulphate,  3  or  4  grams  of  which  are  placed 
in  the  crucible,  which  is  heated  just  high  enough  to  melt  the  acid  sulphate. 
Several  hours'  heating  may  be  required  to  dissolve  a  large  precipitate. 
When  everything  is  dissolved,  except  possibly  a  little  silica,  the  crucible 
is  cooled  and  the  contents  dissolved  in  dilute  sulphuric  acid  and  the  solu- 
tion evaporated  until  fumes  of  sulphuric  acid  are  evolved.     Hot  water  is 
added  and  the  silica  is  filtered  off,  ignited,  and  weighed.     Its  weight  is  to 
be  subtracted  from  the  weight  of  the  alumina  and  ferric  oxide.     The  solu- 
tion is  placed  in  an  Erlenmeyer  flask  of  about  250  c.c.  capacity.     The  flask 
is  fitted  with  a  two-holed  rubber  stopper,  a  glass  tube  extending  to  the 
bottom  of  the  flask  being  passed  through  one  hole  while  a  short  glass  tube 
is  inserted  in  the  other.     The  solution  is  heated  nearly  to  boiling  and  hydro- 
gen sulphide  passed  to  reduce  the  iron.     The  excess  of  hydrogen  sulphide  is 
expelled  by  boiling  the  solution  and  passing  a  stream  of  carbon  dioxide. 
When  the  hydrogen  sulphide  is  entirely  expelled,  as  indicated  by  lead- 
acetate  paper,  the  solution  is  cooled,  while  the  stream  of  carbon  dioxide  is 
passing  and  titrated  with  standard  potassium-permanganate  solution.     The 
iron  cannot  be  reduced  by  zinc,  since  the  titanium  would  then  be  reduced 
also.     After  titrating  the  iron,  the  solution  is  suitable  for  the  determina- 
tion of  titanium  as  described  in  the  following  paragraph. 

277.  Titanium.  —  The   titanium   is   estimated   by   comparing  the   color 
of  the  unknown  solution,  after  oxidation  with  hydrogen  peroxide,  with 
the  color  of  a  standard  solution  of  titanium.     The  standard  solution  of 
titanium  sulphate  should  contain  1  gram  of  Ti02  per  liter  or  the  equivalent 
amount  of  titanium  sulphate.     5%   or  more  of  sulphuric  acid  must  be 
present  in  this  solution.     10  c.c.  are  carefully  measured  out  and  mixed 
with  2  c.c.  of  5%  hydrogen  peroxide  free  from  fluorides  and  diluted  to  100  c.c. 
in  a  measuring-flask.     The  solution  to  be  tested  is  evaporated  to  about 


ANALYSIS  OF  FELDSPAR.  195 

75  c.c.,  transferred  to  a  100-c.c.  flask,  hydrogen  peroxide  added  as  long  as 
the  color  is  intensified,  and  the  solution  finally  made  up  to  100  c.c.  with  dilute 
sulphuric  acid.  If  the  color  of  the  solution  is  much  more  intense  than 
that  of  the  standard  solution,  it  should  be  transferred  to  a  larger  flask  and 
diluted  with  water  and  sulphuric  acid.  50  c.c.  of  the  standard  solution  is 
placed  in  a  Xessler  tube.  The  unknown  solution  is  introduced  from  a 
burette  into  a  second  Nessler  tube  until  the  color  seems  equal  to  that  of  the 
standard.  It  is  then  diluted  to  50  c.c.  After  shaking,  the  color  is  again 
compared  with  that  of  the  standard.  If  the  colors  are  not  identical,  a  third 
Xessler  tube  is  taken  and  a  little  more  or  less  of  the  unknown  solution  is  in- 
troduced as  indicated  by  the  first  test.  After  diluting  to  50  c.c.,  the  colors 
are  again  compared.  If  the  unknown  solution  is  more  dilute  than  the 
standard,  the  process  is  reversed.  As  50  c.c.  of  the  standard  solution  con- 
tains 5  mg.  of  TiO2,  the  volume  of  the  unknown  solution  which  gives  the 
same  depth  of  color  will  also  contain  5  mg.  of  titanium  oxide.  The  total 
amount  may  then  be  easily  computed.  The  amount  of  aluminium  oxide 
is  found  by  subtracting  the  weight  of  iron  found  computed  as  Fe2O3  plus 
the  weight  of  Ti02  from  the  total  weight  of  the  precipitate. 

278.  The  Manganese,  Calcium,  and  Magnesium  are  separated  and  deter- 
mined as  given  in  Exercise  38,  page  181. 

DETERMINATION    OF   THE    ALKALIES. 

279.  Decomposition  of  the  Mineral. — For  the  estimation  of  the  alkalies, 
1  gram  of  the  finely  ground   mineral  is  intimately  mixed  in  a  platinum 
crucible  having  a  closely  fitting  cover  with  about  1  gram  of  resublimed 
ammonium  chloride  and  8  grams  of  calcium  carbonate  which  is  free  from 
alkalies.     The  thorough  mixing  required  is  best  effected  by  grinding  together 
in  a  large  agate  or  porcelain  mortar  the  mineral  and  the  ammonium  chloride. 
The  carbonate  of  lime  is  divided  into  four  parts.     Three  of  the  portions 
are  successively  added  and  thoroughly  mixed.     The  material  is  then  trans- 
ferred to  the  platinum  crucible.     The  mortar  is  rinsed  with  the  fourth 
portion  of  the  carbonate  of  lime.     The  glazed  paper,  which  should  from  the 
beginning  be  under  the  mortar,  is  then  brushed  off.     The  contents  of  the 
crucible  are  settled  down  by  gently  tapping  it  on  the  table.     If  a   large 
enough  crucible  is  not  at  hand,  half  the  quantities  may  be  used. 

The  crucible  is  covered  and  placed  in  an  inclined  position  on  a  pipe-stem 
triangle.  It  is  at  first  heated  gently  with  the  small  flame  of  a  Bunsen 
burner,  placed  so  that  the  flame  does  not  touch  the  crucible.  As  soon  as 
the  odor  of  ammonia  is  no  longer  perceptible,  which  should  be  after 
about  ten  minutes'  heating,  the  crucible  is  heated  to  dull  redness  for  not 
more  than  two-fifths  of  its  height  for  forty  or  fifty  minutes. 

280.  Removal  of  Calcium. — After  cooling,  the  sintered  cake,  which  is  gen- 
erally detached  very  readily  from  the  crucible,  is  placed  in  a  porcelain  dish 
and  60  to  80  c.c.  of  hot  water  added.     The  crucible  is  washed  out  with 


196  ANALYSIS  OF  MINERALS. 

hot  water  and  the  cake  is  broken  up  with  a  blunt  rod.  The  water  is  brought 
to  a  boil  and  after  being  allowed  to  settle,  the  solution  is  decanted  through 
a  filter-paper.  The  material  in  the  dish  is  washed  two  or  three  times  with 
hot  water  by  decantation,  finally  brought  on  the  filter-paper,  and  washed 
free  from  chlorides.  The  residue  may  be  tested  for  unattacked  mineral 
by  dissolving  in  hydrochloric  acid. 

The  calcium  in  the  filtrate  is  precipitated  by  neutralizing  with  ammonia 
and  adding  ammonium  carbonate  and  ammonia.  The  precipitation  should 
be  carried  out  in  a  platinum  or  porcelain  dish  and  the  solution  should  be 
heated  nearly  to  boiling.  After  digesting  some  time,  the  precipitate  is 
filtered  off  and  well  washed  with  hot  water.  The  filtrate  is  evaporated 
to  dryness  in  a  platinum  dish  and  the  ammonium  sa^.ts  expelled.  The 
residue  is  dissolved  in  a  few  cubic  centimeters  of  water,  a  few  drops  of 
ammonium-oxalate  solution  added,  and  the  small  precipitate  of  calcium 
oxalate  filtered  off  and  washed. 

281.  Potassium  and  Sodium.  —  The  filtrate,  which  now  contains  only 
sodium  and  potassium  as  chlorides  and  a  little  ammonium  oxalate  and 
chloride,  is  evaporated  to  dryness  in  a  weighed  platinum  dish.  The  ammo- 
nium salts  are  expelled  by  gently  heating  the  dish,  covered  with  a  watch- 
crystal,  first  on  the  hot-plate  and  then  with  the  flame  of  the  Bunsen  burner. 
The  dish  must  not  be  heated  higher  than  to  dull  redness,  and  that  only  for 
a  moment.  The  burner  is  held  in  the  hand  and  waved  under  the  dish. 
(See  p.  187,  §  266.)  The  residue  is  moistened  with  hydrochloric  acid  to 
convert  any  alkali  carbonate  into  chloride.  It  is  again  evaporated  to 
dryness,  ignited  and  weighed. 

When  the  alkali  chlorides  have  been  brought  to  constant  weight  they 
are  dissolved  in  a  few  cubic  centimeters  of  water  and  enough  10%  platino- 
chloride  solution  added  to  convert  all  of  the  alkali  chlorides  present  into 
the  double  platinum  salt.  In  this  calculation,  the  amount  of  platinum 
solution  necessary  to  convert  the  precipitate  into  the  double  chloride,  if 
it  were  all  sodium  chloride,  should  be  found  and  a  little  more  than  this 
amount  added.  The  solution  is  evaporated  to  a  syrupy  consistency  and 
about  50  c.c.  of  80%  alcohol,  added.  The  evaporation  and  subsequent 
manipulation  must  be  carried  out  in  an  atmosphere  which  is  free  from 
ammonia  fumes.  The  solid  material  is  occasionally  well  stirred  with  a 
glass  rod  until  no  more  solvent  action  can  be  observed.  The  solution  is 
decanted  through  a  Gooch  crucible,  which  has  been  dried  at  110°  to  115°. 
More  80%  alcohol  is  added,  the  precipitate  digested  and  stirred,  and 
the  alcohol  decanted.  When  the  alcohol  no  longer  becomes  strongly  col- 
ored, the  precipitate  is  transferred  to  the  crucible  and  washed  with  small 
portions  of  the  80%  alcohol  until  it  comes  through  colorless.  Dry  the 
crucible  at  110°  to  115°  and  weigh.  The  amount  of  potassium  chloride  is 
now  computed  and  subtracted  from  the  weight  of  the  two  chlorides  to  obtain 
the  amount  of  sodium  chloride  present. 


ELECTROLYTIC  METHODS. 
|    CHAPTER  XVII. 

THE  IONIC  THEORY;  ELECTROLYTIC  APPARATUS  AND 
MANIPULATIONS. 

THE  separation  of  metals  from  their  solutions  and  their 
electrolytic  deposition  in  films  which  can  be  dried  and  weighed 
offers  in  the  case  of  many  metals  rapid  and  accurate  methods  of 
determination,  and  while  in  many  cases  the  time  necessary  for 
complete  deposition  of  the  metal  is  considerable,  little  or  no  atten- 
tion from  the  operator  is  required.  The  methods  in  use  have 
generally  been  empirically  discovered  without  reference  to  the 
prevailing  theories  of  the  nature  of  electrolysis.  The  subject  is 
made  more  intelligible  when  studied  in  the  light  of  theories  which 
serve  to  combine  and  harmonize  all  of  the  facts  known. 

282.  The  Ionic  Theory  of  Electrolysis. — It  is  now  generally 
believed  that  when  the  salt  of  a  metal  is  dissolved  in  water,  the 
molecules  are  separated  by  the  water  into  at  least  two  parts;  which 
are  electrically  charged  and  are  called  ions.  In  the  case  of  copper 
sulphate,  the  copper  atom  is  separated  as  an  ion  from  the  remainder 
of  the  molecule,  the  sulphur  and  oxygen  remaining  in  combination 
and  constituting  a  second  ion.  The  molecule  of  cupric  chloride 
separates  into  three  ions,  one  of  which  is  the  copper  atom,  while 
the  other  two  are  the  chloride  atoms.  When  an  electrical  current 
is  passed  through  the  solution,  the  metallic  ions  move  with  the 
current  and  tend  to  accumulate  around  the  cathode,  and  are 
therefore  called  CATHIONS,  while  the  ions  composed  of  the  acid 
radicles  move  in  a  direction  opposite  to  that  in  which  the  current 
is  moving  and  tend  to  accumulate  around  the  anode,  and  are 
therefore  called  ANIONS.  This  separation  of  the  constituent  parts 
of  a  metallic  salt  is  called  electrolysis. 

An  ION  may  therefore  be  defined  as  a  metallic  atom,  or  an  acid 
radicle,  which  is  charged  with  positive  or  negative  electricity  and 

197 


198  ELECTROLYTIC  METHODS. 

exists  in  the  solution  of  salts,  bases,  and  acids,  in  water  and  a  few 
other  solvents. 

283.  Amount  of  the  Electrical  Charge  on  Ions. — According  to 
Faraday's  law  the  amount  of  a  given  metal  which  is  deposited 
from  a  solution  is  proportional  to  the  amount  of  current  passed, 
provided  no  other  decomposition  is  effected  by  the  current.  For 
example,  a  current  of  one  ampere  passing  for  one  hour  will  deposit 
4.026  grams  of  silver,  any  change  in  the  amount  of  current  passed 
producing  a  proportionate  change  in  the  amount  of  silver  depos-  t 
ited.  This  would  be  the  case  if  each  metallic  atom  or  ion  bore  a 
fixed  and  definite  amount  of  electricity,  since  the  current  is  carried 
through  the  solution  only  by  means  of  the  ions  and  when  a  given 
metallic  ion  gives  up  its  load  of  electricity  at  the  cathode  it  is 
deposited  in  metallic  form. 

As,  still  further,  the  amount  of  a  given  metal  deposited  by  a 
current  is  inversely  proportional  to  the  valence  of  the  metal,  the 
charge  on  each  ion  must  be  directly  proportional  to  the  valence.  A 
current  of  one  ampere  passing  for  one  hour  through  a  solution  of 
cupric  copper  will  deposit  1.184  grams  of  copper,  while  the  same 
current  passing  for  one  hour  through  a  solution  of  cuprous  copper 
will  deposit  twice  as  much,  or  2.368  grams  of  copper.  An  atom 
of  copper  present  as  cuprous  chloride  must  therefore  carry  only 
half  as  much  electricity  as  an  atom  of  copper  existing  as  cupric 
chloride.  As  the  valence  of  the  chlorine  atom  in  these  com- 
pounds is  the  same,  and  it  is  charged  with  negative  electricity, 
the  amount  of  this  negative  charge  on  each  of  the  chlorine  atoms 
must  be  the  same  since  the  solutions  of  both  cuprous  and  cupric 
chloride  are  uncharged;  that  is,  they  contain  the  same  amount 
of  positive  and  negative  electricity. 

While  the  amount  of  a  given  metal  deposited  is  proportional 
to  the  current  used,  the  amounts  of  different  metals  deposited  by 
the  same  current  are  directly  proportional  to  the  atomic  weights 
and  inversely  proportional  to  the  valence  of  the  metals.  The 

M  M'   M" 

quantities  — ,  — ,  —77,  etc.,  express  the  relative  amounts  of  dif- 
ferent metals  deposited  by  the  same  current,  where  M,  M',  M" ', 
etc.,  represent  the  atomic  weights  of  the  metals  used  and  n,  n',  n" 
represent  the  valences  of  the  metals  as  they  existed  in  the  solu- 


THE  IONIC   THEORY.  199 

tions  used.     Taking  silver,  cupric  copper,  and  gold  as  illustrations, 
the  relative  amounts  of  the  metals  deposited  will  be  in  the  ratio  of 

/>O    1O  I  Q£»    >J 

107.66,   — 2~-,   and   — ^-,   or   107.66,   31.59,   and  65.57.     These 

numbers  are  in  the  same  ratios  as  4.026,  1.184,  and  2.458,  the 
amounts  deposited  by  one  ampere-hour  of  current. 

In  the  language  of  the  present  electrolytic  theory,  an  atom  of 
any  metal  in  the  ionic  condition  carries  a  positive  charge  of 
electricity  the  amount  of  which  is  proportional  to  the  valence  of 
the  metal  and  is  the  same  for  all  metals  of  equal  valence.  All 
acid  atoms  or  radicles  carry  a  charge  of  negative  electricity 
which  is  proportional  to  the  valence  and  independent  of  the 
nature  of  the  acid.  Atoms  of  sodium,  potassium,  silver,  and 
ammonium  carry  the  same  amount  of  positive  electricity,  while 
monovalent  acid  radicles  such  as  chlorine,  bromine,  iodine,  N03, 
C103,  etc.,  carry  the  same  amount  of  negative  electricity. 

284.  Potential  of  the  Electrical  Charge  on  Ions.  —  While  the 
amount  of  the  charge  on  all  univalent  atoms  is  the  same,  the 
potential  is  characteristic  of  the  metal  concerned.  This  may 
readily  be  shown  by  electrolyzing  solutions  of  the  various  metals 
in  which  the  same  acid  anion  is  present.  The  minimum  differ- 
ence in  potential  which  must  be  impressed  on  the  electrodes 
before  a  given  metal  will  be  deposited  will  be  found  to  be  character- 
istic of  that  metal. 

The  following  minimum  decomposition  tensions  were  deter- 
mined by  LeBlanc  for  normal  solutions : 

Volts.  Volts.  Volts. 

ZnS04 2.35          NiCl2 1.85          Cd(N03)2. .  .  1.98 

NiS04 2.09          CdCl2. 1.88          Pb(N03)2  .  .  1.52 

CdS04 2.03          CoCl2 1.78          AgN03 0.70 

CoS04 1.92 

This  peculiarity  of  the  metals  has  been  used  to  effect  their 
separation.  The  metal  which  separates  at  the  lowest  potential 
is  taken  out  first.  On  increasing  the  electromotive  force  of  the 
current  other  metals  may  be  deposited. 

In  a  sulphuric-acid  solution  of  cobalt  and  zinc,  for  example, 
the  cobalt  could  be  deposited  by  a  current  of  an  electromotive 
force  less  than  2.35  volts  and  greater  than  1.92.  After  all  of  the 


200  ELECTROLYTIC  METHODS. 

cobalt  had  been  deposited  the  zinc  could  be  deposited  by  increas- 
ing the  electromotive  force  of  the  current  to  more  than  2.35  volts. 

285.  Significance  of  Current  Density. — Not  only  must  the  poten- 
tial of  the  current  used  exceed  a  definite  quantity  before  a  given 
metal  will  be  deposited,  but  the  amount  also  of  current  used 
must  be  sufficiently  large.     The  necessity  for  this  is  found  in 
the  fact  that  the  deposited  metal  tends  to  redissolve  in  most  of 
the  solutions  from  which  it  is  deposited.     The  rate  at  which  the 
metal  is  deposited  must  exceed  that  at  which  it  redissolve s.     As  the 
rate  at  which  the  metal  dissolves  is  proportional  to  the  amount 
of  surface  exposed,  the  amount  of  current  necessary  to  deposit  the 
metal  will  depend  on  the  area  of  the  cathode  surface.     The  amount 
of  current  per  unit  area,  known  as  the  current  density,  is  there- 
fore   of    importance    in    electrolytic    determinations.     The    area 
generally  used  as  the  unit  is  100  sq.  cm.,  and  statements  of  the 
amount  of  current  necessary  refer  to  the  amount  needed  over 
such  an  area.     The  symbol  ND100  is  used  for  such  a  current  den- 
sity.    The  expression  ND1QO  =  1.5  amperes  means  that  1.5  amperes 
of  current  should  be  used  for  each  100  sq.  cm.,  of  cathode  area. 

As  metals  differ  greatly  in  the  ease  with  which  they  dissolve 
in  a  given  acid,  it  is  possible  to  so  choose  the  strength  of  current 
to  be  used  that  with  a  given  concentration  of  acid  in  a  solution 
of  two  metals  one  will  be  deposited  while  the  other  will  remain 
in  solution.  Copper  and  iron  are  generally  separated  in  this 
manner. 

While  the  tendency  of  the  metal  to  redissolve  in  the  solution 
fixes  a  lower  limit  to  the  amount  of  current  which  may  be  used, 
the  tendency  of  most  metals  to  form  deposits  which  are  spongy 
and  non-coherent  when  rapidly  deposited  prevents  the  use  of  an 
indefinitely  large  current.  As  the  condition  of  the  deposited 
metal  is  affected  by  the  solution  from  which  it  is  deposited,  espe- 
cially by  the  amount  of  acid  present,  and  the  temperature,  the 
amount  of  current  which  may  be  used  must  be  determined  by 
experiment,  the  temperature,-  concentration,  acidity,  etc.,  of  the 
solution  being  fixed. 

286.  Secondary  Electrolytic  Reactions  of   Acid  Radicles. — The 
secondary  reactions  produced  by  the  current  frequently  influence 
the  electrolysis  to  a  marked  extent.     Nitric  acid  is  so  rapidly 


SECONDARY  ELECTROLYTIC  REACTIONS.  201 

reduced  to  ammonia  by  a  current  of  moderate  strength,  that  the 
reaction  may  become  strongly  alkaline  during  the  course  of  an 
ordinary  electrolytic  determination  if  a  too  dilute  acid  is  used. 
Solutions  of  oxalic  acid  are  entirely  decomposed,  hydrogen  being 
given  off  at  one  electrode,  while  carbon  dioxide  is  liberated  at  the 
other,  the  decomposition  taking  place  according  to  the  following 
equation: 

H2C204  =  H2  (cathode) +  2C02  (anode). 

If  an  oxalate  of  one  of  the  alkali  metals  is  present,  the  carbon 
dioxide  is  not  evolved,  but  remains  in  combination  with  the  alkali, 
the  transformation  from  oxalate  to  carbonate  being  ultimately 
complete.  The  reaction  with  ammonium  oxalate  is  as  follows: 

2H20  +  (NH4)2C  A  -  2NH4HC03 + H2. 

The  chlorine  set  free  during  the  electrolysis  of  hydrochloric 
acid  or  a  chloride  reacts  with  the  water  to  form  hypochlorous, 
chloric,  and  perchloric  acids,  which  combine  with  the  bases  present 
to  form  salts.  No  secondary  products  are  formed  by  the  electroly- 
sis of  sulphuric  acid  and  sulphates. 

287.  The  Secondary  Reactions  Produced  by  the  Metals  are  less 
numerous  and  complicated.  The  alkali-  and  alkaline-earth  metals 
when  deposited  by  the  current  react  with  the  water  present  to 
form  hydroxides,  which  combine  with  any  acids  present.  Silver 
is  almost  always  deposited  from  a  solution  of  the  double  cyanide 
KAgCy2.  In  this  compound  the  silver  undoubtedly  forms  a 
part  of  the  acid  or  negative  radicle  which  is  carried  by  the  current 
to  the  anode.  The  silver,  however,  is  found  deposited  on  the 
cathode.  This  is  explained  on  the  assumption  that  the  potassium 
which  is  liberated  at  the  cathode  displaces  the  silver  in  the  cyanide 
radicle,  one  atom  of  potassium  thus  liberating  one  atom  of  silver. 
The  deposits  of  some  of  the  heavy  metals,  such  as  copper,  are 
found  contaminated  with  oxygen,  while  with  others,  as  well  as 
copper,  hydrogen  is  occluded  under  certain  conditions.  Some 
of  the  metals,  notably  lead  and  manganese,  are  found  deposited 
on  the  anode.  These  metals  form  negative  ions  which  are  depos- 
ited on  the  anode  as  most  metals  are  on  the  cathode,  or  are  oxidized 


202  ELECTROLYTIC  METHODS. 

by  the  nascent  oxygen  liberated  at  the  anode.  The  oxides  formed 
in  the  case  of  manganese  and  lead,  being  insoluble  in  the  nitric  acid 
present,  are  deposited  on  the  anode.  The  peroxides  of  manganese 
and  lead  formed  in  this  manner  may  be  dried  and  weighed  as 
such.  This  property  of  manganese  and  lead  offers  an  excellent 
method  of  separating  these  from  many  other  metals.  In  this 
way  the  determination  of  two  metals  may  be  carried  on  simul- 
taneously, one  being  deposited  on  the  anode  and  the  other  on  the 
cathode. 

288.  Reducing  Current  Strength  by  Means  of  Incandescent 
Lamps. — The  electric  current  for  quantitative  work  may  be 
obtained  in  various  ways.  The  ordinary  commercial  direct- 
current  circuit  of  110  volts  is  undoubtedly  the  most  convenient, 
and  where  available  the  work  is  accomplished  by  such  a  current 
most  satisfactorily  and  with  the  least  labor.  The  current  must, 
of  course,  be  available  the  entire  twenty-four  hours  of  the  day, 
as  it  is  often  most  convenient  to  allow  the  electrolysis  to  continue 
overnight.  The  voltage  mentioned  is  much  greater  than  is  ever 
required,  and,  as  in  reducing  it  part  of  the  energy  is  wasted,  a  cur- 
rent of  smaller  voltage  is  more  economical.  The  reduction  of  the 
current  is  very  conveniently  accomplished  by  means  of  incandes- 
cent lamps.  If  the  current  is  passed  through  a  16-candle-power 
lamp  about  0.4  ampere  will  be  obtained.  A  32-candle-power  lamp 
will  give  a  current  of  0.8  ampere,  while  a  50-candle-power  lamp  will 
give  a  current  of  1.2  amperes.  The  resistance  of  most  solutions 
prepared  for  electrolysis  is  so  small  that  when  placed  in  series 
with  the  lamp  the  current  is  not  materially 
reduced.  A  very  convenient  arrangement 
of  the  lamp-sockets  is  shown  in  Fig.  27. 
The  lamp-sockets  L  and  U  are  connected 
to  binding-posts  a,  6,  and  c,  as  shown  in 
the  figure,  the  various  parts  being  screwed 
to  a  convenient  sized  board.  By  connecting 
at  a  and  b,  or  at  a  and  c,  the  current  is 
.p  passed  through  one  lamp,  while  by  con- 

necting   at    b    and    c     the    current    passes 

through  both  lamps  in  series,  and  by  connecting  b  and  c  with  a 
short  piece  of  wire  and  passing  the  current  from  a  to  6,  it  passes 


REGULATION  OF  CURRENT.  203 

through  the  lamps  in  parallel.  If  16-candle-power  lamps  are 
used,  the  single  lamps  give  0.4  ampere,  the  lamps  in  series  give  0.2 
ampere,  while  the  two  lamps  in  parallel  give  0.8  ampere.  The  32- 
candle-power  lamps  connected  in  the  same  manner  give  0.8,  0.4, 
and  1.6  amperes  respectively,  while  the  50-candle-power  lamps  give 
currents  of  1.2,  0.6,  and  2.4  amperes.  Most  electrolytic  work 
can  be  done  by  these  strengths  of  current. 

289.  Reduction  of  the  Voltage  of  a  Current. — If  very  small  cur- 
rents are  desired  they  may  most  easily  be  obtained  by  making 
the  electrolytic  solution  a  shunt.     If  a  16- 
candle-power  lamp  is  connected  by  means  of 

a  Gorman-silver  or  other  wire  of  several  ohms 

resistance  to  the  main  circuit  of  110  volts, 

any  small  number  of  volts  desired  may  be 

sent  through  the  solution  by  attaching  the  pIG<  28. 

wires  to  two  points  on  the  resistance  wire. 

If  the  resistance  between  the  two  points  A  and  B  is  about  2J  ohms 

the  difference  of  potential  between  these  points  will  be  about  1 

volt.    If  about  5  ohms  are  enclosed  between  A  and  B,  the  difference 

in  potential  will  be  about  2  volts.    In  this  manner  any  desired  small 

voltage  may  be  obtained.     As  OHM'S  LAW  holds  for  the  solution,  the 

current  produced  will  be  inversely  proportional  to  the  resistance. 

If,  for  instance,  the  resistance  of  the  solution  is  30  ohms  and  a 

difference  of  potential  of  2  volts  is  desired  at  the  electrodes  the  cur- 

1  f1 

rent  obtained  will  be  —  ampere  according  to  Ohm's  law,  C=#, 
lo  .  J\ 

C  being  the  current,  E  the  voltage,  and  R  the  resistance.  If  it 
is  desired  to  increase  the  current  while  maintaining  the  difference 
of  potential  at  2  volts,  the  resistance  of  the  solution  must  be 
decreased  by  warming  it,  or  adding  salts  or  acids  or  bringing 
the  electrodes  closer  together  or  increasing  their  size. 

290.  Primary  and  Storage  .  Cells. — A  very  convenient  method 
of  producing  small  currents  is  by  the  use  of  primary  cells.     Any 
desired  voltage  may  be  obtained  by  this  means  if  a  sufficient  num- 
ber of  cells  are  provided.    The  cells  used  for  this  purpose  should  be 
those  that  give  a  current  which  is  constant  for  a  long  period  of 
time.     The  ordinary  DANIELL  gravity  cell  is  cheap  and  gives  a 
very  steady  current.     The  most  satisfactory  primary  cell  on  the 


204 


ELECTROLYTIC  METHODS. 


market  is  the   EDISON  PRIMARY  CELL,  formerly  known  as  the 
EDISON  LALANDE  CELL.     The  form  known  as  type  "S"  is  shown 
in  Fig.  29.     It  has  an  electromotive  force  of  0.667  volt,  and  a  ca- 
pacity of  300  ampere-hours.    The  positive 
plate  is  made  of  copper  oxide    and    is 
placed   between   two    zinc    plates.     The 
liquid  is  a  solution  of  caustic  soda  which 
is  protected  from  the  carbon  dioxide  of 
the  air  by  a  layer  of  heavy  paraffine-oil, 
which  also  prevents  the   solution  from 
"  creeping."     The  jar  is  made  of  porce- 
lain, and  is  entirely  unacted  on  by  the 
alkaline  liquid.     No  gases  of  any  kind 
are    evolved.     The    chemical    reactions 
taking  place  may  be  represented  as  fol- 
lows: Zn+2NaOH=Na2Zn02+H2.     The 
hydrogen  is  oxidized  by  the  copper  oxide 
as    follows:     CuO +H2=Cu+H20.     The 
current     produced     is     very     constant, 
although  the    potential    rises   slowly  to 
ab  >ut  one  volt  as  the  copper  oxide  be- 
comes reduced  to  metallic  copper. 
When  a  dynamo  current  is  available  which  is  not  generated 
continuously  STORAGE  BATTERIES  must  be  provided.      If  prop- 
erly cared  for,  these  cells  give  a  very  steady  current,  the  small 
drop  from  about  2  volts  when  charged  to  1.8  volts  when  discharged 
not  affecting  most  electrolytic  work,  especially  since  adjustment 
may  be  made  by  resistance  coils.     Portable  storage-cells  may  be 
purchased  for  use  in  laboratories  where  a  dynamo  current  is  not 
available.  These  cells  may  be  carried  to  a  power-station  for  charging. 
When  either  primary  or  storage  cells  are  used  the  strength  of 
the  current  may  be  modified  by  varying  the  number  of  cells  used, 
the  voltage  in  every  case  being  equal  to  the  sum  of  the  voltage 
given  by  the  individual  cells  which  are  connected  in  series.     The 
current  may  be  still  further  regulated  by  inserting  or  removing 
resistance  from  the  circuit.     This  is  most  easily  done  by  means 
of  a  resistance-box  connected  in  series  with  the  solution  to  be 
electrolyzed. 


FIG.  29. 


ELECTROLYTIC  APPARATUS.  205 

A  simple  form  of  AMMETER  and  VOLT-METER  are  indispensable 
for  much  electrolytic  work,  especially  in  separations  where  a 
slight  change  of  the  potential  may  spoil  a  determination. 

291.  Use  of  Platinum  Dishes  as  Cathodes. — Various  forms  of 
electrodes  are  in  use  upon  which  to  deposit  the  metals  determined. 
The  most  commonly  used  is  an  ordinary  platinum  evaporating- 
dish,  such  as  is  kept  in  most  laboratories  for  other  uses.     Dishes 
of  a  capacity  of  about  150  c.c.  and  weighing  35  to  40  grams  are 
most  convenient.     They  should  be  free  from  scratches  or  dents 
produced  by  wear.     Old  dishes  should  be  sent  to  the  manufacturer 
to  be  reformed.     The  deposits  of  manganese  and  lead  peroxides 
adhere  more  firmly  to  a  platinum  surface  which  has  been  rough- 
ened with  a  sand-blast.     Such  a  rough  surface  may  also  be  pro- 
duced by  depositing  platinum  electrolytically  from  a  solution  of 
potassium-platino-chloride  by  a  rather  large  current.     The  deposit 
does  not  adhere  very  firmly  unless,  after  washing,  the  dish  is  heated 
to  redness  for  some  time  with  the  blast-lamp.     Such  a  roughened 
dish  is  equally  available  for  use  with  other  metallic  deposits  than 
lead  and  manganese.    As  the  dish  cannot  be  employed  for  general 
laboratory  purposes,  it  is  advisable  to  roughen  the  surface  of  a 
platinum  cone  which  is  available  only  for  electrolytic  work. 

The  ANODES  used  with  the  platinum  dishes  are  either  disks 
of  platinum  to  which  a  rod  of  the  same  metal  has  been  securely 
fastened,  or  a  stiff  platinum  wire  the  end  of  which  has  been  made 
into  a  flat  coil  at  right  angles  to  the  wire. 

292.  Electrolytic  Stands. — The  dish  and  anode  are  held  by  an 
electrolytic  stand  which  is  shown  in  Fig.  30,  and  consists  of  an 
iron  base  into  which  a  stout  glass  rod  has  been  fastened.     A  brass 
ring   to  which   several    platinum  points  have    been   soldered  is 
attached  to  a  clamp  so  that  it  may  be  fastened  to  the  glass  rod  at 
any  convenient  height.     A  small  binding-post  is  also  fastened  to 
the  clamp  for  connection  with  the  source  of  the  electric  current; 
A  brass  rod  is  fitted  with  a  similar  clamp  for  attachment  to  the 
glass  rod.     A  binding-post  for  the  second  wire  from  the  battery, 
or  other  source  of  electricity,  is  attached  to  the  clamp.     Another 
binding-post  which  can  be  moved  along  the  brass  rod  holds  the 
platinum  anode. 

For  this  rather  expensive  electrolytic  stand  various  devices 


206 


ELECTROLYTIC  METHODS. 


made  from  ordinary  laboratory  apparatus  may  be  substituted. 
An  ordinary  iron  ring-stand  is  easily  adapted  to  the  purpose.  A 
clean  copper  wire  is  wound  around  an  iron 'ring  of  suitable  size. 
The  platinum  dish  is  placed  on  this  ring,  the  electric  current  being 
led  to  the  dish  by  means  of  the  copper  wire.  The  stiff  wire  forming 


FIG.  30. 

part  of  the  anode  is  forced  through  a  cork  which  is  held  in  a  clamp 
which  in  turn  is  fastened  to  the  iron  rod  of  the  ring  stand. 

As  considerable  loss  may  result  from  the  spattering  of  the  solu- 
tion due  to  the  gases  liberated  by  the  electrolysis,  it  is  advisable 
to  cover  the  platinum  dish  with  a  watch-crystal  through  the 
centre  of  which  a  hole  has  been  drilled.  The  watch-crystal  is  then 
split  so  that  it  may  be  placed  on  the  dish  with  the  anode  passing 
between  the  halves  where  the  hole  was  drilled.  This  watch- 
crystal  must  be  rinsed  off  into  the  dish  a  short  time  before  inter- 
rupting the  electrolysis.  A  sheet  of  mica  of  suitable  size  is  also 
convenient  for  this  purpose. 

293.  Platinum   Cylinders    as  Electrodes. — Platinum   cylinders 


ELECTROLYTIC  APPARATUS. 


207 


made  of  thin  foil  and  fastened  to  a  stiff  platinum  wire  are  very 
largely  used  in  laboratories  where  many  electrolytic  determinations 
must  be  conducted.  The  cylinders  are  made  of  two  sizes,  so  that 
the  one  of  smaller  diameter  may  be  placed  within  the  other.  The 
cylinders  are  of  the  same  height.  By  carefully  centering  the  cylin- 
ders, the  space  between  them  becomes  of  uniform  width  at  all 
points,  thus  distributing  the  current  so  that  the  metal  is  deposited 
very  uniformly.  The  larger  cylinder  is  always  connected  so  as  to 
receive  the  metallic  deposit.  Instead  of  the  inner  cylinder,  a 


FIG.  31. 

platinum  wire  either  straight,  or  twisted  into  a  coil,  may  be  used. 
In  place  of  the  outer  cylinder  a  truncated  cone  with  the  smaller 
end  up  has  been  used.  Eecently,  cylinders  made  of  platinum 
gauze  have  been  found  to  offer  many  advantages  over  the  forms 
already  described.  Not  only  is  the  time  necessary  for  depositing 
the  metal  greatly  shortened,  but  a  much  more  firmly  adhering 
deposit  is  obtained.  The  rapid  deposition  is  undoubtedly  due  to 
the  free  circulation  of  the  electrolyte  permitted  by  the  wire  gauze. 
The  firm  adherence  of  the  deposit  is  probably  due  to  its  cylindrical 
form,  which  does  not  give  any  point  for  the  peeling  process  to  start. 
Rotation  of  one  of  the  electrodes,  especially  that  on  which  the 


208  ELECTROLYTIC  METHODS. 

metal  is  being  deposited,  has  been  found  to  improve  greatly  the 
quality  of  the  deposit  and  shorten  the  time  for  complete  deposi- 
tion. 

Supports  for  the  cylindrical  electrodes  are  made  with  an  iron 
foot  and  glass  standard,  exactly  like  those  used  for  the  platinum 
dishes.  Two  brass  rods  similar  to  the  one  used  for  holding  the 
anode  for  the  dish  are  provided  for  holding  the  two  cylinders. 
The  use  of  cylinders  has  the  advantage  that  the  solution  to  be 
electrolyzed  may  be  prepared  in  beakers,  and  in  many  cases 
insoluble  matter  need  not  be  filtered  off,  nor  the  volume  reduced 
by  evaporation  to  that  of  the  platinum  dish. 

294.  Electrolysis  of  Warm   Solutions. — The  time  required  for 
many  electrolytic  determinations  is  greatly  reduced  by  warming 
the  solution.    Temperatures  from  60°  to  80°  are  generally  found 
suitable.    This  temperature  need  not  generally  be  taken  with  a 
thermometer,  since  most  chemists  are  able  to  estimate  the  tempera- 
ture closely  enough  by  touching  the  beaker  with  the  hand.    The 
small  flame  needed  for  this  purpose  may  most  easily  be  obtained 
by  screwing  off  the  tube  of  the  Bunsen  burner,  and  using  the  small 
white  flame  which  is  then  produced  by  lighting  the  burner,  and 
turning  the  gas  nearly  off.    This  white  flame  must  not  be  allowed 
to  come  in  contact  with  the  bottom  of  the  platinum  dish.     Starting 
the  electrolysis  with  the  solution  warm  will  frequently  insure  a 
better  deposit  than  can  be  obtained  from  a  cold  ; solution.     The 
solution  should  be  tested  for  the  metal  before  interrupting  the 
electrolysis  unless  the  time  required  for  similar  determinations 
has  been  carefully  noted.     It  is  usually  not  advisable  to  allow  the 
current  to  pass  for  a  longer  time  than  necessary  to   completely 
deposit  the  metal. 

295.  Washing  and  Drying  Deposited  Metals. — In  many  cases 
the  deposited  metal  redissolves  quite  rapidly  in  the  solution   from 
which  it  has  been  deposited.    In  most  cases  the  loss  is  very  slight 
if  the  cylinders  are  removed  from  the  solution  while  the  current  is 
passing  and  the  acid  is  removed  by  instantly  dipping  them  into 
distilled  water  or  washing  them  in  a  stream  of  water.      More  accu- 
rate results  are  obtained,  especially  by  the  beginner,  if  the  acid 
solution  is  replaced  by  water  while  the  current  is  still  passing. 
For  this  purpose,  the  acid  solution  is  removed  by  a  siphon,  while  a 


WASHING  AND  DRYING  DEPOSITED  METALS.  209 

stream  of  distilled  water  is  poured  in  from  the  wash-bottle.  If  an 
incandescent  lamp  is  used  as  resistance  in  circuit  with  the  electrodes, 
the  removal  of  the  acid  solution  is  indicated  by  the  disappearance 
of  light  in  the  lamp.  When  the  acid  solution  has  been  removed  in 
this  manner,  the  deposited  metal  is  washed  with  d.  stilled  water 
and  then  with  alcohol  to  remove  the  water.  Ether  is  sometimes 
used  for  the  same  purpose,  but  it  is  much  more  liable  to  contain 
fatty  substances  in  solution,  which  remain  after  evaporation  of 
the  ether.  If  ether  is  used,  the  metal  will  dry  much  more  rapidly. 
The  removal  of  the  water  is  necessary,  as  moist  metals  oxidize 
rapidly  and  increase  in  weight.  After  washing  with  alcohol  or 
ether,  the  metal  is  dried  for  a  few  minutes  in  the  steam-bath,  or 
in  the  air-bath  kept  at  100°,  and  after  cooling,  is  weighed.  The 
removal  of  the  alcohol  and  the  drying  may  be  accomplished  much 
more  rapidly  by  burning  the  portion  adhering  to  the  surface.  After 
cooling,  the  metal  is  weighed.  A  trifling  amount  of  oxidation  takes 
place  when  this  method  is  used. 

296.  Rotation  of  the  Anode  or  the  Cathode  offers  several  very 
decided  advantages.  The  metal  is  generally  deposited  in  a  much 
smoother  and  more  coherent  film,  so  that  a  number  of  metals 
which  cannot  be  deposited  in  coherent  films  with  stationary 
electrodes  can  be  successfully  determined  by  rotating  one  of 
the  electrodes.  This  character  of  the  film. also  allows  the  use  of 
a  much  larger  current,  so  that  the  deposition  is  much  more 
rapid.  The  agitation  of  the  solution  also  increases  the  rate  of 
deposition  because  the  'layer  of  liquid  near  the  cathode  ife  very 
soon  deprived  of  its  metal  by  the  electric  current  when  the  solu- 
tion is  allowed  to  remain  at  rest,  while  when  it  is  kept  in  constant 
motion,  more  metal  is  constantly  being  brought  up  to  the  electrode. 
For  the  same  reason,  heating  the  solution  increases  the  rate  of 
deposition  because  convection  currents  are  set  up  in  the  liquid. 
Rotation  of  the  cathode  or  anode  produces  a  much  more  vigor- 
ous circulation  and  consequently  hastens  the  deposition,  so  that 
15  minutes  is  in  many  cases  a  sufficient  time  for  the  complete 
deposition  of  the  metal. 

297.  Electrodes  for  Rotation.  —  Cylinders  are  not  suitable 
electrodes  for  rotation.  Platinum  dishes,  however,  are  admir- 
ably adapted  for  this  purpose.  The  platinum  spiral  used  as  anode 


210 


ELECTROLYTIC  METHODS. 


can  be  very  conveniently  rotated,  care  being  taken  to  accurately 
center  the  spiral  and  not  to  rotate  it  so  fast  that  the  solution 
is  thrown  out  of  the  dish.  Ordinary  platinum  crucibles  may 
also  be  used  for  this  purpose.  A  rubber  or  cork  stopper  is 
selected  which  fits  tightly  into  the  crucible.  The  shaft  of  the 
rotator  is  passed  through  a  hole  in  the  middle  of  the  stopper, 
electrical  contact  being  made  between  the  shaft  and  the  crucible. 
For  this  purpose  a  copper  wire  is  wound  around  the  shaft  and 
passes  through  the  hole  in  the  stopper,  projecting  far  enough  to 


FIG.  32 

touch  the  bottom  of  the  crucible.  Contact  is  made  between 
the  shaft  and  a  binding  post  which  is  connected  with  the  nega- 
tive terminal.  The  positive  terminal  is  connected  with  the  anode, 
which  may  consist  of  platinum  foil  or  wire.  This  arrangement 
is  shown  in  Fig.  32.  The  metal  is  deposited  on  the  outside  of  the 
crucible  to  within  about  a  quarter  of  an  inch  of  the  top.  When 
being  weighed  the  crucible  is  inverted  on  the  scale  pan. 

298.  The  Motors  for  rotation  may  be  electric  or  water  motors. 
A  velocity  of  300  to  600  revolutior.s  per  minute  will  be  required. 


MOTORS  FOR  ROTATION. 


211 


FIG.  33. 


212 


ELECTROLYTIC  METHODS. 


A  system  of  pulleys  of  different  sizes  is  convenient  for  regulating 
the  speed  of  rotation.  In  addition  to  this  means  of  regulation, 
the  current  supplied  to  electric  motors  may  be  varied  by  chang- 
ing the  resistance  in  the  circuit, 
and  with  water  motors  the  amount 
of  water  supplied  is  easily  con- 
trolled. An  apparatus  designed 
for  utilizing  a  water  motor  is 
shown  in  Fig.  32.  The  upright 
shaft  of  the  motor  rotates  on  a 
needle  point,  while  the  shaft  hold- 
ing the  crucible  rotates  on  the 
ball-bearings  of  a  bicycle  hub. 
An  apparatus  designed  for  using 
an  electric  motor  is  shown  in 
Fig.  33.  R  is  the  resistance  in 
circuit  with  the  solution  being 
electrolyzed  and  controls  the 
amount  of  current  used,  as  indi- 
cated by  the  ammeter  A,  which  is 
in  the  same  circuit.  The  fuses  F 
protect  the  ammeter  from  being 
burned  out.  The  motor  M  is 
attached  by  means  of  the  flexible 
chord  C  at  the  socket  T.  The 
motor  may  be  instantly  stopped 
by  pulling  the  chord  out  of  the 
socket  T.  The  motor  requires  no 
outside  resistance.  The  switch  S 
controls  both  the  motor  circuit 
and  the  electrolytic  circuit,  so 
that  the  electrolysis  and  the  rota- 
tion may  be  started  or  interrupted 
simultaneously.  The  wiring  of  this  board  is  shown  in  Fig.  34, 
which  is  a  diagrammatic  representation  of  the  reverse  of  the  slate 
board  upon  which  the  apparatus  is  mounted.  A  volt  meter  has 
not  been  permanently  connected  with  the  other  apparatus  because 
in  ordinary  work  it  is  not  needed.  The  resistance  R  is  in  series 
with  the  electrolytic  circuit,  but  not  with  the  motor  circuit. 


FIG.  34. 


CHAPTER  XVIII. 
ELECTROLYTIC  DETERMINATION  OF  METALS. 

DETERMINATION  OF  COPPER. 

299.  Deposition  from  a  Nitric  or  Sulphuric  Acid  Solution. — The 
electrolytic  determination  of  copper  is,  no  doubt,  the  most  accurate 
as  well  as  the  most  rapid  method  of  determining  this  metal.    If  the 
solution  of  copper  is  free  from  other  metals,  a  great  many  of  the 
methods  which  have  been  proposed  may  be  used.     If,  as  most  gener- 
ally occurs  in  practice,  the  copper  must  be  determined  in  the  pres- 
ence of  other  metals  but  one  or  two  methods  are  available.      By 
these  methods  the  copper  is  deposited  from  a  solution  containing 
FREE  NITRIC  or  SULPHURIC  ACID.     As  the  copper  is  apt  to  be 
deposited  in  a  spongy  form  when  only  sulphuric  acid  is  present, 
solutions  containing  only  nitric  acid,  or  nitric  and  sulphuric  acids 
are  generally  used.    Not  more  than  5%  of  concentrated  nitric  acid 
may  be  present,  and  if  the  same  amount  of  concentrated  sulphuric 
is  present,  J%  of  concentrated  nitric  acid  is  sufficient.     The  sul- 
phuric acid  solution  is  used  when  the  presence  of  the  nitric  acid  or 
the  ammonium  salts  formed  by  its  decomposition  would  interfere 
with  subsequent  operations,  such  as  the  precipitation  of  zinc  as 
sulphide. 

The  nitric  acid  may  be  entirely  omitted  if  1  gram  of  urea  is 
added  to  the  solution  containing  7%  to  10%  of  concentrated 
sulphuric  acid.  By  using  a  current  of  about  one  ampere,  the 
decomposition  is  then  effected  in  less  than  two  hours.* 

300.  Separation  and   Determination   of  Lead. — The   sulphuric 
acid  solution  is  used  for  copper  when  much  lead  is  present.    This 
metal  is  then  separated  as  sulphate,  in  which  form  it  may  be  fil- 
tered off  and  weighed.     If  only  a  small  amount  of  lead  is  present, 
it  may  be  deposited  on  the  anode  from  a  nitric  acid  solution,  and 

*  Classen,  Quan.  Chein.  Analysis  by  Electrolysis,  page  160. 

213 


214  ELECTROLYTIC  METHODS. 

the  peroxide  washed,  dried  at  180°,  and  weighed.  If  the  lead  is  to 
be  separated  and  determined  in  this  manner,  sulphuric  acid  should 
not  be  introduced,  as  the  lead  peroxide  will  be  contaminated  with 
this  acid. 

301.  Separation  of  Copper  from  Other  Metals. — ZINC,  COBALT, 
NICKEL,  CADMIUM,  MANGANESE,  and  moderate  amounts  of  IRON 
may  be  present  in  the  nitric  acid  solution  without  interfering  with 
the  accuracy  of  the  determination  of  copper.  Large  amounts  of 
iron  are  objectionable  because  the  ferric  salts  tend  to  redissolve 
the  precipitated  copper.  The  difficulty  is  partially  overcome  by 
diluting  the  solution  and  increasing  the  current  density.  The 
copper  may  also  be  completely  separated  from  iron  by  precipita- 
tion with  hydrogen  sulphide  or  sodium  thiosulphate.  The  precipi- 
tate is  filtered  off,  washed,  and  redissolved  in  nitric  acid.  From 
this  solution  it  may  be  precipitated  electrolytically  and  weighed. 

SILVER,  MERCURY,  BISMUTH,  ARSENIC,  ANTIMONY,  and  TIN  are 
deposited  with  the  copper.  Silver  should  be  precipitated  as  chlo- 
ride before  electrolyzing  the  solution,  an  excess  of  hydrochloric 
acid  being  avoided.  Mercury  may  be  expelled  from  the  ore  by  roast- 
ing. When  alloys  or  ores  of  copper  are  dissolved  in  nitric  acid 
the  tin  and  almost  all  of  the  antimony  remains  undissolved  while 
most  of  the  arsenic  goes  into  solution.  The  removal  of  these 
metals  may  be  accomplished  by  passing  hydrogen  sulphide  through 
the  solution  until  no  more  precipitate  is  formed,  and  then  dissolving 
the  sulphides  of  arsenic,  antimony,  and  tin  in  sodium  sulphide 
solution.  The  copper  sulphide  may  then  be  dissolved  in  nitric 
acid  and  the  copper  deposited  and  weighed. 

Copper  may  also  be  readily  and  completely  separated  from 
BISMUTH,  ARSENIC,  ANTIMONY,  and  IRON  by  precipitation  as 
cuprous  sulphocyanate.  The  solution  of  the  copper  salt  is  made 
slightly  acid  with  hydrochloric  acid,  and  the  copper  reduced  by 
saturating  the  solution  with  sulphur  dioxide.  A  solution  of 
ammonium  thiocyanate  is  then  added  until  precipitation  is  com- 
plete, and  the  mixture  heated  nearly  to  the  boiling-point  for  a 
few  minutes.  The  copper  may  also  be  precipitated  by  the 
addition  of  a  solution  of  equal  parts  of  acid  ammonium  sulphite 
and  ammonium  thiocyanate.  The  precipitate,  which  is  colored 
when  first  formed,  quickly  becomes  white,  and  settles  very  readily. 


DETERMINATION  OF  COPPER.  215 

It  is  washed  with  warm  water  until  free  from  the  other  metals 
present.  The  copper  is  dissolved  in  a  little  warm  concentrated 
nitric  acid,  and  then  deposited  as  usual. 

COPPER  ORES  containing  ARSENIC,  ANTIMONY,  and  TIN  may 
be  decomposed  by  fusion  with  six  times  the  weight  of  a  mixture 
of  equal  parts  of  sulphur  and  sodium  carbonate  or  the  same  amount 
of  dry  sodium  thiosulphate.  The  ore  must  be  very  finely  pow- 
dered and  the  fusion  conducted  in  a  porcelain  crucible.  After 
heating  with  a  small  Bunsen-burner  flame  until  sulphur  is  no 
longer  evolved,  the  melt  is  allowed  to  cool,  and  then  extracted 
with  hot  water,  and  the  residue  washed  with  water  containing  a 
little  ammonium  sulphide.  By  this  treatment  the  arsenic,  anti- 
mony, and  tin  are  dissolved  while  the  copper  remains  as  sulphide, 
which  is  dissolved  in  nitric  acid,  and  the  copper  in  the  solution 
determined. 

302.  Deposition  of  Copper  from  an  Ammonium-oxalate  Solu- 
tion.— A  very  excellent  method  of  determining  copper  is  based 
on  its  separation  from  a  solution  containing  excess  of  ammonium 
oxalate.  Classen*  has  improved  this  method  so  that  the  determina- 
tion may  be  carried  out  in  two  hours.  As  most  of  the  metals  are 
precipitated  from  an  oxalate  solution,  this  method  cannot  be  used 
unless  the  copper  is  first  separated  from  the  other  metals,  or  the 
amount  present  in  a  pure  salt  is  to  be  determined. 

EXERCISE  40. 
Determination  of  Copper  in  Copper  Sulphate^  CuSO4-5H  O. 

Clean,  dry  at  100°,  and  weigh  a  platinum  dish  or  platinum  cone  or  cylin- 
der. Weigh  out  1  gram  of  pure  recrystallized  copper  sulphate.  If  the 
cone  or  cylinder  is  used,  transfer  the  copper  sulphate  to  a  250-c.c.  beaker, 
otherwise  place  it  in  the  weighed  dish.  Dissolve  in  a  few  cubic  centimeters 
of  water,  add  a  solution  of  4  grams  of  ammonium  oxalate,  and  dilute  the 
solution  to  about  120  c.c.  Make  a  saturated  solution  of  oxalic  acid  by 
dissolving  4  grams  of  the  crystals  in  50  c.c.  of  water,  and  filter  if  necessary. 
Place  the  dish  on  the  ring  of  the  electrolytic  stand,  and  adjust  the  anode 
so  that  it  is  about  £  inch  below  the  surface  of  the  liquid.  If  a  beaker  is 
used,  it  must  be  placed  on  a  tripod  or  other  support,  so  that  the  solution 
can  be  heated.  The  cylinder  or  cone  should  be  adjusted  so  that  it  very 
nearly  touches  the  bottom  of  the  beaker.  Fasten  a  burette  by  means  of  a 

*  Classen,  Quant.  Anal,  by  Electrolysis,  page  154. 


216 


ELECTROLYTIC  METHODS. 


clamp  and  ring-stand,  so  that  the  solution  of  oxalic  acid  which  is  placed 
in  the  burette  will  drop  on  the  split  watch-crystal  which  covers  the  dish 
or  beaker.  A  thermometer  is  also  suspended  with  the  bulb  in  the  copper 
solution,  which  is  heated  with  a  small  flame  from  a  Bunsen  burner  to  80°. 
The  current  of  electricity  used  should  be  so  adjusted  that  about  1  ampere 
passes  through  the  solution,  and  the  tension  at  the  electrodes  is  from  2.5 
to  3.2  volts. 

As  soon  as  a  film  of  copper  is  seen  to  cover  the  cathode,  the  stop-cock 
of  the  burette  is  so  adjusted  that  the  oxalic-acid  solution  falls  at  the  rate 


FIG.  35. 

of  10  drops  per  minute.  The  copper  comes  down  as  a  bright-red  deposit.  It 
it  becomes  dark-colored  or  spongy,  the  oxalic  acid  should  be  allowed  to 
drop  faster.  The  electrolysis  will  be  completed  within  two  hours.  When 
the  solution  has  become  colorless,  portions  may  be  taken  out  and  tested 
for  copper  by  acidifying  with  hydrochloric  acid  and  adding  potassium 
ferrocyanide.  When  all  of  the  copper  has  been  deposited,  siphon  off  the 
acid  liquid  while  water  from  the  inverted  wash-bottle  is  allowed  to  flow 
over  the  copper  deposit.  The  copper  is  then  washed  with  alcohol,  dried 
at  100°,  and  weighed.  The  theoretical  percentage  of  copper  in  copper 
sulphate  is  25.41. 


DETERMINATION   OF  SILVER.  217 

EXERCISE  41. 
Determination  of  Copper  from  a  Nitric-acid  Solution. 

The  copper  in  copper  sulphate  may  also  be  precipitated  from  a  nitric-  ' 
acid  solution.  This  method  is  well  adapted  to  the  determination  of 
copper  in  the  refined  metal.  About  0.3  gram  of  the  metal  is  carefully  weighed 
out,  and  dissolved  in  3  c.c.  concentrated  nitric  acid  and  a  little  water.  The 
solution  is  diluted  to  about  100  c.c.,  and  warmed  to  50°  or  60°,  and  the 
copper  precipitated  with  a  current  of  0.5  to  1  ampere.  No  arrangement 
need  be  made  to  keep  the  solution  warm,  as  in  the  preceding  method.  The 
deposition  of  the  metal  is  more  rapid,  and  a  better  deposit  is  obtained  if 
the  solution  is  warm  at  first.  After  four  to  five  hours,  all  of  the  copper 
will  be  deposited.  If  a  cylinder  made  of  wire  gauze  is  used,  about  half  this 
time  will  be  required.  If  the  rotating  cathode  or  anode  is  used,  the  amount 
of  nitric  acid  may  be  reduced  to  1  c.c.,  while  the  current  may  be  increased 
to  from  3  to  5  amperes.  Most  of  the  copper  will  be  deposited  in  10  min- 
utes. The  small  amount  still  remaining  in  solution  will  be  very  rapidly 
deposited  if  the  solution  is  nearly  neutralized  by  the  addition  of  ammonia. 
When  all  of  the  copper  is  deposited,  as  shown  by  the  ferrocyanide  test, 
the  solution  is  displaced  with  distilled  water  while  the  current  is  passing, 
and  the  deposit  washed  with  alcohol,  dried,  and  weighed  exactly  as  directed 
in  the  first  method. 

303.  Deposition  of  Silver  from  a  Potassium-cyanide  Solution. — 
Silver  is  determined  electrolytically   almost  exclusively  from  a 
cyanide  solution.     The  potassium  cyanide  used  must  be  pure. 
Failure  to  obtain  good  results  is  frequently  due  to  the  use  of 
impure  cyanide.     Another  sample  of  cyanide  should  therefore  be 
obtained  and  used.    The  current  density  should  be  less  than  0.5 
ampere.     The   electrolysis   may   be    conducted   at    the   ordinary 
temperature,   though  when   heated  to   about  65°,   the  time   re- 
quired for  a  determination  is  very  much   reduced,  so    that   two 
or  three  hours  is  generally  sufficient.     3  grams  of  potassium  cyanide 
are  sufficient  for  0.5  gram  of  silver.     If  other  metals  are  present, 
the  silver  may  be  precipitated  as  chloride,  which  is  then  dissolved 
in  the  potassium  cyanide.     It  is  doubtful,  however,  if  in  this  case 
the  result  would  be  more  accurate  or  obtained  more  quickly  than 
if  the  silver  chloride  were  directly  weighed. 

304.  Separation   of  Silver  from  Lead  and  Copper. — Silver  may 
be  separated  from  lead  in  consequence  of  the  deposition  of  the  lat- 


218  ELECTROLYTIC  METHODS. 

ter  on  the  anode  as  peroxide,  when  a  solution  containing  20%  of 
concentrated  nitric  acid  is  electrolyzed.  The  separation  of  silver 
from  copper  is  effected  by  electrolyzirg  a  solution  of  the  two  metals 
in  potassium  cyanide  by  a  current  giving  a  tension  between  the 
electrodes  of  about  1.4  volts.  When  all  of  the  silver  has  been 
deposited,  the  copper  may  be  determined  by  adding  nitric  acid  to 
the  solution,  and  increasing  the  tension  between  the  electrodes. 

EXERCISE  42. 
Determination  of  Silver  in  Silver  Nitrate. 

Weigh  out  0.5  gram  of  pure  dry  silver  nitrate,  and  dissolve  in  a  few  cubic 
centimeters  of  water.  Dissolve  about  2  grams  of  potassium  cyanide  in  a 
small  amount  of  water,  and  pour  the  silver  solution  into  the  cyanide  solution. 
Either  the  platinum  dish  or  the  cylinders  may  be  used  as  described  under 
the  determination  of  copper.  Three  Edison-Lalande  cells  connected  in 
series  or  other  source  of  electricity  giving  about  3  volts,  should  be  used 
The  electrolysis  requires  about  six  hours,  but  the  time  necessary  may  be 
reduced  by  warming  the  solution  to  65°,  which  is  about  the  temperature 
the  hand  can  bear  for  a  moment  without  much  discomfort.  If  the  anode 
is  rotated,  a  current  of  2  to  3  amperes  may  be  used,  and  the  deposition  will 
be  complete  in  10  to  15  minutes.  The  end  of  the  operation  is  ascertained 
by  testing  a  few  drops  of  the  solution  by  adding  hydrochloric  acid.  The 
test  should  be  made  under  the  hood.  Siphon  off  the  solution  while  add- 
ing distilled  water,  rinse  with  alcohol,  dry  at  100°,  and  weigh.  If  the 
cyanide  solution  becomes  quite  dark  on  passing  the  current,  the  potassium 
cyanide  is  impure  and  the  result  is  not  apt  to  be  correct.  The  determina- 
tion should  be  repeated  with  another  sample  of  the  cyanide.  The  theo- 
retical percentage  of  silver  in  silver  nitrate  is  63.51. 

EXERCISE  43. 
Analysis  of  a  Silver  Coin, 

305.  Solution  of  Coin.— When  results  within  0.1  or  0.2%  of  the  theo- 
retical have  been  obtained  with  silver  nitrate,  a  coin  is  analyzed.  Thor- 
oughly clean  a  dime  or  other  silver  coin  by  rubbing  with  alcohol,  ammonia, 
etc.  Cut  it  into  four  or  five  pieces  so  that  each  piece  weighs  about  0.4 
gram.  Weigh  one  of  the  pieces  carefully,  and  dissolve  in  a  few  cubic 
centimeters  of  a  mixture  of  equal  parts  of  water  and  concentrated  nitric 
acid.  This  should  be  done  in  a  small  covered  beaker.  Warm  the  beaker 
on  the  water-bath,  and  when  solution  is  complete  remove  the  watch- 
crystal,  rinsing  it  if  necessary,  and  let  the  solution  go  to  dryness. 


DETERMINATION  OF  SILVER,  219 

306.  Silver.— In  the  mean  time  weigh  out  roughly  and  dissolve  in 
water  two  grams  of  potassium  cyanide.  Dissolve  the  copper  and  silver 
nitrates  in  a  little  water,  and  pour  the  solution  into  the  cyanide  solution. 
If  difficulty  is  experienced  in  dissolving  the  salts  of  copper  and  silver  which 
have  dried  on  the  sides  of  the  beaker,  a  little  potassium  cyanide  solution 
may  be  added  after  the  water  solution  has  been  removed.  The  silver 
may  be  deposited  on  a  platinum  dish  or  on  a  cylinder.  The  solution  should 
be  warmed  to  about  65°,  and  electrolyzed  with  the  current  from  two  Edison- 
Lalande  or  similar  cells  connected  in  series.  The  difference  of  potential 
between  the  two  platinum  electrodes  must  be  measured  with  a  volt-meter 
and  must  not  exceed  1.5  volts.  If  the  two  cells  give  a  higher  voltage  than 
this,  the  potential  must  be  reduced  by  introducing  resistance.  A  resistance- 
box  may  be  used,  or  the  connection  between  the  cells  and  one  of  the  electrodes 
may  be  made  by  means  of  a  few  feet  of  German-silver  wire.  By  drawing 
more  or  less  of  this  wire  through  one  of  the  binding-posts  the  resistance 
may  be  changed  until  the  potential  between  the  two  electrodes  is  a  little 
less  than  1.5  volts. 

The  dish  or  beaker  is  covered  with  a  split  watch-crystal.  As  the  evapo- 
ration at  65°  is  considerable,  water  must  be  added  from  time  to  time.  In 
about  five  hours  the  electrolysis  will  be  completed,  as  may  be  ascertained 
by  adding  water  until  an  unplated  portion  of  the  platinum  dish  is  covered. 
If  no  silver  is  deposited  on  this  portion  of  the  dish  in  half  an  hour,  the 
deposition  of  the  silver  is  complete.  The  solution  is  now  siphoned  off 
into  a  beaker  and  replaced  with  water,  care  being  taken  to  lose  none.  The 
silver  is  rinsed  with  alcohol,  dried  at  100°,  and  weighed. 

307.  Copper. — The  copper  in  the  filtrate  is  deposited  on  a  cone  or  cylin- 
der after  the  addition  of  5  c.c.  concentrated  nitric  acid.  The  operation  is 
repeated  until  analytical  duplicates  are  obtained.  The  rotating  anode  may 
be  used  in  this  determination  as  well  as  in  the  determination  of  the  silver. 

DETERMINATION  OF  NICKEL  AND  COBALT. 

Both  nickel  and  cobalt  are  most  easily  determined  electro- 
lytically.  The  same  methods  serve  for  both  metals.  Where  the 
two  metals  occur  together,  they  are  separated  from  the  other 
metals  present  by  the  methods  given  in  Chapter  XIV.  Both 
metals  are  then  precipitated  electrolytically,  and,  after  weighing, 
the  deposit  is  dissolved  in  nitric  acid,  one  of  the  metals  being 
precipitated  and  weighed  by  methods  given  in  the  chapter 
mentioned. 

308.  Ammonium-oxalate  Method.  —  According  to  Classen  * 
these  metals  may  be  readily  deposited  from  a  solution  of  the 

*  Classen,  Quan.  Anal,  hy  Electrolysis,  page  141. 


220  ELECTROLYTIC  METHODS. 

double-ammonium  oxalate.  Nitrates  and  chlorides  should  be 
removed  by  evaporation  with  sulphuric  acid.  The  excess  of 
acid  is  neutralized  with  ammonia,  and  4  to  5  grams  of  ammonium 
oxalate  added,  which  is  dissolved  by  warming  the  solution. 
Water  is  then  added  until  the  volume  is  from  ICO  to  120  c.c.  If 
the  temperature  of  the  solution  is  kept  at  60°  to  70°,  the  elec- 
trolysis will  be  complete  in  three  to  four  hours,  a  current  of 
about  1  ampere  and  an  electrode  tension  of  3  to  4  volts  being  used. 
309.  Ammonia  Method. — By  the  method  of  Fresenius  and 
Bergmann  *  chlorides  and  nitrates  are  removed  by  evaporation 
with  sulphuric  acid.  The  excess  of  acid  is  then  neutralized  with 
ammonia,  and  unless  at  least  6  c.c.  of  strong  ammonia  (sp.  gr.  0.90) 
are  used  for  this  purpose,  6  grams  of  ammonium  sulphate  should 
be  added.  15  c.c.  of  strong  ammonia  are  then  added,  or  if  more 
than  0.5  gram  of  metal  is  present,  20  c.c.  should  be  used.  The 
solution  is  then  diluted  to  150-170  c.c.,  and  electrolyzed  with  a 
current  of  0.7  ampere.  The  separation  is  not  hastened  by  warm- 
ing the  solution.  The  deposition  is  complete  in  about  six  hours. 


EXERCISE  44. 
Analysis  of  a  Nickel  Coin. 

310.  Solution  of  Coin. — Clean  a  nickel  coin  thoroughly  by  rubbing  with 
sand  or  "  Sapolio."      Cut  it  into  small  pieces  weighing  about  0.4  gram. 
After  weighing  one  of  these  pieces  place  it  in  a  small  beaker  (about  150  c.c.), 
and  add  10  c.c.  dilute  nitric  acid.     Warm  the  covered  beaker  on  the  water- 
bath  until  the  metal  is  dissolved.     Rinse  off  the  watch-crystal,  add  16  c.c. 
dilute  sulphuric  acid,  and  evaporate  on  the  water-bath  until  no  more  nitrous 
fumes  come  off. 

311.  Copper.— If  a  cylinder  is  available,   dilute  the    solution  untii  the 
cylinder  is  nearly  covered  when  immersed  so  as  to  very  nearly  touch  the 
bottom  of  the  beaker.     Electrolyze  with  a  current  of  about  0.8  ampere. 
If  the  solution  is  warmed  to  about  60°,  the  copper  will  be  deposited  in  from 
four  to  six  hours.      If  it  is  convenient  to  conduct  the  electrolysis  overnight, 
a  current  of  about  0.4  ampere  should  be  used,  and  the  electrolysis  begun 
with  %  warm  solution,  which  is  then  allowed  to  cool  off.     The  end  of  the 
operation  is  ascertained  by  testing  a  small  portion  of  the  solution  for  copper 
by  passing  hydrogen  sulphide.     After  boiling  for  a  few  minutes  to  expel 

*Zeit.  Anal.  Chem.,  vol.  19,  page  329. 


DETERMINATION  OF   TIN.  221 

the  hydrogen  sulphide,  this  portion  may  be  returned  to  the  beaker.  The 
solution  may  also  be  tested  for  copper  by  adding  water  so  that  a  clean  por- 
tion of  the  cylinder  is  immersed.  If  no  copper  is  deposited  on  this  portion 
after  half  an  hour,  all  of  the  metal  has  been  deposited.  When  all  of  the 
copper  has  been  deposited,  the  solution  containing  the  nickel  is  siphoned 
off  and  replaced  with  water.  The  copper  is  washed  with  water  and  alcohol, 
dried  at  100°,  and  weighed.  The  anode  or  cathode  may  be  rotated,  and 
the  current  largely  increased,  and  the  time  required  for  the  deposition 
reduced  to  less  than  half  an  hour. 

312.  Nickel. — The  solution  containing  the  nickel  should  be  evaporated 
if  the  volume  is  more  than  250  c.c.  It  is  neutralized  with  ammonia,  15  c.c. 
of  concentrated  ammonia  added,  and  the  nickel  deposited  with  a  current  of 
about  0.7  ampere.  About  five  to  six  hours  will  be  required.  The  solution 
is  tested  for  nickel  by  adding  to  a  small  portion  a  little  colorless  ammo- 
nium sulphide,  or  passing  hydrogen  sulphide.  If  the  nickel  is  deposited 
slowly,  enough  ammonia  is  not  present  and  more  must  be  added.  The  solu- 
tion need  not  be  siphoned  off.  The  nickel  is  washed  with  water  and  alcohol, 
dried  at  100°,  and  weighed.  The  determination  is  repeated  with  other  por- 
tions of  the  coin  until  duplicates  are  obtained.  The  rotating  anode  or 
cathode  may  be  used  for  the  deposition  of  the  nickel.  The  current  may 
then  be  increased  to  5  amperes,  when  the  deposition  will  be  complete  in 
less  than  half  an  hour. 

DETERMINATION  OF  TIN. 

Tin  may  be  deposited  in  a  bright,  crystalline,  closely  adherent 
form  from  solutions  of  the  double-ammonium  oxalate  containing 
free  oxalic  acid.  If  the  tin  is  obtained  as  the  dioxide,  it  may  be 
dissolved  by  fusing  with  8  times  its  weight  of  caustic  soda  in  a 
silver  dish.  The  melt  is  dissolved  in  water,  an  excess  of  oxalic 
acid  added  and  the  solution  heated  until  clear.  Hydrogen  sulphide 
is  then  passed  to  precipitate  the  silver  which  is  filtered  off,  after  which 
the  solution  is  boiled  to  expel  the  hydrogen  sulphide.  Ammonium 
oxalate  is  then  added,  the  solution  warmed  to  about  65°,  and 
electrolyzed  with  a  current  of  4  to  5  volts  and  from  0.5  to  1.5 
amperes.  For  0.3  gram  of  tin  9  to  10  grams  oxalic  acid  and 
4  grams  of  ammonium  oxalate  should  be  used.  The  tin  is  com- 
pletely deposited  in  about  4J  hours.  It  is  washed  as  usual  with 
water  and  alcohol,  and  dried  in  the  steam-bath.  Excellent  results 
are  obtained  with  the  rotating  ai.ode. 

313-  Dissolving  Tin  Deposits  on  Platinum. — Some  difficulty 
is  experienced  in  dissolving  the  l!n  which  has  been  deposited  on 


222  ELECTROLYTIC  METHODS. 

the  platinum.  This  metal  tends  to  form  an  alloy  with  the  platinum 
so  that  acids  do  not  readily  attack  the  tin.  The  difficulty  may  be 
overcome  by  first  coating  the  dish  or  other  electrode  with  silver  or 
copper  or  a  silver  dish  may  be  used  for  these  determinations. 
The  silver  is  deposited  from  a  solution  of  silver  nitrate  in  potassium 
cyanide,  while  the  copper  may  be  deposited  from  a  solution  of 
copper  sulphate  acidified  with  nitric  acid.  After  a  sufficiently  thick 
coating  has  been  obtained  by  either  method  the  solution  is  de- 
canted and  the  deposited  metal  washed  and  dried  as  usual  for  the 
determination  of  these  metals.  The  best  solvent  for  the  tin  is 
prepared  by  diluting  concentrated  nitric  acid  with  an  equal  volume 
of  water  and  then  saturating  with  oxalic  acid.  After  most  of  the 
tin  has  been  dissolved  by  this  solution  the  nitric  acid  is  washed  off 
with  water  and  the  cylinder  or  dish  immersed  in  concentrated  C. 
P.  hydrochloric  and  allowed  to  remain  for  5  to  6  hours. 

It  is  advisable  to  keep  cylinders  used  for  the  determination  of 
tin  immersed  in  concentrated  C.  P.  hydrochloric  acid  when  not  in 
use.  This  acid  will  dissolve  all  the  tin  if  allowed  to  act  for  a 
sufficient  time. 

DETERMINATION   OF  LEAD. 

This  element  cannot  be  determined  by  deposition  in  the  metal- 
lic form,  because  during  the  process  of  washing  and  drying  it 
invariably  oxidizes  to  a  greater  or  less  extent.  Advantage  is 
therefore  taken  of  the  fact  that  when  a  current  of  electricity  is 
passed  through  a  nitric  acid  solution  of  lead,  it  is  converted  into 
the  peroxide  which  is  deposited  on  the  anode,  and  may  be 
dried  by  heating  to  180°-200°.  The  platinum  anode  should  be 
roughened  according  to  the  suggestion  of  A.  Classen  by  means  of 
a  sand-blast,  otherwise  the  deposit  readily  flakes  off  unless  depos- 
ited very  slowly  and  in  small  amount.  By  this  method  the  lead  is 
separated  from  zinc,  cobalt,  nickel,  iron,  aluminium,  copper,  gold, 
mercury,  antimony,  and  cadmium.  Traces  of  silver  and  bismuth,  if 
present,  are  deposited  as  peroxide  with  the  lead. 

314.  Conditions  of  Deposition  of  Lead. — The  amount  of  nitric 
acid  which  must  be  present  depends  on  the  temperature  of  the  solu- 
tion and  the  amount  of  current  used.  If  the  electrolysis  is  carried 
out  at  the  ordinary  temperature,  about  10%  by  volume  of  nitric 


DETERMINATION  OF  LEAD.  223 

acid  of  sp.  gr.  1.38  must  be  present,  and  a  current  of  .05  amperes 
used,  while  if  a  current  of  0.5  amperes  is  employed  20%  by  vol- 
ume of  the  nitric  acid  must  be  present.  The  depositon  of  the  lead 
is  much  more  rapid  at  a  temperature  of  60°  to  65°,  but  the  amount 
of  acid  necessary  at  this  temperature  for  a  current  of  .05  amperes 
is  2%  by  volume;  while  for  a  current  of  0.5  amperes  7%,  and  for 
1.5  to  1.7  amperes  20%  is  necessary.  The  small  current  must  be 
used  when  only  smooth  dishes  or  electrodes  are  available.  If  too 
little  nitric  acid  is  present,  metallic  lead  will  be  deposited  on  the 
cathode.  Hydrochloric  acid  should  be  absent  from  the  solution, 
and  only  small  amounts  of  sulphuric  acid  should  be  present,  as  it 
is  partly  precipitated  with  the  lead  peroxide.  The  rotating  anode 
or  cathode  may  be  used  to  advantage. 

The  end  of  the  operation  is  ascertained  by  adding  water,  so 
as  to  expose  a  fresh  portion  of  the  electrode.  If  lead  is  still 
present,  a  dark-brown  deposit  will  be  formed  after  a  quarter  to 
half  an  hour.  The  solution  need  not  be  siphoned  off.  The  lead 
peroxide  is  washed  with  hot  water  and  then  with  alcohol,  and  is 
dried  for  half  an  hour  in  an  air-bath  heated  to  200°.  The  lead 
dioxide  may  be  dissolved  in  hot  dilute  nitric  acid  to  which  a  little 
oxalic  acid  has  been  added,  or  in  dilute  hydrochloric  acid  which 
has  been  saturated  with  sulphur  dioxide.  It  may  also  be  dis- 
solved in  dilute  nitric  acid  after  being  converted  into  litharge  by 
heating  for  a  few  minutes  in  the  oxidizing  flame  of  the  blast-lamp 
or  of  a  Bunsen  burner. 

315.  Determination  of  Lead  in  Ores. — The  electrolytic  method 
is  well  suited  to  the  determination  of  lead  in  ores.  If  possible 
the  ore  should  be  decomposed  without  the  use  of  nitric  acid, 
which  converts  any  sulphur  present  into  sulphuric  acid.  As  has 
already  been  said,  the  presence  of  this  acid  in  the  solution  intro- 
duces a  slight  error  in  the  determination.  A  method  of  treating 
the  ore,  which  is  suitable  for  galena,  is  given  by  Medicus.*  The 
finely  powdered  ore  is  dissolved  in  concentrated  hydrochloric 
acid.  Excess  of  caustic-potash  solution  (1:3),  and  if  antimony  is 
present,  1  to  2  grams  of  tartaric  acid  are  added.  The  solution  is 
warmed  for  a  few  minutes  at  100°,  3,nd,  after  cooling  and  diluting 
with  water,  the  lead  is  precipitated  by  passing  carbon  dioxide. 

*Ber.  d.  Chem.  Ges.,  1892,  p.  2490. 


224  ELECTROLYTIC  METHODS. 

This  will  require  about  one  and  one-half  hours.  The  lead  car- 
bonate is  washed  with  hot  water  until  free  from  chlorine,  and  is 
then  dissolved  in  dilute  nitric  acid  and  the  paper  well  washed. 
The  lead  in  the  solution  is  then  deposited  as  the  peroxide,  as 
already  directed. 

DETERMINATION   OF   ZINC. 

A  large  number  of  electrolytic  methods  have  been  proposed 
for  zinc,  very  few  of  which  have  been  found  practicable.  The 
difficulties  attending  the  gravimetric  determination  of  this  metal 
have  led  to  continued  effort  in  this  direction,  until  methods  which 
appear  to  be  successful  have  been  devised  which  depend  on  the 
deposition  of  the  zinc  from  a  solution  kept  slightly  acid  with  a 
weak  organic  acid. 

316.  Difficulty  of  Dissolving  Zinc. — In  the  electrolytic  deter- 
minations of  zinc,  the  same  inconvenience  is  met  with  as  in  the 
determination  of  tin,  that  the  metal  is  dissolved  with  difficulty 
from  the    platinum   electrode.     The    same    remedy   is  adopted: 
namely,  to  coat  the  platinum  with  silver  or  copper,  or  to  use  silver 
dishes.     The  zinc  may  be  dissolved,  leaving  the  copper  or  silver, 
by  warming  with  dilute  sulphuric  acid. 

317.  Ammonium-oxalate  Method.— In  the  method  proposed  by 
Classen,*  the  zinc  is  deposited  from  a  solution  of  the  double  oxa- 
late  of  zinc  and  potassium  or  ammonium.     To  the  neutral  and 
concentrated  solution  of  zinc  about  4  grams  of  potassium  or  ammo- 
nium oxalate  are  added.     Solution  is  effected  by  warming,  and, 
if  necessary,  adding  a  little  water.     The  solution  is  diluted  to 
about  120  c.c.,  warmed  to  50°  or  60°,  and  electrolyzed  with  a  cur- 
rent of  0.5  to  1  ampere  with  an  electrode  tension  of  3.5  to  4.8  volts. 
After  the    current  has  passed  for  three  to  five  minutes,  a  cold- 
saturated  solution  of  oxalic  acid,  or  better,  a  6%  solution  of  tar- 
taric  acid  is  allowed  to  flow  from  a  burette  at  the  rate  of  about 
10  drops  per  minute  upon  the  watch-crystal  which  covers  the 
dish  or  beaker.     The  end  of  the  operation  is  ascertained  by  test- 
ing small  portions  of  the  solution  with  potassium  ferrocyanide. 

*  Classen,  Quan.  Analysis  by  Electrolysis,  p.  146. 


DETERMINATION  OF  ZINC.  225 

The  time  required  is  about  two  hours.     The  deposit  must  be 
washed  without  interrupting  the  current. 

318.  Acetic-acid  Methods. — A  method  proposed  by  Riche  and 
modified  by  Smith  *  involves  the  use  of  a  solution  of  zinc  acidified 
with  acetic  acid.    To  the, neutral  solution  of  the  zinc,  from  which 
ammonium  salts  should  be  absent,  1  gram  of  sodium  acetate,  J  to 
1  gram  of  ammonium  oxalate,  and  0.3  c.c.  of  99%  acetic  acid  are 
added.    The  solution  is  heated  to  65°,  and  a  current  of  about  0.36 
ampere  and  4  volts  passed,  and  the  anode  or  cathode  rotated. 
Add  acetic  acid  occasionally,  as  zinc  forms  the  best  deposits  from 
a  nearly  neutral  solution.    At  the  end  of  an  hour  the  current  is 
increased  to  about  0.7  ampere  and  5  volts.     Water  is  added  so  as 
to  cover  a  clean  silver  surface  with  the  solution.    If  no  more  zinc 
is  deposited  and  the  solution  is  acid,  it  is  carefully  neutralized  by 
the  addition  of  ammonia  and  the  current  passed  for  about  fifty 
minutes  longer.    The  condition  of  the  solution  which  requires  the 
addition  of  ammonia  is  usually  indicated  by  the  solution  appearing 
as  if  full  of  bubbles.    The  complete  precipitation  of   the  zinc  is 
ascertained  by  testing  a  portion  of  the  solution  with    potassium 
ferrocyanide.     The  solution  is  then  siphoned  off  and  the  depos- 
ited metal  washed  with  water  and  alcohol,  dried   at  100°,  and 
weighed. 

319.  Determination   of    Zinc   in    Ores. — In  carrying   out  the 
determination  of  zinc  in  zinc  ores,  the  finely  powdered  material  is 
weighed  out  and  dissolved  in  hydrochloric  acid,  or,  if  necessary,  in 
nitric  acid.     In  the  latter  case  the  nitric  acid  is  removed  by  evapo- 
ration with  a  little  sulphuric  acid  and  the  residue  dissolved  in 
water,  :  If  considerable  quantities  of  copper,  arsenic,  antimony, 
etc.,  are  present,  hydrogen  sulphide  is  passed  through  the  solution 
and  the  precipitate  washed  with  water  containing  hydrogen  sul- 
phide.    If  iron,  cobalt,  and  nickel  are  absent,   the   zinc  may  be 
determined  m  the  filtrate  by  the  method  of  Classen.     Iron  does 
not  interfere  if  the  method  of  Smith  is  used,  since  small  quantities 
are  not  carried  down  with  the  zinc,  and  large  amounts  would  be 
precipitated  as  basic  acetate  on  warming  the  solution. 

*  Jour.  Am.  Chem.  Soc.,  Vol.  XXIV,  p.  1073, 


VOLUMETRIC  METHODS. 

CHAPTER  XIX. 

CALIBRATION   OF  APPARATUS. 

320.  Definition.  —  Volumetric  are  distinguished  from  gravi- 
metric methods  by  the  fact  that  in  the  former  the  amount  of  a 
given  constituent  is  ascertained  by  MEASURING  the  VOLUME  of  a 
LIQUID,  while  in  gravimetric  methods,  the  element  to  be  determined 
is  brought  to  a  condition  where  it  may  be  placed  on  the  balance- 
pan,  and  WEIGHED.  Solutions  of  reagents  which  have  been  made 
to  contain  an  accurately  determined  amount  of  a  given  chemical 
substance  are  called  STANDARD  SOLUTIONS.  The  volume  of  such 
a  solution,  which  will  just  complete  the  reaction  with  a  given 
constituent  in  a  known  amount  of  a  substance  to  be  analyzed,  will 
give  the  amount  of  the  constituent  to  be  determined. 

We  may,  for  instance,  weigh  out  58.50  grams  of  pure  sodium 
chloride  and  dissolve  in  a  liter  of  water.  The  amount  of  silver 
contained  in  a  given  solution  may  be  ascertained  by  adding  the 
salt  solution  until  all  of  the  silver  has  been  precipitated  as  silver 
chloride,  and  further  addition  of  the  salt  solution  gives  no  further 
precipitate.  The  volume  of  the  salt  solution  having  been  carefully 
noted,  the  amount  of  silver  present  may  be  computed  from  the 
known  amount  of  salt  solution  used.  From  the  equation, 

Nad  +        AgN03         =        AgCl  +  NaN03, 

(23.05  +  35.45  =  58.50)  (Ag  =  107.93) 

we  know  that  58.50  parts  of  sodium  chloride  can  precipitate 
107.93  parts  of  silver.  As  the  salt  solution  contained  58.50 
grams  of  sodium  chloride  per  liter,  if  100  c.c.  were  used  to  pre- 
cipitate the  silver,  then  10.793  grams  of  silver  were  present  in 
the  unknown  solution. 

226 


VOLUMETRIC  APPARATUS. 


227 


VOLUMETRIC  APPARATUS. 

321.  Flasks. — The  apparatus  which  has  been  devised  for  meas- 
uring the  solutions  consists  in  the  first  place  of  so-called  standard 
flasks.     These  flasks  are  made  of  such  a  capacity  that  when  filled  to 
a  mark  made  on  the  stem,  they  contain  definite  volumes  of  liquid, 
such  as  1  liter  or  1000  c.c.,  J  liter  or  500  c.c.,  \  liter  or  250  c.c.,  ect. 

322.  Pipettes  are  tubes  constructed  to  deliver  definite  volumes 
having,  generally,  a  bulb  at  the  centre  and  a  mark  on  the  upper 
part  of  the  stem  to  indicate  the  point  to  which  the  pipette  must  be 
filled  to  deliver  the  volume  marked  on  the  instrument.    The  liquid 


s.  / 


FIG.  36. 

is  sucked  up  into  the  pipette,  the  upper  end  closed  with  the  finger, 
and  after  allowing  the  liquid  to  flow  down  to  the  mark,  the  contents 
of  the  pipette  are  allowed  to  flow  into  the  desired  vessel. 


228 


VOLUMETRIC  METHODS. 


323.  Burettes  are  glass  tubes  of  uniform  bore,  having  a  stop-cock 
a  t  the  lower  end,  and  marked  so  that  any  desired  number  of  cubic 
centimeters  may  be  measured  out.  The  total  capacity  is  usually 


FIG.  37. 

50  c.c.,  and  the  smallest  divisions  indicate  either  the  fifth  or  the 
tenth  of  a  cubic  centimeter.  It  is  customary  to  estimate  the  tenth 
of  the  smallest  division  so  that  volumes  may;  be  measured  with 
burettes  to  the  -fa  or  the  y^  of  a  cubic  centimeter. 

324.  Reading  Burettes. — Unices  special  care  is  taken  in  making 
readings,  errors  of  tV  °f  a  cubic  centimeter  may  easily  be  made. 
Generally  the  lowest  part  of  the  meniscus  is  the  point  at  which 
readings  are  made.  As  this  point  is  in  the  centre  of  the  tube,  a 
different  reading  will  be  made  according  to  the  position  of  the 
eye,  as  will  be  seen  from  Fig.  38.  One  of  the  simplest  devices  for 
securing  the  correct  position  of  the  eye,  corresponding  to  the 
point  b,  consists  of  a  piece  of  paper  or  card  with  a  perfectly  straight 
edge  folded  double  so  as  to  completely  surround  the  tube.  The 
card  is  held  so  that  the  folded  upper  edges  meet  exactly,  as  shown 


READING  BURETTES. 


229 


in  Fig.  39.  The  eye  is  placed  in  such  a  position  that  the  edge  of  the 
card  on  the  front  of  the  burette,  the  bottom  of  the  meniscus,  and 
the  edge  on  the  back  of  the  burette  are  in  the  same  line.  The 
reading  of  the  burette  is  then  taken  at  the  upper  edge  of  the  card. 
The  eye  may  also  be  held  in  the  same  straight  line  with  the 
upper  edges  of  the  meniscus.  The  lowest  point  of  the  meniscus 


FIG.  38. 


FIG.  39. 


is  then  read.  This  method  will  give  a  reading  which  is  slightly 
too  high,  but  if  the  burette  is  always  read  in  this  manner,  no 
error  will  result,  as  the  volumes  measured  out  are  always  found  by 
taking  the  difference  between  two  readings. 

325.  Floats. — Another  method  of  securing  accurate  readings  is 
by  the  use  of  floats,  which  are  small  glass  tubes  sealed  at  both  ends 
and  containing  enough  mercury  to  cause  them  to  be  very  nearly 
immersed  in  dilute  water  solutions.     A  circle  is  etched  on  the 
float  at  right  angles  to  its  length,   and  the    readings  are  made 
by  the  line  which  is  produced  by  holding  the  eye  in  the  plane 
of  this  circle.     While  this  line  gives  very  exact  readings,  serious 
error  is  often  occasioned  by  the  tendency  of  floats  to  go  down 
irregularly  by  a  series  of  jumps  instead  of  following  the  level  of 
the  liquid.     Before  being  used  a  float  should  be  carefully  tested 
by  placing  it  in  the  burette    filled  with  water.      The  water   is 
allowed  to  flow  out  at  a  moderate  rate,  while  the  float  is  closely 
observed  to  see  if  it  descends  at  a  perfectly  regular  rate. 

326.  Errors   in  Reading    Burettes.— When  skill  has  been  ac- 
quired in  using  one  or  the  other  of  these  devices  and  in  estimating 


230 


VOLUMETRIC  METHODS. 


the  tenth  of  one  of  the  smallest  divisions  on  the  burette,  the  accu* 
racy  acquired  in  -practice  is  still  never  greater  than  T^  of  a  cubic 
centimeter;  that  is,  the  reading  will  be  doubtful  to  the  tenth  of 
the  smallest  division  of  the  burette.  As  an  error  of  this  magnitude 
is  possible  at  both  of  the  readings,  in  measuring  a  given  volume  of 
liquid  the  total  error  may  be  at  least  two-hundredths  of  a  cubic 
centimeter.  Careful  experiments  show  that  in  careful  work 


FIG.  40. 

errors  of  three-hundredths  of  a  cubic  centimeter  occur.  The 
percentage  error  of  a  measurement  is  greater,  therefore,  the 
smaller  the  volume  of  liquid  measured  out,  being  3%  for  1  c.c., 
0.3%  for  10  c.c.,  and  0.1%  for  30  c.c.  As  the  possible  errors  in  the 
other  parts  of  volumetric  analysis  are  not  less  than  0.1%,  about 
25  c.c.  is  the  minimum  volume  of  liquid  which  should  be  used  in 
a  volumetric  analysis.  Greater  accuracy  is  seldom  obtained  by 
using  a  larger  volume  of  liquid. 


CORRECT  USE  OF  PIPETTES.  231 

327.  Method  of  Using  Pipettes. — If  a  pipette  is  properly  used, 
liquids  may  be  measured  with  greater  accuracy  than  is  possible 
with  a  burette.    This  arises  from  the  fact  that  the  tube  in  which 
the  initial  and  final  readings  are  made  is  so  much  smaller  than  in 
the  case  of  a  burette.    The  errors  in  using  a  pipette  arise  from 
allowing  the  liquid  to  drain  for  varying  intervals  of  time,  and  also 
from  leaving  a  drop  of  liquid  on  the  lower  end  of  the  tube.     This 
drop  should  be  taken  off,  when  the  liquid  has  been  brought  to  the 
mark  on  the  upper  part  of  the  stem,  by  drawing  the  point  of  the 
pipette  along  the  glass  surface  of  the  vessel  from  which  the  liquid 
has  been  drawn.    The  liquid  should  be  allowed  to  flow  out  of  the 
pipette  at  its  maximum  speed,  and  the  point  should  be  touched 
against  a  glass  surface  when  the  pipette  is  empty.     If  it  has  a 
mark  on  the  lower  stem,  the  liquid  must  be  allowed  to  flow  to  this 
mark ;  otherwise  the  pipette  is  completely  emptied  with  the  excep- 
tion of  the  drop  which  always  remains  in  the  point.    Some  chem- 
ists blow  this  drop  out  of  the  pipette.    This  is  unnecessary,  but 
the  amounts  of  liquid  delivered  will  be  identical  if  this  practice  is 
uniformly  adhered  to.     More  uniform  results  will  be  obtained  if 
the  hole  at  the  point  of  the  pipette  is  not  over  2  mm.  in  diameter, 
so  that  the  contents  of  the  pipette  may  flow  out  in  about  ten  seconds. 
If  the  opening  is  too  large,  it  may  be  diminished  by  heating  the 
point  to  redness  in  the  flame  of  a  Bunsen  burner.    The  upper  end 
of  the  stem  as  well  as  the  finger  used  in  closing  it  must  be  dry  or  the 
flow  of  liquid  cannot  be  properly  controlled. 

328.  Standard    Temperature. — The    volumes    of    the    glass- 
measuring  apparatus  vary  appreciably  with  changes  in  temper- 
ature, while  the  changes  in  the  volumes  of  liquids  is  very  marked 
indeed.     It  is  necessary,  therefore,  in  volumetric  work  to  keep 
solutions   and   apparatus   at   some   standard   temperature.    The 
most  convenient  temperature  is  that  of  the  air  in  the  laboratory. 
This  changes  within  quite  wide  limits  and  varies  in  different  coun- 
tries.    The  German  chemists  have  chosen  15°  Centigrade  as  the 
average  temperature   of  the  laboratory.    American  laboratories 
are  kept  much  warmer  than  this,  so  that  20°  is  a  much  better 
standard  for  this  country.     This  will  be  used  as  the  standard 
temperature  for  the  work  described  in  this  book,  the  glassware 
being  calibrated  for  this  temperature. 


232  VOLUMETRIC  METHODS. 

329.  Expansion   of  Glass,  Water,  etc. — The  change  in  volume 
of  a  liter  flask  due  to  a  change  in  temperature  of  5°  C.  is  0.15  c.c. 
As  this  is  .015%  of  the  total  volume,  it  is  much  less  than  other 
experimental  errors  and  may,  therefore,  be  entirely  disregarded. 
The  change  in  volume  of  water  is  much  greater.     A  liter  of  water 
measured  at  20°  will  contract  0.9  c.c.  if  cooled  to  15°,  while  if 
heated  to  25°  it  will  expand  1.144  c.c.     The  error  in  this  case  for 
a  change  in  temperature  of  5°  will  therefore  be  about  0.1%.     The 
change  in  volume  of  solutions  is  generally  much  greater,  the  more 
concentrated   the   solution   the   greater  being  the   coefficient   of 
expansion. 

330.  Errors  of  Graduation. — A  very  common  source  of  error 
in  volumetric  analysis  is  the  inaccuracy  of  the  graduations  of  the 
measuring  apparatus  used.     It  is  by  no  means  uncommon  to  find 
errors  amounting  to  1%  in  apparatus  bought  of  the  dealers,  while 
J%  would,  perhaps,  fairly  represent  the  error  in  using  most  un- 
tested apparatus.     The  errors  are  due  to  several  causes,  but  chiefly 
to  the  fact  that  various  standards  of  volume  are  in  use. 

331.  Definition    of    Units  of    Volume. — Volumetric   apparatus 
is  usually  constructed  on  the  basis  of  the  metric  system,  the  units 
of  volume  being  the  LITER  and  the  CUBIC  CENTIMETER.     In  this 
system  the  unit  of  length  was  first  selected  and  prepared.     The 
unit  of  volume  was  obtained  from  the  unit  of  length,  a  liter  being 
defined  as  the  cube  whose  sides  are  a  decimeter  in  length.     A 
cubic  centimeter  is  the  one-thousandth  part  of  this  volume.     The 
unit  of  weight  was  obtained  by  preparing  pure  water  and  calling 
the  weight  of  one  liter  weighed  in  a  vacuum  at  4°  C.  a  kilogram. 

332.  Various  Standard  Liters  in  Use. — On  account  of  the  ease 
of  preparing   pure   water,    measuring  its   temperature   correctly 
and  weighing  it  with  accuracy,  the  custom  has  arisen  of  using 
accurately  weighed  water  as  the  standard  of  volume.     This  cus- 
tom is  still  further  justified  by  the  fact  that  it  is  comparatively 
easy  to  obtain  weights  which  are  correct  far  beyond  the  accuracy 
attainable  in  measuring  volumes.     The  errors  found  in  commer- 
cial volumetric  apparatus  have  arisen  from  a  desire  to  avoid  the 
trouble  of  making  the  weighings  of  the  water  under  the  standard 
conditions  of  temperature  and  absence  of  an  atmosphere.     In 
many  instances  the  suggestion  made  by  Mohr  has  been  adopted. 


STANDARDS  OF    VOLUME.  233 

He  proposed  to  adopt  ~as  a  liter  the  volume  of  1  kilogram  of  water 
at  4°  C.,  weighed  in  air.  He  adopted  17.5°  C.  as  the  standard 
temperature.  This  liter  would  differ  from  the  true  liter  by  1.2 
cubic  centimeters.  Other  workers  have  used  as  a  liter  the  vol- 
ume of  one  kilogram  of  water  weighed  in  the  air  at  15°  C.,  while 
still  others  weigh  at  17.5°  C.  or  20°  C. 

With  these  various  standards  in  use  it  is  not  surprising,  espe- 
cially when  taken  in  connection  with  the  possible  errors  of  work- 
manship, that  the  use  of  uncalibrated  apparatus  frequently  pro- 
duces very  erroneous  results.  The  possibility  of  error  is  consider- 
ably reduced  if  all  of  the  volumetric  apparatus  in  a  laboratory 
has  been  made  by  the  same  manufacturer,  since  no  error  will 
result  from  the  use  of  a  standard  other  than  the  true  liter  if  all  of 
the  apparatus  is  graduated  according  to  the  same  standard,  the 
cubic  centimeter  being  exactly  the  one-thousandth  part  of  the 
liter.  Apparatus  used  for  gas  analysis  must,  however,  be  grad- 
uated according  to  the  true  liter.  All  confusion  would  be  avoided 
if  the  true  liter  were  generally  adopted.  The  German  chemists 
have  taken  the  lead  in  this  direction.  The  German  Imperial 
Standards  Commission  has  made  it  illegal  to  use  any  other  than 
the  true  liter  for  official  purposes.  The  directions  given  for  cali- 
bration in  this  book  will  be  based  on  the  true  liter. 

CALIBRATION. 

There  are  in  use  two  general  methods  of  calibrating  volumetric 
apparatus.  By  the  first  method  the  weight  and  temperature  of 
the  water  which  just  fills  the  apparatus  to  be  calibrated  is  found 
and  the  volume  calculated  from  the  known  density  of  water.  By 
the  second  method  standard  bulbs  whose  capacity  has  been  care- 
fully determined  are  used.  The  water  which  just  fills  the  bulb  is 
allowed  to  flow  into  the  flask  to  be  calibrated.  Where  many  pieces 
of  apparatus  must  be  calibrated  this  method  is  to  be  preferred. 

333.  Calibration  of  Flasks  by  Weighing  Water. — Flasks  which 
are  to  be  calibrated  are  first  cleaned  and  dried,  then  carefully 
counterpoised  or  weighed.  Distilled  water  at  20°  C.  is  poured  in 
until  the  flask  is  filled  to  the  mark.  The  additional  weights  neces- 
sary to  again  counterbalance  the  flask  give  the  weight  of  the 


234  VOLUMETRIC  METHODS. 

water.  The  weight  of  the  water  in  vacuo  must  now  be  calculated. 
The  air  displaced  by  a  kilogram  of  water  may  be  taken  as  a  liter, 
which  under  ordinary  temperatures  and  pressures  weighs  1.2 
grams.  When  in  the  air  the  water  is  lighter  by  this  amount. 
The  weights,  however,  are  also  lighter  in  the  air  than  in  a  vacuum. 
The  specific  gravity  of  brass  weights  is  8.4.  A  kilogram  of  brass 
weights  would  displace  -|£  of  a  liter  of  air  which  would  weigh 
.14  gram.  As  the  buoyant  force  on  the  water  is  1.2  grams  and 
on  the  weights  .14  gram,  the  diminution  hi  weight  in  the  air  is 
1 .06  grams  (1 .20  — .  14) .  A  kilogram  of  water,  therefore,  weighed  in 
the  air  with  brass  weights  would  weigh  998.94  grams  (1000  —  1.06), 
the  weight  in  a  vacuum  being  exactly  1000  grams.  The  general 
correction  for  air  displacement  is  therefore  +.106%.  The  weight 
of  the  water  in  a  vacuum  divided  by  the  density  at  20°  gives  the 
volume.  In  this  manner  the  volume  of  the  flask  when  filled  to 
the  mark  is  obtained.  If  a  flask  is  to  be  calibrated  to  deliver  a 
definite  volume  it  should  be  rinsed  with  distilled  water  and 
allowed  to  drain  for  the  same  length  of  time  employed  in  ordinary 
use.  It  is  then  weighed  and  the  calibration  carried  out  as  already 
described. 

If  it  is  desired  to  ascertain  the  point  on  the  stem  to  which  the 
flask  must  be  filled  in  order  to  contain  one  liter,  it  should  be  dried 
and  counterbalanced  on  the  scale-pan,  and  then  weights  equal  to 
997.2  *  grams  placed  on  the  opposite  pan  and  distilled  water  at 
20°  C.  poured  into  the  flask  until  the  weights  are  again  counter- 
balanced. A  mark  may  now  be  made  with  a  file  on  the  neck  of 
the  flask  opposite  the  lowest  point  of  the  meniscus,  and  then  a 
circular  mark  may  be  etched  on  by  means  of  hydrofluoric  acid. 
For  this  purpose  a  thin  layer  of  paraffine  or  beeswax  should  be 
melted  on  the  neck  of  the  flask  and  a  circle  traced  with  a  sharp 
steel  point  while  the  flask  is  rotated  in  a  lathe  or  some  similar 
device.  The  acid  may  be  applied  by  means  of  a  piece  of  filter- 
paper  and  should  be  allowed  to  remain  from  three  to  five  minutes. 

334.  Calibration  of  Burettes  by  Weighing  Water. — The  burette 
is  filled  with  distilled  water  at  about  20°  C.  A  small  flask  or 

*  1000  c.c.  water  at  20°  in  a  vacuum  weigh  998.26  grams.  From  this 
weight  the  loss  in  weight  of  1.06  grams  due  to  air  displacement  must  be  sub- 
tracted. 


CALIBRATION  BY  WEIGHING  WATER.  235 

weighing-tube  capable  of  holding  50  c.c.  is  weighed.  The  water 
in  the  burette  is  now  brought  exactly  to  the  zero-point  and  then 
about  5  c.c.  is  allowed  to  flow  into  the  flask.  The  exact  reading 
of  the  burette  should  be  taken  and  the  flask  weighed.  Another 
5-c.c.  portion  of  water  is  then  allowed  to  flow  into  the  flask  and 
the  burette  again  read  and  the  flask  weighed.  This  process  is 
repeated  until  the  last  reading  on  the  burette  is  exactly  50  c.c. 
and  the  flask  contains  the  water  delivered  by  the  burette  between 
zero  and  the  50-c.c.  mark.  The  water  need  be  weighed  only  to 
the  hundredth  of  a  gram,  as  volumes  smaller  than  .01  c.c.  cannot 
be  measured  on  ordinary  volumetric  apparatus. 

The  volumes  corresponding  to  the  weights  of  water  are  now 
calculated  by  the  method  given  for  the  calibration  of  a  flask, 
.997  gram  of  water  being  equal  to  1  c.c.  The  difference  between 
the  volume  indicated  on  the  burette  and  that  calculated  from 
the  weight  of  the  water  is  the  correction  to  be  applied  at  that 
point.  The  correction  for  intermediate  points  is  found  by  plot- 
ting a  curve,  as  shown  in  Fig.  44,  in  which  the  ordinates  are  the 
numbers  1  to  50  representing  the  readings  on  the  burette,  while 
the  abscissas  are  the  corrections  found  by  the  calibration.  The 
assumption  is  made  that  the  variations  in  volume  are  uniform 
between  the  points  calibrated.  Generally  this  does  not  lead  to 
serious  error.  If  the  curve  is  very  irregular,  the  burette  'should 
be  calibrated  for  every  2  or  3  c.c. 

CALIBRATION    BY   MEANS    OF    THE    MORSE-BLALOCK 

BULBS. 

335.  Description  of  Bulbs. — If  many  pieces  of  apparatus  are  to 
be  calibrated  the  Morse-Blalock  calibrating  bulbs  should  be  used. 
These  authors  recommend  the  use  of  6  bulbs  of  different  sizes 
combined  into  3  pieces  of  apparatus,  as  shown  in  Fig.  41.  Three 
bulbs  of  capacities  2  c.c.,  3  c.c.,  and  50  c.c.,  respectively,  are  all 
blown  on  one  stem,  various  parts  of  which  are  graduated.  Two 
bulbs  of  50  c.c.  and  200  c.c.  capacity  are  connected  on  a  single 
stem,  which  is  also  graduated,  while  the  500-c.c.  bulb  is  on  a  sepa- 
rate graduated  stem.  Bulbs  of  other  sizes  could  be  made  as 
desired.  The  intention  of  Morse  and  Blalock  being  to  adapt  the 
bulbs  for  use  at  a  considerable  range  of  temperature,  they  were 


236 


VOLUMETRIC  METHODS. 


made  of  such  a  capacity  that  the  volume  of  a  bulb  at  30°  should  not 
be  greater  than  that  marked  on  it,  while  at  0°  the  combined  vol- 
ume of  the  stem  and  bulb  should  not  be  less  than  that  indicated 
on  the  bulb. 

336.  Calibration  of  the  Bulbs. — The  exact  capacity  of  each  bulb 


FIG.  41. 

is  ascertained  by  filling  it  with  distilled  water  and  weighing  the 
amount  delivered  on  emptying  it.  The  temperature  of  the 
water  is  noted  and  the  volume  of  the  bulb  for  this  temperature 
is  calculated.  The  temperature  of  the  water  need  not  be  that  at 
which  the  bulb  is  to  be  used,  since  the  volume  of  the  bulb  having 


MORSE-BLALOCK  CALIBRATING  BULBS.  237 

been  ascertained  for  a  given  temperature,  by  using  the  coeffi- 
cient of  expansion  of  the  glass  of  which  volumetric  apparatus  is 
made,  .000026,  the  volume  at  any  other  temperature  can  be  cal- 
culated. The  volume  of  the  graduated  stem  is  ascertained  in 
the  same  manner.  In  using  a  bulb  for  calibration  it  is  filled  with 
distilled  water,  which  is  allowed  to  flow  into  the  dry  and  empty 
flask  until  the  bulb  and  a  sufficient  number  of  divisions  on  the 
stem  are  emptied  to  give  the  required  volume.  An  example  will 
make  this  clearer. 

Weight  of  Water  from  Bulb.  Calculated  Volume. 

At  17.5°.  At  14.3°.  At  20°.  At  15°. 

498.340  gr 499.51  c.c. 

498. 456  gr.     499.43  c.c. 

Av.  499 . 47  c.c.   499 . 40  c.c. 

Weight  of  Water  from  Stem. 
At  14.6°.  At  14.7°. 

3 . 177  gr 3 . 182  c.c. 

3.180gr.  3. 185  c.c. 


Av.  3. 184  c.c. 

Volume  of  1  division  of  stem  =  .032  c.c. 

Bulb +17  divisions  of  stem  =500.00  c.c.  at  20°  (499.47+17 
X.032). 

Bulb+19  divisions  of  stem =500.00  c.c.  at  15°  (499.40+19 
X.032). 

337.  Temperature  of  Water  Used  in  Calibration. — If  it  is  de- 
sired to  calibrate  a  flask  to  hold  500  c.c.  at  20°,  the  water  con- 
tained in  the  bulb  and  17  divisions  of  the  stem  is  allowed  to  flow 
into  the  flask  and  a  mark  made  on  the  stem  opposite  the  bottom 
of  the  meniscus.  The  temperature  of  the  water  need  not  be  20°. 
If  the  temperature  of  the  water  and  consequently  of  the  bulb  is 
greater  than  20°,  the  volume  of ,  the  bulb  and  17  divisions  of  the 
stem  will  be  greater  than  500  c.c.,  but  on  cooling  to  20°  it  will 
be  exactly  500  c.c.  The  temperature  of  the  flask  will  be  raised 
to  that  of  the  water,  so  that  the  volume  of  the  flask  will  be  greater 
than  500  c.c.,  but  .the  flask  also  will  contract  on  cooling,  and  as 
the  bulbs  are  made  of  the  same  kind  of  glass  as  the  flasks,  the  con- 
traction of  the  flask  will  be  exactly  equal  to  the  contraction  of  the  bulb. 


238 


VOLUMETRIC  METHODS. 


The  same  line  of  reasoning  will  show  that  the  temperature  of 
the  water  may  be  below  the  standard  temperature  without  affect- 
ing the  accuracy  of  the  calibration. 

338.  Calibration  of  Flasks.— By  means  of  the  50-c.c.  and 
200-c.c.  bulbs,  flasks  of  capacities  which  are  multiples  of  50  c.c. 
may  be  calibrated.  For  flasks  whose  capacities  are  multiples  of 
500  c.c.,  the  large  bulb  is  used. 


FIG.  42. 

339.  Calibration    of   Burettes. — The   50-c.c.   bulb  with  2-c.c. 
and  3-c.c.  bulbs  are  used  for  calibrating  burettes.    For  this  work 


CALIBRATION  OF  BURETTES. 


239 


as  well  as  for  calibrating  flasks  it  is  necessary  to  have  a  bottle 
holding  several  liters  placed  on  a  shelf  which  is  above  the  top  of 
the  burette  or  bulb  when  fixed  in  clamps  for  use.  A  Friedrich- 


FIG.  43. 

Greiner  two-way  stop-cock  is  attached  to  the  bulb  so  that  water 
may  be  withdrawn  from  the  bulb  by  two  openings  in  the  stop- 
cock. One  of  these  tubes  is  connected  with  one  arm  of  a  T-tube, 
the  second  arm  being  connected  by  a  tube  to  the  large  bottle  on 


240  VOLUMETRIC  METHODS. 

9 

the  shelf,  and  the  third  one  is  connected  with  the  burette.  The 
latter  must  be  clamped  in  such  a  position  that  the  point  giving 
the  last  reading  is  above  the  highest  point  of  the  calibrating  bulb. 
All  joints  must  be  absolutely  water-tight,  and  the  connecting 
tubes  should  be  of  glass  as  far  as  possible.  A  pinch-cock  should 
be  placed  on  the  tube  leading  to  the  large  bottle,  which  is  filled 
with  distilled  water.  By  opening  this  pinch-cock  water  may  be 
admitted  either  to  the  burette  or  to  the  bulb.  All  bubbles  of  air 
must  be  displaced  from  the  connecting  tubes. 

340.  Cleaning     Mixture. — The   burette    and    calibrating   bulb 
must  be  cleaned  so  that  a  continuous  film  of  water  is  left  on 
emptying  them.    A  most  excellent  cleaning  mixture  is  made  by 
adding  a  few  grams  of  potassium  dichromate  to  a  half  liter  of 
concentrated  commercial  sulphuric  acid.    This  mixture  should  be 
kept  in  a  glass-stoppered  bottle  to  which  portions  which  have 
been  used  may  be  returned  unless  largely  diluted  with  water. 
Apparatus  filled  with  this  mixture  and  allowed  to  stand  for  some 
time  will  be  perfectly  clean  on  pouring  out  the  cleaning  mixture 
and  washing  with  water.     The  cleaning  mixture  is  much  more 
efficient  if  warmed.    When  it  has  become  green  because  of  reduced 
chromium,  it  must  be  discarded  or  more  chromate  added.    Burettes 
and  pipettes  must  always  be  kept  clean  or  errors  arise  in  their 
use    because  of   the  drops  of   liquid  which  adhere  to  the  glass. 
If  burettes  are  filled  with  distilled  water  when  not  in  use,  they 
keep  cleaner  than  if  left  empty. 

341.  Total    Capacity. — The  bulb  and  burette  are  filled  with 
water  which  is  allowed  to  flow  out  at  a  moderate  rate.     The  burette 
is  again  filled  with  water,  and  by  cautiously  turning  the  two-way 
stop-cock,  water  is  allowed  to  flow  from  the  burette  into  the  bulb 
until  the  reading  in  the  former  is  exactly  zero.    The  stop-cock 
is  now  turned  through  180°,  and  the  50  c.c.  bulb  emptied  until  the 
meniscus  is  opposite  the  mark  on  the  stem.     By  again  turning  the 
stop-cock  the  water  from  the  burette  is  allowed  to  flow  into  the 
bulb  until  the  reading  on  the  burette  is  exactly  50  c.c.    The 
number  of  divisions  of  the  stem  of  the  bulb  filled  by  the  water  is 

"now  noted.  From  this  the  total  capacity  of  the  burette  can  be 
calculated.*  Duplicates  should  agree  within  1  or  2  divisions  of 
the  stem.  The  50-c,c.  bulb  should  be  so  constructed  that  its 


CALIBRATION  OF  BURETTES.  241 

volume  is  not  greater  than  49.75  c.c.,  while  the  capacity  of  the 
stem  should  be  at  least  .75  C.G.,  so  that  the  total  capacity  of  the 
stem  and  bulb  shall  be  about  50.50  c.c.  If  the  capacity  of  the 
burette  is  greater  than  50.50  c.c.,  the  stem  may  be  emptied,  and 
water  again  allowed  to  flow  in  from  the  burette. 

342.  Irregularities  of  Bore. — The  burette  is  once  more  filled 
with  water  which  is  brought  to  the  zero-mark.  It  is  then 
allowed  to  fill  either  the  2-c.c.,  3-c.c.,  or  both  bulbs,  depending 
upon  the  accuracy  desired  in  the  calibration.  The  reading  of  the 
burette  is  carefully  taken,  the  small  bulb  emptied,  and  the  opera- 
tion repeated  until  the  burette  has  been  emptied.  The  capacity 
of  the  small  bulbs  should  be  a  trifle  less  than  that  indicated,  so 
that  the  last  reading  on  the  burette  may  always  be  less  than  50  c.c. 
The  method  of  calculating  the  results  can  best  be  given  by  the 
results  of  an  actual  calibration. 

Total  capacity  =50  c.c.  bulb +  78  divisions  of  stem. 

Total  capacity  =50  c.c.  bulb +  80  divisions  of  stem. 

Volume  of  50  c.c.  bulb  =49.62  c.c. 

Volume  of  stem  (100  divisions)  =.7807  c.c. 

Total  capacity  of  burette  =50.24  c.c.  (49.62 +.0078X79). 


Readings. 

0.00 

True  Capacity. 

0.00 

Corrections. 

0.00 

1.99 

1.98 

-0.01 

3.98 

3.96 

-0.02 

5.92 

5.94 

+  0.02 

7.90 

7.92 

+  0.02 

9.89 

9.89 

0.00 

11.87 

11.87 

0.00 

13.83 

13.85 

+  0.02 

15.81 

15.83 

+  0.02 

17.79 

17.81 

+  0.02 

19.77 

19.79 

+  0.02 

21.71 

21.77 

+  0.06 

23.68 

23.75 

+  0.07 

25.64 

25.72 

+  0.08 

27.60 

27.70 

+  0.10 

29.58 

29.68 

+  0.10 

242  VOLUMETRIC  METHODS. 

Readings.  True  Capacity.  Corrections. 

31.55  31.66  +0.11 

33.50  33.64  +0.14 

35.48  35.62  +0.14 

37.45  37.60  +0.15 

39.40  39.58  +0.18 

41.38  41.56  +0.18 

43.32  43.53  +0.21 

45.29  45.51  +0.22 

47.28  47.49  +0.21 

49.24  49.47  +0.23 

50.00  50.24  +0.24 

The  last  reading  being  49.24  and  the  true  capacity  at  50.00 
being  50.24,  the  true  capacity  at  49.24  is  found  by  the  proportion 

50 : 50.24 : :  49.24 :  x,  where  x  =49.47. 

As  the  2-c.c.  bulb  has  been  filled  twenty-five  times  with  water, 
the  volume  of  which  is  49.47  c.c.,  the  capacity  of  the  bulb  must- 
be  ^5-  of  49.47,  or  1.979.  As  at  each  successive  reading  of  the 
burette  one  bulb-full  of  water  has  been  taken  out,  the  true  volumes, 
at  these  points  are  found  by  multiplying  1.979  by  the  numbers, 
from  1  to  25.  These  values  are  given  in  the  second  column.  The 
differences  between  these  values  and  the  readings  of  the  burette 
are  the  corrections  to  be  applied  at  these  points.  The  correc- 
tions are  plotted  on  co-ordinate  paper  as  abscissa,  while  the  cor- 
responding burette  readings  are  the  ordinates,  as  shown  in  Fig.  44. 

EXERCISE  45. 
Calibration  of  Morse  and  Blalock  Bulbs. 

Clean  one  of  the  bulbs  by  filling  with  bichromate  solution  and,  after  ten 
minutes,  empty  it  and  wash  with  water.  Clamp  it  in  position  with  the 
Greiner-Friedrich  stop-cock,  which  is  connected  with  the  large  bottle  of 
distilled  water,  as  shown  in  Fig.  42.  Fill  with  water  and  allow  it  to 
flow  out  at  a  moderate  rate.  If  the  film  of  water  left  on  the  glass  breaks, 
leaving  drops,  the  cleaning  with  the  bichromate  solution  must  be  repeated. 
When  the  bulb  is  clean,  fill  again  with  distilled  water  and  bring  the  lower 
part  of  the  meniscus  exactly  on  a  line  with  the  upper  mark  on  the  bulb. 


CALIBRATION  CURVE. 


243 


CORRECTIONS  '"  CUBIC  CENTIMETERS 
-Q.I       go     -t-ai     +0.2       0.0     +0.1      +02     +03        go     +01 


FIG.  44. 


244  VOLUMETRIC  METHODS. 

Let  the  water  flow  at  a  moderate  rate  into  a  weighed  flask  until  the  menis- 
cus is  at  the  lower  mark  on  the  bulb.  Weigh  the  water  to  milligrams. 
Draw  out  another  portion  of  the  water  into  a  flask  and  take  its  tempera- 
ture. Repeat  the  determination  until  duplicates  are  obtained  which  are 
identical  to  at  least  1  part  in  1000.  Calculate  the  volume  using  the  specific 
gravity  of  water  at  the  temperature  noted  and  a  correction  of  +.106%  for 
air  displacement.  Calculate  the  volume  at  20°.  In  the  same  manner  ascer- 
tain the  volume  of  the  graduated  stem  attached  to  the  bulb.  Calculate 
the  number  of  divisions  of  the  stem  which  must  be  used  to  make  the  volume 
equal  to  that  marked  on  the  bulb. 


EXERCISE  46. 
Calibration  of  Flasks. 

Clean  and  dry  the  standard  flasks  you  have.  Clean  all  calibrating 
bulbs  to  be  used  according  to  the  directions  given  in  the  preceding  exer- 
cise. Clamp  the  ^-liter  bulb  in  position  as  shown  in  Fig.  42.  Fill  the 
bulb  to  the  upper  mark  and  allow  the  water  to  flow  into  the  liter  flask 
until  the  bulb  and  the  number  of  divisions  on  the  stem  giving  500  c.c. 
have  been  emptied.  Repeat  the  operation  and  immediately  make  a 
mark  on  the  neck  of  the  flask  opposite  the  bottom  of  the  meniscus.  Cali- 
brate the  other  flasks  in  a  similar  manner.  A  thin  layer  of  beeswax  or  par- 
affine  should  be  melted  on  the  neck  and  a  circular  line  passing  through 
the  file-mark  made  with  a  sharp  steel  point.  The  flask  should  be  held  in  a 
lathe  for  this  purpose.  The  mark  is  etched  on  by  dipping  a  piece  of  filter- 
paper  into  hydrofluoric  acid  and  moistening  the  mark  with  the  acid.  After 
about  three  minutes  the  acid  is  washed  off  with  water.  After  wiping  the 
flask  dry,  the  paraffine  is  melted  by  waving  the  neck  of  the  flask  in  the 
Bunsen-burner  flame.  The  neck  of  the  flask  may  then  be  cleaned  by  wiping 
with  dry  filter-paper. 


EXERCISE  47. 
Calibration  of  Burettes. 

After  cleaning  your  burette,  set  it  up  with  the  50-c.c.  bulb  as  shown  in 
Fig.  43,  p.  239.  Test  the  apparatus  for  leaks  by  filling  with  water  and  wiping 
all  joints  dry  with  filter-paper.  No  drops  of  water  should  be  visible  after 
fifteen  minutes.  All  joints  except  a  and  b  may  be  made  tight  by  painting 
the  glass  tubing  with  shellac  varnish  before  the  rubber  tubing  is  put  on. 
The  total  capacity  of  the  burette  is  first  found  by  filling  it  with  water  by 
opening  the  stop-cock  c  and  closing  d.  The  water  is  carefully  brought 
to  the  zero-mark  by  opening  a  (which  should  now  remain  open)  and  d. 


* 

CALIBRATION  OF   BURETTES.  245 

The  stop-cock  d  is  now  turned  so  that  water  can  flow*  into  the  beakei 
until  it  is  brought  to  the  lower  mark  on  the  50-c.c.  bulb,  d  is  again  turned 
so  that  water  is  slowly  admitted  from  the  burette  until  it  reads  exactly 
50  c.c.  The  number  of  divisions  on  the  stem  of  the  50-c.c.  bulb  is  now 
read.  The  determination  is  repeated  until  duplicates^agreeing  within  2 
or  3  cU visions  of  the  stem  are  obtained.  From  the  values  already  found 
for  the  50-c.c.  bulb,  and  for  one  division  of  the  stem,  the  total  capa^fty  of 
the  burette  is  calculated. 

The  burette  is""aga"m  filled  with  water  and  the  reading  brought  to  the 

.zero-mark.  If  the  total  capacity  is  not  more  than  0.2  c.c.  over  50  c.c.,  the 
burette  may  be  calibrated  every  5  c.c.  For  thisr- purpose  the  meniscus 
of  the  water  in  the  calibrating  bulb  is  brought  to  the  mark  below  the  2-c.c. 
bulb  and  water  allowed  to  flow  in  from  the  burette  until  the  3-c.c.  bulb  is 
filled  to  the  mark  on  the  stem.  The  reading  on  the  burette  is  now  taken 
and  the  operation  repeated  until  the  burette  is  empty.  Repeat  the  calibra- 
tion until  duplicates  are-  obtained  for  each  reading  agreeing  within  less 
than  .02  c.c.  10  portions  of  approximately  5  c.c.,  but  exactly  equal, 
will  now  have  been  withdrawn  from  the  burette.  The  method  of  calcu- 
lating the  re'sult  is  best  given  by  the  following  example:  A  burette 
whose  total  capacity  was  found  to  be  50.15  c.c.  gave  as  the  last  reading 
with  the  5-c.c.  portions  49.90  c.c.  The  volume  at  this  point  is  found  by 
the  following  proportion:  50.00  :  49.90  :  :  50.15  :x,  from  which  x,  OP  the 
volume  of  10  of  the  5-c.c.  portions  withdrawn,  equals  50.05  c.c.  Each 
5-c.c.  portion  is  therefore  equal  to  5.005  c.c.  This  number  multiplied  by 
the  numbers  from  1  to  10  gives  the  true  volumes  at  the  burette  readings 

*  taken.  The  differences  between  the  readings  and  the  true  volumes  give  the 
corrections  to  be  applied  at  the  points  calibrated.  To  find  the  corrections 
at  intermediate  points,  plot  a  curve  with  the  corrections  as  the  abscissas, 
and  the  readings  on  the  burette  as  ordinates,  as  shown  in  Fig.  44.  If 
the  difference  in  the  corrections  between  any  two  consecutive  readings  is 
more  than  .05  c.c:,  this  portion  of  the  burette  should  be  recalibrated  with 
the  2-c.c.  or  the  3-c.c.  bulb. 

PROBLEMS. 

Problem  i.  The  weight  of  water  at  20°  C.,  which  just  fills  a  flask,  is 
249.2  grams.  Calculate  (a)  the  volume  of  the  flask  at  20°,  (6)  its  volume 
at  1£,  (c)  its  volume  at  25°. 

Problem  2.  A  burette  was  calibrated  with  a  Morse  and  Blalock  bulb, 
of  which  the  50-c.c.  bulb  had  a  capacity  of  49.55,  and  the  stem  of  100  divi- 
sions had  a  capacity  of  0.73  c.c.  When  obtaining  the  total  capacity  of  the 
burette,  the  bulb  was  filled  and  67  divisions  of  the  stem.  When  calibrating 
the  bore  of  the  burette,  5-c.c.  portions  were  withdrawn  from  the  burette  by 
means  of  the  2-  and  3-c.c.  bulbs.  The  following  readings  were  obtained: 
5.05,  10.09,  15.09,  20.19,  25.19,  30.19,  35.15,  40.19,  45.19,  50.10.  Calculate 
the  total  capacity  of  the  burette  and  the  corrections  at  the  readings  given. 


• 

246  VOLUMETRIC  METHODS. 

Problem  3.  Another  burette  was  calibrated  with  the  same  bulb.  For 
the  total  capacity,  the  bulb  full  plus  70  divisions  of  the  stem  was  obtained. 
When  calibrating  the  bore  of  the  burette,  the  following  readings  were  taken: 
5.00,  10.11,  15.09,  20.09,  25.08,  30.09,  35.05,  40.09,  45.10,  50.10.  Calculate 
the  total  capacity  of  the  burette  and  the  corrections  at  the  readings  given. 

The  accuracy  of  the  calibration  of  these  burettes  may  be  judged  by  a 
comparison  of  the  volumes  calculated  for  the  5-c.c.  portions  withdrawn 
from  the  burettes.  These  values  should  not  differ  by  more  than  .03  c.c. 


> 


•-• 


CHAPTER  XX. 
ACIDIMETRY. 

343.  A  Standard  Solution  has  already  been  defined  as    one 
which  has  been  made  so  as  to  contain  an  accurately  determined 
amount  of  a  given  chemical  substance  in  a  definite  volume.    It 
has  been  found  that  the  acidity  of  a  solution  depends  on  the 
amount  of  replaceable  or  acid  hydrogen  present,  irrespective  of 
the  amount  of  other  substances  present.     A  standard  solution  of 
an  acid  may  therefore  be  defined  as  one  which  contains  a  known 
amount  of  acid  or  replaceable  hydrogen  in  a  definite  volume.    In 
practice  it  has   been  found   very  convenient    to    use   standard 
solutions  of  acids  which  contain  1  gram  *  of  replaceable  hydro- 
gen per  liter.    Such  a  standard  solution  is  called  a  NORMAL  solu- 
tion.    The  strength  of  any  standard  solution  may  be  given  in 
terms  of  a  normal  solution.    It  may  for  instance  be  J,  J,  fa 
etc.,  normal,  or  3,  3J,  1.362,  etc.,  times  normal. 

344.  Normal  Acids. — The  amount  of  a  given  acid  to  be  dissolved 
in  a  liter  to  make  a  normal  solution  will  vary  directly  as  its  molec- 
ular weight,  and  inversely  as  the  number  of  acid  or  replaceable 
hydrogens  in  the  molecule.     HYDROCHLORIC  ACID  having  a  molec- 
ular weight  of  36.458  (Cl  =35.45,  H  =  1.008),  a  liter  of  a  normal 
solution  of  this  acid  must  contain  36.458  grams  of  the  acid.    The 
molecular  weight  of  SULPHURIC  ACID  is  98.076  (S  =32.06,  04 =64.00, 
H2  =2.016).    As  the  molecule,  however,  contains  two  replaceable 
hydrogen  atoms,  one-half  of  the  molecular  weight,  or  49.038  grams, 
must  be  dissolved  in  a  liter  to  make  a  normal  solution.     Normal 
solutions  of  all  monobasic  acids  must  therefore  contain  per  liter 
the  weight  of  acid  equal  to  the  molecular  weight  in  grams,  while 

*  Since  the  system  of  atomic  weights  having  oxygen  as  16.00  as  the  basis 
has  come  into  use,  1.008  grams  of  hydrogen  per  liter  has  become  the  standard. 

247 


248  VOLUMETRIC  METHODS. 

one-half  the  molecular  weight  in  grams  must  be  taken  per  liter  for 
dibasic  acids. 

As  the  acid  value  of  a  solution  depends  wholly  on  the  amount 
of  replaceable  hydrogen  present,  and  as  the  designation  of  the 
strength  of  a  standard  acid  in  terms  of  normal  gives  the  amount 
of  hydrogen  per  liter,  the  weight  of  the  acid  radicle  need  not  be 
known  for  calculating  the  results  of  a  titration.  If  caustic  soda 
is  titrated  with  normal  hydrochloric  acid,  one  liter  of  the  acid 
will  neutralize  exactly  40.058  grams  of  NaOH(Na  =23.05, 0  =  16.000, 
H=1.008),  according  to  the  equation  HCl+NaOH=NaCl+H20. 
If  normal  sulphuric  acid  were  used  the  reaction  would  take  place 
according  to  the  equation  H2S04 + 2NaOH  =  Na2S04 + 2H20. 
Two  molecules  of  caustic  soda  or  80.116  grams  per  liter  would  be 
neutralized  by  one  molecule  of  sulphuric  acid  or  98.076  grams  per 
liter.  But  half  of  the  latter  amount  having  been  taken  for  a  liter 
of  normal  sulphuric  acid,  only  one  molecule  of  the  caustic  soda 
or  40.058  grams  per  liter  of  acid  will  be  neutralized.  It  may  be 
said  then  in  general  that  a  liter  of  a  normal  acid  will  neutralize 
40.058  grams  of  caustic  soda. 

If  sodium  carbonate  were  being  titrated  only  half  of  the  molec- 
ular weight  must  be  taken,  as  one  molecule  of  this  base  is  capable 
of  replacing  two  atoms  of  acid  hydrogen,  the  molecular  weight 
in  grams,  106.10,  being  able  to  replace  2  grams  of  acid  hydrogen. 
One  liter  of  normal  acid  is  therefore  equal  to  53.05  grams  of  sodium 
carbonate.  One- thousandth  of  this  weight  or  the  amount  per 
cubic  centimeter  is  more  convenient  for  use  in  calculation,  being 
.040058  gram  of  caustic  soda  and  .05305  gram  of  sodium  carbonate. 
If  the  strength  of  the  acid  must  be  expressed  in  fractions  or  deci- 
mal parts  of  normal,  the  value  per  cubic  centimeter  will  be  found 
by  multiply  ing  the  value  per  cubic  centimeter  of  a  normal  acid  by 
the  fractions  or  decimals.  The  value  in  caustic  soda  per  cubic 
centimeter  of  TV  normal  acid  will  be  TV  of  .040058  or  .0040058. 

When  many  titrations  of  a  given  substance  must  be  made  the- 
calculation  is  simpler  if  the  acid  is  so  made  that  1  c.c.  is  equal  to 
an  amount  of  base,  which  can  be  expressed  by  a  simple  number 
such  as  .050  gram  of  sodium  carbonate.  If  such  an  acid  is  used 
for  titrating  any  other  substance,  the  calculation  is  not  as  simple 
as  when  the  value  of  the  acid  is  expressed  in  terms  of  normal. 


ACIDIMETRY.  249 

345.  Percentage  Given  by  Number  of  Cubic  Centimeters  of  Acid 
Used. — The  calculation  of  the  result  for  a  single  substance  may 
be  made  very  simple  indeed  if  the  acid  is  made  of  such  a  strength 
that  the  percentage  is  given  by  the  number  of  cubic  centimeters 
used  in  titrating  a  given  weight  of  the  substance.  For  instance, 
1  gram  of  sodium  carbonate  requires  for  its  neutralization  .6872 
gram  of  hydrochloric  acid.  This  follows  from  the  fact  that  1 
molecule  of  sodium  carbonate  (106.10)  is  neutralized  by  2  mole- 
cules of  hydrochloric  acid  (72.916).  If  a  standard  solution  of 
hydrochloric  acid  is  made  of  such  a  strength  that  it  contains  6.872 
grams  of  acid  per  liter,  then  100  c.c.  of  this  acid  will  just  neutral- 
ize 1  gram  of  pure  sodium  carbonate.  If  mixtures  of  sodium 
carbonate  and  neutral  salts  are  analyzed,  the  percentage  of  sodium 
carbonate  will  be  given  by  the  number  of  cubic  centimeters  of 
the  acid  necessary  to  neutralize  1  gram  of  the  mixture.  The 
strength  of  the  acid  in  terms  of  normal  may  be  found  from  the 
ratio  of  6.872  to  36.458,  which  gives  .1885  normal.  As  burettes 
holding  50  c.c.  are  commonly  used,  it  will  generally  be  found 
more  convenient  when  mixtures  containing  more  than  50%  of 
sodium  carbonate  are  to  be  analyzed  to  make  the  acid  twice 
as  strong.  Then  the  percentage  will  be  twice  the  number  of  cubic 
centimeters  used.  The  same  object  will  also  be  accomplished  if 
acid  of  the  same  strength  is  used  and  £  gram  of  the  material  is 
weighed  out  for  each  determination. 

The  percentage  may  also  be  given  by  the  number  of  cubic 
centimeters  of  a  normal  acid,  provided  a  definite  weight  of  the 
material  is  taken.  In  the  case  of  sodium  carbonate,  100  c.c.  of  a 
normal  acid  will  be  equal  to  5.305  grams  of  the  pure  material,  since, 
as  has  already  been  shown,  one  liter  of  the  normal  acid  is  equal  to 
53.05  grams  of  sodium  carbonate.  If  5.305  grams  were  weighed, 
the  number  of  cubic  centimeters  of  normal  acid  used  would  be 
equal  to  the  percentage  of  sodium  carbonate.  It  would  be  more 
convenient  to  weigh  out  2.652  grams  and  multiply  the  number  of 
cubic  centimeters  used  by  2.  If  a  fifth-normal  acid  were  used, 
one-fifth  or  one-tenth  of  5.305  grams  would  be  taken. 


250  VOLUMETRIC  METHODS. 


INDICATORS. 

346.  Effect  of  Carbon  Dioxide  on  Indicators. — The  results 
obtained  by  titrating  a  given  base  with  a  given  standard  acid 
will  differ  according  to  the  indicator  used  to  give  the  end-point. 
This  difference  is  due  to  the  varying  degree  of  sensitiveness  of 
the  indicators.  For  instance,  a  given  amount  of  sodium  carbo- 
nate will  require  about  half  as  much  standard  hydrochloric  acid 
for  its  neutralization  if  phenolphthalein  is  used  as  the  indicator  as 
when  methyl  orange  is  employed.  This  arises  from  the  fact  that 
when  hydrochloric  acid  is  added  to  a  solution  of  sodium  carbonate 
sodium  chloride  is  produced  and  carbonic  acid  liberated.  As 
carbonic  acid  is  too  weak  an  acid  to  change  the  color  of  methyl 
orange,  the  addition  of  the  strong  hydrochloric  acid  is  continued 
until  a  slight  excess  is  present,  which  gives  the  acid  color  to  the 
indicator.  Phenolphthalein,  on  the  other  hand,  is  sensitive  to 
carbonic  acid.  Sodium  carbonate  reacts  alkaline  with  phenol- 
phthalein until,  by  the  addition  of  a  strong  acid  like  hydrochloric, 
one-half  of  the  sodium  has  been  converted  into  sodium  chloride, 
and  the  remainder  is  present  as  the  bicarbonate.  Another  drop 
of  hydrochloric  acid  liberates  a  little  carbonic  acid,  which  changes 
the  color  of  the  phenolphthalein.  If  the  solution  is  concentrated 
or  hot,  carbon  dioxide  will  escape,  so  that  if  the  solution  is 
boiled  the  titration  of  sodium  carbonate  will  give  the  same  result 
with  either  indicator. 

As  carbon  dioxide  is  present  in  the  air,  in  ordinary  distilled 
water,  and  in  most  solutions  of  acids  as  well  as  alkalies,  unless 
special  precautions  are  taken  to  exclude  it,  the  analyst  must  be 
thoroughly  familiar  with  the  behavior  towards  this  acid  of  all 
indicators  used.  As  has  already  been  stated,  methyl  orange 
is  almost  entirely  unaffected  by  it,  while  phenolphthalein  is  sen- 
sitive to  one  of  the  two  hydrogen  atoms  of  the  acid.  Carbonic 
acid  may  therefore  be  said  to  be  monobasic  to  phenolphthalein 
in  cold  dilute  solution.  Its  basicity  is  zero  in  boiling  solution 
with  phenolphthalein,  as  well  as  with  methyl  orange  both  hot  and 
cold.  Much  valuable  work  on  indicators  has  been  carried  out 
by  R.  T.  Thomson,*  who  has  divided  them  into  three  classes. 

*  C.  N.,  v.  47,  p.  123;  185,  v.  49,  pp.  32,  119.    J.  S.  C.  I.,  v.  6,  pT  195. 


IN DIC A  TORS.  251 

The  first  class,  of  which  methyl  orange  is  the  best  example  and 
the  one  most  commonly  used,  comprises  lacmoid,  dimethyl  amido- 
benzene,  cochineal,  and  congo  red.  This  class  is  most  sensitive  to 
alkalies  and  can  be  used  with  strong  acids  only,  being  unaffected 
by  weak  acids  such  as  carbonic,  silicic,  etc.  The  second  class, 
of  which  phenolphthalein  is  the  type  and  the  one  most  commonly 
used,  includes  also  turmeric.  These  indicators  are  extremely  sensi- 
tive to  acids,  and  are  therefore  especially  adapted  to  the  titration 
of  weak  acids  among  which  most  organic  acids  must  be  placed. 
Strong  bases  must  be  used  with  these  indicators,  and  carbon 
dioxide  must  be  carefully  excluded.  The  third  class,  of  which 
litmus  is  the  type,  includes  rosolic  acid  and  phenacetolin.  This 
class  of  indicators  is  about  midway  between  the  other  two 
in  sensitiveness,  being  fairly  sensitive  to  both  weak  acids  and 
weak  bases. 

347.  Basicity  of  Acids  with  Various  Indicators. — The  table  on 
page  252  prepared  by  Thomson  gives  the  number  of  molecules 
of  a  univalent  base,  such  as  caustic  soda  or  potash,  neutralized 
by  a  molecule  of  each  acid  in  the  presence  of  the  indicator  at  the 
head  of  the  column.    A  blank  indicates  the  absence  of  a  sharp 
end-point.    The  same  results  are  obtained  with  calcium  or  barium 
hydrate  where  insoluble  compounds  are  not  formed. 

348.  Litmus,  although  one  of  the  earliest  indicators  used,  has 
by  no  means  been  superseded  by  the  many  indicators  recently 
introduced,  especially  by  the  workers  in  organic  chemistry.    It 
is  considerably  more  sensitive  than  methyl  orange,  not  only  being 
more  sensitive  to  weak  acids,  but  giving  an  end-point  more  readily 
distinguished  by  most  workers.     Carbon  dioxide  interferes  seri- 
ously, so  that  to"  obtain  the  best  results  this  acid  must  be  quite 
completely  excluded.    This  may  be  readily  done  by  boiling  the 
solution.    This  operation  does  not  destroy  the  indicator,  but  as 
it  is  considerably  more  sensitive  cold  than  hot,  the  solution  should 
be   cooled  before   the   titration  is   completed.    For  this  reason 
methyl  orange  is  more  commonly  used  when  considerable  carbon 
dioxide  is  present. 

As  obtained  in  commerce  litmus  consists  of  small  cubes  com- 
posed of  powdered  chalk  saturated  with  the  coloring-matter, 
which  is  a  vegetable  extract.  The  coloring-matter  is  not  a  single 


252 


VOLUMETRIC  METHODS. 


Acids. 

Methyl 
Orange. 

Phenolphthalein. 

Litmus. 

Name. 

Formula. 

Cold. 

Cold. 

Boiling. 

Cold. 

Boiling. 

Sulphuric.      .    . 

H2S04 
HC1 
HN03 
H2S203 
H2CO3 
H2S03  ' 
H2S 
H3P04 
H3AsO4 
H3AsO, 
HN02 

H4Si04 
H3B03 

H2Cr(V 
H2C204 
HC2H302 
HC4H702 
H2C4H404 
HC,H8Of 
H2C4H406 
H3C6H507 

2 
1 
1* 

2 
0 
1 
0 

1 
1 
0 
indicator 
destroyed 
0 
0 
1 

2 
1 
1 
2 
1  dilute 
2 
1  dilute 
2 
2 

1 

t 
2 

2 

1 
1 
2 
1 
2 
3 

2 
1 
1 
2 
0 

0 

2 
2 

2 
1 
1 
2 

0 
1 

0 

2 
1  nearly 
1  nearly 
2  nearly 
1 
2 

2 
1 
1 
2 

0 
0 

'o 

0 

.  i 
2 

Hydrochloric  
Nitric  

Thiosulphuric  .... 
Carbonic  

Sulphurous  
Hydrosulphuric.  . 
Phosphoric.  .  .  . 

Arsenic  

Arsenious  .... 

Nitrous  

Silicic  

Boric  

Chromic  

Oxalic  

Acetic  

Butyric  

Succinic  

Lactic  

Tartaric  

Citric  

*  Concentrated  nitric  acid  sometimes  contains  oxides  of  nitrogen,  producing 
on  dilution  nitrous  acid  which  destroys  methyl  orange. 

f  1  in  a  solution  containing  30%  of  glycerine,  or  a  few  grams  of  mannitol. 

substance.  Various  methods  have  been  proposed  for  obtaining 
the  pure  coloring-matter  for  use  as  a  sensitive  indicator.  One  of 
the  simplest  and  best  methods  consists  in  extracting  the  cubes 
two  or  three  times  with  hot  85%  alcohol.  By  this  treatment  a 
violet  coloring-matter  is  removed.  The  residue  is  then  extracted 
several  times  with  water,  the  first  portion  being  rejected,  as  it 
contains  the  bulk  of  the  alkali  carbonates.  The  remainder  of 
the  water-extract  is  acidified  with  dilute  sulphuric  acid  and  boiled 
for  some  time  to  completely  expel  carbonic  acid.  The  excess  of 
sulphuric  acid  is  then  neutralized  with  barium  hydrate.  The 
neutral  color  is  violet,  which  a  drop  of  alkali  turns  to  a  decided 
blue,  and  a  drop  of  acid  to  a  decided  red.  The  solution  should  be 


INDICATORS. 

preserved  from  mould  by  the  addition  of  a  few  drops  of  chloroform. 
If  kept  in  a  closely  stoppered  bottle,  the  color  disappears,  but 
is  restored  on  exposure  to  the  air.  The  solution  is  best  pre- 
served in  a  wide-mouthed  bottle  partially  filled,  and  protected 
from  dust  by  a  cover  of  filter-paper,  or  a  loose  cork  stopper,  through 
which  passes  a  glass  tube  with  which  to  withdraw  the  solution  as 
needed. 

349.  Cochineal   solution   is  very  similar   to   litmus   solution. 
When  acidified  the  color  is  red,  which  is  turned  to  violet  by  alkalies. 
It  is  not  as  sensitive  to  carbonic  and  weak  organic  acids  as  litmus. 
This  gives  it  an  advantage  over  the  latter,  as  carbon  dioxide  need 
not  be  so  rigidly  excluded.     It  cannot  be  used,  however,  in  the 
presence  of  even  traces  of  compounds  of  iron  or  aluminium  0** 
acetates.    It  is  used  largely  in  the  titration  of  ammonia  in  the 
Kjeldahl  method  for  determining  nitrogen.     The  solution  is  made 
by  digesting  1  part  of  the  crushed  cochineal  with  10  parts  of  25% 
alcohol. 

350.  Methyl  Orange  is  the  sodium  or  ammonium  salt  of  para- 
dimethyl-aniline-azo-benzene-sulphonic  acid.     It    is    a    synthetic 
product,  and  can  generally  be  purchased  in  a  sufficiently  pure 
condition  for  use.    It  is  dissolved  in  water,  1  gram  per  liter  giving 
a  solution  of  which  1  drop  is  sufficient  for  most  titrations.    A 
large  amount  of  this  indicator  decreases  the  sharpness  of  the  end- 
point.    An  amount  just  sufficient  to  give  a  faint  yellow  color  to 
the  alkaline  solution  should  be  used.    The  solution  should  be  kept 
in  a  bottle  provided  with  a  stopper,  through  which  a  drawn-out 
glass  tube  passes  so  that  the  indicator  may  be  introduced  by 
drops. 

Methyl  orange  is  very  largely  used  because  it  is  comparatively 
unaffected  by  carbonic  acid  as  well  as  by  sulphuretted  hydrogen, 
hydrocyanic,  silicic,  boric,  arsenious,  oleic,  stearic,  palmitic,  and 
carbolic  acids,  etc.  The  base  in  the  salts  of  these  acids  may  be 
titrated  without  removal  of  the  acid.  It  has  found  a  very  ex- 
tended use  in  the  standardization  of  acids  by  means  of  pure  sodium 
carbonate.  In  the  presence  of  carbonic  acid  the  change  from 
the  yellow  alkaline  to  the  red  acid  color  is  somewhat  gradual.  It 
is  difficult  to  remember  the  exact  shade  corresponding  to  the 
neutral  point,  especially  as  a  slight  reddish  tint  is  imparted  by 


254  VOLUMETRIC  METHODS. 

the  carbon  dioxide.  It  is  therefore  advisable  to  prepare  a  solu- 
tion for  comparison  which  is  given  this  tint  by  using  distilled 
water  saturated  with  carbon  dioxide  which  has  been  freed  from 
mineral  acid  by  passing  through  a  solution  of  sodium  bicarbonate. 
The  beaker  containing  this  solution  should  be  of  the  same  size 
and  contain  the  same  volume  of  water  as  the  beaker  in  which 
the  titration  is  made.  The  same  amount  of  indicator  is  also 
added.  At  the  end-point  of  the  titration  a  color  a  little  deeper 
than  that  of  the  carbon  dioxide  solution  must  be  obtained.  The 
indicator  is  most  sensitive  in  a  cold  solution.  The  presence  of 
alcohol  greatly  decreases  its  sensitiveness.  It  cannot  be  used  in 
solutions  containing  nitrous  acid,  as  this  acid  destroys  the  indi- 
cator. If  only  small  amounts  are  present,  the  titration  may  be 
completed  before  the  indicator  is  destroyed.  If  large  amounts 
are  present,  excess  of  standard  alkali  must  be  added,  and  the 
solution  titrated  back  with  standard  acid. 

351.  Phenolphthalein  is  a  synthetic  organic  product  which 
may  readily  be  purchased  in  a  pure  condition.  It  is  dissolved  in 
90  to  95%  alcohol,  1  or  2  grams  per  liter.  It  is  advisable  to  first 
distil  the  alcohol  after  adding  a  little  caustic  soda  or  potash,  as 
the  commercial  product  is  seldom  free  from  organic  acids.  One 
or  two  drops  of  this  solution  are  used.  In  neutral  or  acid 
solutions  it  is  colorless,  but  the  faintest  excess  of  alkali  gives  a 
sudden  change  to  purple-red.  Carbon  dioxide  must  be  entirely 
absent,  as  in  its  presence  the  indicator  reacts  acid  until  it  is  con- 
verted ^o  the  bicarbonate  by  the  alkali.  Small  amounts  of 
carbonataBm  presence  of  alkali  hydrates  may  be  titrated  accu- 
rately by  means  of  phenolphthalein.  Acid  is  added  until  the 
pink  color  has  disappeared.  All  of  the  alkali  hydrate  will  then 
be  neutralized,  and  the  alkali  carbonate  converted  into  bicarbo- 
nate. On  adding  methyl  orange  and  standard  acid  until  the 
red  acid  color  of  this  indicator  is  developed  the  bicarbonate  will 
be  decomposed.  From  the  amount  of  acid  added  for  this  purpose 
the  amount  of  carbonate  originally  present  may  be  calculated. 
The  amount  of  carbonate  must  be  small,  as  even  the  bicarbonate 
gives  a  slight  color  to  the  phenolphthalein.*  Ammonia  must 

*  Kiister,  Zeit.  f.  anorg.  Ch.,  13,  127. 


INDICATORS.  255 

also  be  absent,  as  in  its  presence  the  end-point  is  not  sharp.  Phe- 
nolphthalein  is  especially  valuable  for  the  titration  of  weak  acids, 
especially  the  organic  acids.  Many  of  these  acids  being  insoluble 
in  water  are  dissolved  in  alcohol  and  then  titrated.  This  does 
not  seem  to  be  quite  as  accurate  as  titration  in  water  solution. 
A  strong  base  must  always  be  used  with  phenolphthalein.  As 
water  solutions  of  caustic  soda  and  potash  always  contain  carbon 
dioxide,  an  alcoholic  solution  of  the  latter  is  sometimes  used,  and 
more  frequently  a  solution  of  barium  hydrate. 

352.  Best  Acid  for  General  Use. — As  has  already  been  said, 
the  acidity  of  a  solution  depends  only  on  the  amount  of  replace- 
able hydrogen  present  in  a  given  volume,  irrespective  of  the 
nature  of  the  acid  radicle.  As  has  been  shown,  the  acid  value 
of  the  different  hydrogen  atoms  differs  when  two  or  more  are 
present  in  the  same  molecule,  so  that  the  acidity  of  such  a  solu- 
tion may  differ  widely  with  different  indicators.  For  general  use, 
therefore,  a  strong  acid  should  be  chosen  so  that  the  strength 
of  the  solution  may  be  the  same  with  all  indicators.  A  stable 
non-volatile  acid  is  also  desirable  so  that  a  solution  once  stand- 
ardized may  be  permanent.  These  considerations  limit  us  to 
the  three  commonly  used  strong  acids,  hydrochloric,  sulphuric, 
and  nitric.  Of  these  three  only  the  first  two  have  found  any 
very  general  use.  The  standardization  of  nitric  acid  offers  some 
difficulties  'because  it  forms  no  insoluble  weighable  salt.  It  is 
also  an  oxidizing  agent,  so  that  titrations  may  be  disturbed  by 
secondary  reactions.  On  the  other  hand,  both  sulphuric  and 
hydrochloric  acids  are  very  stable  and  may  also  be  precipitated 
and  weighed  with  great  accuracy.  Choice  must  therefore  be  made 
between  these  two  acids  for  a  carefully  standardized  perma- 
nent acid  to  be  used  in  the  standardization  of  alkalies  and  other 
acids.  For  general  use  hydrochloric  acid  offers  several  decided 
advantages  over  sulphuric  acid.  It  is  appreciably  stronger,  so 
that  the  end-point  with  most  indicators  is  considerably  sharper 
than  with  sulphuric  acid.  This  is  partly  due  to  the  fact  that  there 
are  two  atoms  of  hydrogen  in  the  sulphuric  acid  molecule,  so 
that  when  the  solution  is  partly  neutralized  an  acid  sulphate  is 
formed  whose  hydrogen  atom  is  not  so  strongly  acid  as  the  hydro- 
gen atom  first  displaced.  The  use  of  sulphuric  acid  is  also  lin> 


256  VOLUMETRIC  METHODS. 

ited  by  the  fact  that  it  forms  insoluble  sulphates  with  the  alkaline- 
earth  metals.  The  gravimetric  standardization  of  hydrochloric 
acid  by  precipitation  as  silver  chloride  is  subject  to  fewer  sources 
of  error  than  the  standardization  of  sulphuric  acid  by  precipita- 
tion as  barium  sulphate. 

353.  Most  Desirable  Strength  of  Standard  Acids. — Various 
strengths  of  acid  are  in  use  among  chemists,  many  using  tenth-  or 
fifth-normal  acids,  while  among  commercial  chemists  the  use  of 
half-  or  full-normal  solutions  is  quite  common.  The  use  of  such 
concentrated  solutions  is  defended  by  the  argument  that  a  sharp 
end-point  may  always  be  obtained  with  one  drop  of  the  acid. 
This  is  advantageous  in  commercial  laboratories,  where  titrations 
must  be  made  rapidly,  and  as  large  amounts  of  material  must  be 
weighed  out  to  use  a  proper  volume  of  acid,  the  weighings  may 
be  more  quickly  made.  This  is  evident  from  the  consideration 
that  an  error  of  5  mg.  on  5  grams  is  the  same  percentage  as  an 
error  of  1  mg.  on  1  gram.  As  methyl  orange  must  frequently 
be  used  in  the  presence  of  carbon  dioxide,  the  strong  acid  gives 
a  sharp  end-point  even  with  one  drop  of  the  standard  solution. 

The  use  of  the  more  dilute  solutions  is  defended  by  the  argu- 
ment that  a  larger  volume  of  the  solution  must  be  used  for  titrat- 
ing a  given  weight  of  material,  and  therefore  the  reading  of  the 
Volume  on  the  burette  is  more  accurate.  The  indefinite  dilution 
of  the  solution  does  not  lead  to  greater  and  greater  accuracy, 
since  a  point  is  soon  reached  where  a  greater  and  greater  volume 
of  the  solution  must  be  taken  to  give  a  distinct  end-point  with  the 
indicator.  This  limit  is  usually  reached  with  fifth-  or  tenth-normal 
solutions.  With  moderate  care  a  distinct  end-point  may  be 
obtained  even  with  methyl  orange  in  the  presence  of  carbon 
dioxide  by  means  of  one  drop  of  fifth-normal  acid.  Another 
decided  advantage  of  the  fifth-normal  acid  is  found  in  the  mod- 
erately small  coefficient  of  expansion  with  change  in  temperature, 
which  with  stronger  acids  becomes  quite  considerable.  In  gen- 
eral, therefore,  more  accurate  work  can  be  done  with  the  more 
dilute  acid,  while  time  is  saved  at  the  expense  of  accuracy  by  the 
use  of  strong  acids.  The  final  decision  must  therefore  be  made 
on  the  requirements  of  the  work  at  hand.  For  the  exercises 
given  in  this  book  fifth-normal  solutions  will  be  uniformly  used. 


PROBLEM.  257 


PROBLEM. 

Problem  4.  Calculate  the  number  of  grams  of  impure  potassium  car- 
bonate which  must  be  weighed  out,  so  that  when  the  solution  is  titrated 
with  N/5  acid,  the  number  of  cubic  centimeters  used  shall  be  equal  to 
the  per  cent  of  potassium  carbonate.  Calculate  the  strength  of  acid  in 
terms  of  normal  to  be  made  up,  so  that  if  1  gram  of  impure  potassium  car- 
bonate is  weighed  out,  the  per  cent  of  potassium  carbonate  will  be  equal 
to  the  number  of  cubic  centimeters  used  multiplied  by  two. 


CHAPTER  XXI. 

STANDARD    ACIDS. 
STANDARDIZATION. 

ONE  of  the  most  important  points  in  volumetric  work  is  the 
accurate  standardization  of  the  solutions.  No  subsequent  care 
can  overcome  an  error  in  this  part  of  the  work.  Although  many 
excellent  methods  of  standardizing  acids  have  been  proposed, 
considerable  skill  and  experience  are  necessary  in  order  to  secure 
absolutely  reliable  results. 

354-  Sodium  Carbonate. — One  of  the  oldest  as  well  as  most 
reliable  methods  of  standardization  is  by  means  of  sodium  car- 
bonate. It  may  be  purchased  in  a  high  state  of  purity,  so  that 
it  frequently  needs  only  to  be  heated  to  completely  dehydrate 
it  and  decompose  any  bicarbonate  which  may  be  present.  For 
this  purpose  it  need  not  be  heated  over  300°.*  It  is  better  to  heat 
to  this  temperature  than,  as  has  long  been  the  custom,  to  heat 
to  dull  redness  and  below  the  fusing-point  of  the  carbonate,  as 
small  amounts  of  the  carbonate  are  apt  to  be  converted  into  the 
hydroxide  by  this  treatment. 

The  carbonate  must  first  be  tested  for  impurities.  It  should 
dissolve  completely  in  water.  A  portion  of  2  or  3  grams  is  tested 
for  chlorides  by  dissolving  in  water,  acidifying  with  dilute  nitric 
acid,  and  adding  silver  nitrate.  Only  a  slight  opalescence  is  per- 
missible. Another  2-  or  3-gram  portion  is  dissolved  in  water, 
acidified  with  hydrochloric  acid,  and  tested  for  sulphuric  acid  by 
means  of  barium  chloride. 

A  very  pure  sodium  carbonate  may  be  obtained  by  heating 
PURE  SODIUM  BICARBONATE  to  270°-300°  for  one-half  to  one  hour. 
The  bicarbonate  must  be  tested  for  impurities  by  the  methods 

*  Lunge,  Zeit.  f.  angew.  Chem.,  1897,  522. 

258 


STANDARD  ACIDS.  259 

given  for  the  carbonate.  Small  amounts  of  chlorides  and  sulphates 
may  be  washed  out  by  repeated  treatment  with  small  amounts  of 
cold  distilled  water.  The  bicarbonate  may  also  be  recrystallized. 
Distilled  water  heated  to  80°  in  a  Jena  beaker  is  saturated  with 
the  bicarbonate  by  adding  small  amounts  and  stirring.  The 
saturated  solution  is  filtered  through  a  folded  filter  in  a  hot-water 
funnel,  and  the  solution  cooled  and  vigorously  stirred  as  the  salt 
crystallizes  out.  It  is  filtered  off  in  a  funnel  closed  with  a 
platinum  cone  or  porcelain  plate,  washed  with  small  portions  of 
cold  water  and  dried  in  a  porcelain  dish.  Impure  sodium  carbon- 
ate may  be  purified  by  making  a  cold  saturated  solution,  filtering 
if  necessary,  and  saturating  with  carbon  dioxide  which  has  been 
washed  by  passing  through  a  solution  of  sodium  bicarbonate. 
The  precipitated  bicarbonate  is  filtered  off  and  washed  as  already 
directed.  It  is  advisable  to  preserve  the  bicarbonate  as  such 
and  convert  suitable  portions  into  carbonate  immediately  before 
weighing  it  out  for  use.  Better  results  will  also  be  obtained  if 
separate  small  portions  are  weighed  out  for  each  titration  instead 
of  making  a  standard  solution,  portions  of  which  are  measured 
out.  The  errors  of  the  volumetric  apparatus  are  thus  avoided. 

355.  Crystallized  Oxalic  Acid. — Extended  use  has  also  been 
made  of  crystallized  oxalic  acid.  A  solution  of  an  alkali,  such  as 
caustic  soda  or  potash,  is  first  standardized  by  means  of  the  pure 
oxalic  acid.  The  acid  solution  to  be  standardized  is  then  care- 
fully compared  with  the  standardized  alkaline  solution.  This 
method  has  the  disadvantage  that  the  errors  of  two  titrations  are 
introduced  into  the  standardization.  Carbon  dioxide  must  also  be 
absent  as  litmus  or  phenolphthalein  must  be  used  as  the  indicator. 

Two  difficulties  are  met  with  in  preparing  pure  oxalic  acid — 
the  removal  of  traces  of  the  alkalies  and  the  drying  of  the  crystal- 
lized product  so  as  not  to  lose  water  of  crystallization.  The 
first  object  is  attained  by  dissolving  the  commercial  article  in  a 
mixture  of  equal  parts  of  alcohol  and  ether,  in  which  the  alkali 
oxalates  as  well  as  other  impurities  are  insoluble.  After  filtering 
the  solution,  the  bulk  of  the  alcohol  and  ether  are  distilled  off, 
water  is  added,  and  the  distillation  continued.  Finally  the 
water  solution  is  transferred  to  a  porcelain  dish  and  kept  near 
the  boiling-point  until  the  last  traces  of  the  alcohol  and  ether  88 


260  VOLUMETRIC  METHODS. 

well  as  any  ethereal  salts  which  may  have  been  formed  are  vola- 
tilized. The  steam  will  then  have  no  odor.  If  the  oxalic  acid 
begins  to  crystallize  out  of  the  hot  solution  during  this  process, 
small  quantities  of  distilled  water  are  added  from  time  to  time. 
When  the  volatile  matter  has  been  expelled,  the  solution  is  allowed 
to  cool  with  constant  stirring.  The  crystals  are  filtered  off  in  the 
usual  manner  and  washed  with  small  quantities  of  cold  distilled 
water.  They  are  then  spread  out  on  unglazed  porcelain  and 
exposed  to  the  air  until  dry.  The  material  is  preserved  in  glass- 
stoppered  bottles.  The  crystals  must  not  be  exposed  to  the  air 
after  they  are  dry,  as  shown  by  their  non-adherence  to  a 
clean  dry  glass  surface.  The  crystals  then  have  the  formula 
H2C204.2H20. 

356.  Anhydrous   Oxalic  Acid. — To  obviate  the  uncertainty  of 
the  degree  of  hydration,  anhydrous  oxalic  acid  has  been  somewhat 
used.     Drying  at  60°  to  80°  for  several  hours  is  said  to  secure  an 
anhydrous  product.     At  100°  considerable  oxalic  acid  is  volatil- 
ized.    As  the  anhydrous  material  is  quite  hydroscopic,  it  must  be 
cooled  in  a  desiccator  and  weighed  as  soon  as  cool.    The  pure 
recrystallized  product  prepared  as  already  directed  must  be  used 
for  this  purpose.     Solutions  of  oxalic  acid  are  not  entirely  stable, 
especially  if  dilute.    For  standardizing  purposes  it  is  therefore 
advisable  to  weigh  out   convenient   amounts,  and,  after  drying 
and  weighing,  to  dissolve  in  water  and  titrate  immediately. 

357.  Potassium  Tetroxalate. — In  place  of  the  crystallized  dehy- 
drated oxalic  acid,  the  use  of  crystallized  potassium  tetroxalate 
KHC204.H2C204.2H20  has  been  recommended.      This   salt  may 
easily  be  prepared  by  making  a  saturated  solution  of  oxalic  acid 
and  filtering  if  necessary.     One-fourth  of  this  solution  is  neutral- 
ized with  pure  potassium  carbonate  and  added  with  stirring  to 
the  remainder  of  the  oxalic  acid  solution.     The  crystals  which 
separate  out  are  recrystallized  from  hot  water  several  times  and 
dried  as  directed  for  crystallized  oxalic  acid.    The  salt  is  pre- 

*  served  in  well-stoppered  bottles. 

358.  Potassium  Bichromate. — The  suggestion  of  Richter  *  to 
use  pure  potassium  dichromate  which  may  be  titrated  against  a 

*2eit.f.anal.Ch..21.205 


STANDARD  ACIDS.  261 

solution  of  caustic  potash,  using  phenolphthalein  as  the  indicator, 
would  seem  to  be  of  value.  Potassium  dichromate  may  readily 
be  prepared  pure  and  anhydrous. 

359.  Gravimetric     Standardization.  —  The    precipitation    and 
gravimetric  estimation  of  the  acid  radicle  offers  a  most  excellent 
method  of  standardizing  acids,  especially  sulphuric  and  hydro- 
chloric.    The  details  of  the  precipitation  and  weighing  of  sul- 
phuric acid  as  barium  sulphate,  and  of  hydrochloric  acid  as  silver 
chloride,  have  already  been  given.     A  very  simple  gravimetric 
method  consists  in  evaporating  to  dryness  in  a  weighed  platinum 
dish  a  measured  volume   of  the  acid   solution  which  has  been 
neutralized  with  ammonia.     Both  ammonium  chloride  and  ammo- 
nium sulphate  may  be  dried  at  100°  without  appreciable  volatili- 
zation.    Very  pure  ammonia  may  be  obtained  for  this  purpose 
by  distilling  the  concentrated  solution  and  absorbing  the  gas  in 
distilled  water.      Test-tubes  may  be  used  for  this  purpose,  and 
the  distilled  ammonia  should  be  used  immediately.    The  weighed 
ammonium  salt  may  be  still  further  tested  for  impurities  by  vola- 
tilizing the  weighed  salt  by  gently  heating  the  dish  with  the 
Bunsen  burner  and  weighing  the  residue. 

360.  Standardization   by  Specific   Gravity. — Standard  solutions 
of  acids,  especially  sulphuric  acid,  may  be  made  with  almost  as 
high  a  degree  of  accuracy  as  by  the  methods  already  given,  by 
carefully  taking  the  specific  gravity  of  the  concentrated  acid  and 
diluting  a  measured  or  weighed  amount  to  a  definite  volume. 
The  C.  P.  concentrated  sulphuric  acid  should  first  be  diluted  to 
about  30%.    Acid  of  this  concentration  is  not  hydroscopic,  and 
the  change  in  specific  gravity  with  change  in  concentration  is 
greater   than  with  the    stronger   acid.      The   specific   gravity  is 
carefully  taken  at  the  standard  temperature,  15°  C.,  by  means 
of  a  Westphal  balance  or  carefully  calibrated  pycnometer.    The 
greatest  care  should  be  taken  to  bring  the  temperature  to  exactly 
15°  C.  with  a  tested  thermometer.     If  the  acid  is  to  be  measured 
out  it  must  be  kept  at  this  temperature  as  carefully  as  when  the 
specific  gravity  is  being  taken.    This  difficulty  may  be  obviated 
by  weighing  the  acid.     This  method  has  the  additional  advantage 
that  the  weight  can  be  taken  far  more  accurately  than  the  volume, 
especially  if  it  is  corrected  for  air  displacement.    The  weighing 


262  VOLUMETRIC  METHODS. 

need  not  be  made  closer  than  a  few  milligrams,  as  an  error  of 
10  mg.  is  negligible.  The  calculation  is  also  very  simple.  The 
number  of  grams  of  acid  required  per  liter  for  the  solution  to  be 
made  is  divided  by  th^  percentage  given  in  the  table  for  the  spe- 
cific gravity  found.  The  quotient  multiplied  by  100  is  the  num- 
ber of  grams  of  the  concentrated  acid  to  be  weighed  out  and 
diluted  to  a  liter. 

As  the  measuring  out  of  concentrated  hydrochloric  acid  is 
accompanied  by  much  more  uncertainty,  it  is  advisable  to  measure 
out  a  little  more  of  this  acid  than  the  calculated  amount,  so  that 
after  standardization  it  may  be  diluted  to  the  exact  strength. 
A  solution  of  hydrochloric  acid  of  approximately  constant  strength 
may  be  obtained  by  boiling  either  the  dilute  or  concentrated 
acid.  Equilibrium  is  soon  reached  so  that,  when  boiling  at  about 
760-mm.  pressure,  both  acid  and  water  are  volatilized  in  equal 
amount  from  the  residue,  which  contains  20.2%  of  hydrochloric 
acid  and  has  a  specific  gravity  of  1.10.*  180  grams  of  this  acid 
diluted  to  1  liter  give  an  acid  slightly  stronger  than  normal. 

361.  Dilution  of  Acid  to  Exact  Strength. — If  the  sulphuric 
acid  is  not  made  by  diluting  a  strong  acid  whose  specific  gravity 
has  been  very  carefully  taken,  it  should  also  be  made  so  as  to  be 
somewhat  stronger  than  the  acid  required.  At  least  1£  liters 
should  be  made,  placed  in  a  bottle  of  suitable  size  and  well 
mixed  by  shaking.  After  carefully  standardizing  the  acid  by 
two  of  the  methods  already  given,  it  should  be  diluted  to  the 
exact  strength.  Perhaps  the  simplest  method  of  calculating  the 
results  of  each  standardization  is  to  find  the  number  of  grams  of 
acid  present  per  liter.  The  average  of  the  standardizations, 
which  agree  within  at  least  0.2%,  is  taken  and  divided  into  the 
number  of  grams  per  liter  required.  For  normal  hydrochloric 
this  will  be  36.46  grams  and  for  normal  sulphuric  49.04  grams. 
The  quotient  multiplied  by  1000  gives  the  number  of  cubic  centi- 
meters to  be  diluted  to  1  liter.  This  number  subtracted  from  1000 
gives  the  number  of  cubic  centimeters  of  water  to  be  measured 
out  into  the  liter-flask,  which  is  then  filled  to  the  mark  with  the 
acid  to  be  diluted.  The  strength  of  the  diluted  acid  is  verified 

*  Roscoe  and  Dittmar,  Jour.  Chem.  Soc.,  XII,  128,  1860. 


STANDARD  ACIDS.  263 

by  one  of  the  methods  already  used.  This  is  especially  necessary 
if  the  amount  of  water  added  per  liter  is  large.  Frequently  the 
most  satisfactory  method  of  standardization  is  by  the  use  of 
sodium  carbonate,  this  being  a  volumetric  method,  and  is  carried 
out  under  the  same  conditions  that  will  be  met  in  regular  work. 

••'"  :      •'  •'     •'•  •  -Tu     -  :  ' 

EXERCISE  48. 
Preparation  of  Standard  N/s  Hydrochloric  Acid. 

To  75  c.c.  of  concentrated  hydrochloric  acid  an  equal  volume  of  water 
is  added  and  the  solution  boiled  in  a  beaker  or  a  porcelain  dish  for  a  few 
minutes.  About  36  grams  of  this  acid  are  weighed  out  and  transferred 
to  the  liter  flask,  which  should  be  clean,  but  need  not  be  dry.  The  beaker 
or  other  vessel  is  rinsed  out  into  the  flask.  Distilled  water  is  added  until 
the  flask  is  filled  to  the  mark.  The  solution  is  poured  into  a  large  bottle,* 
which,  if  not  dry,  should  first  be  rinsed  with  a  little  of  the  acid.  The  flask 
is  rinsed  with  a  little  distilled  water,  and  a  second  liter  of  acid  made  and 
poured  into  the  large  bottle.  The  bottle  is  filled  from  a  third  liter  of  the 
diluted  acid.  After  thoroughly  mixing  the  solution  by  shaking,  the  acid 
is  standardized  by  two  of  the  following  methods,  duplicate  determinations 
being  made  by  each  method. 

STANDARDIZATION. 

362.  First  Method.  By  Evaporation  with  Ammonia. — Measure  out  with 
a  burette  or  a  calibrated  pipette  50  c.c.  of  the  acid  into  a  weighed  plati- 
num dish.  Fit  a  small  flask  or  a  test-tube  with  a  glass  delivery-tube  bent 
twice  at  right  angles.  Place  10  to  15  c.c.  of  strong  ammonia  in  the  flask 
and  distil  off  the  ammonia,  using  a  small  flask  or  test-tube  containing  15 
to  20  c.c.  distilled  water  as  a  receiver.  The  end  of  the  delivery-tube  should 
dip  into  the  distilled  water.  Neutralize  the  acid  with  this  ammonia,  evapo- 
rate to  dryness  on  the  water-bath,  and  weigh  the  ammonium  chloride. 
Bring  to  constant  weight  by  heating  on  the  water-bath  for  fifteen  to  twenty 
minutes  and  weighing.  When  the  weight  is  constant  volatilize  the  ammo- 
nium chloride  by  gently  heating  the  dish  with  the  Bunsen  burner,  and 
weigh  again.  Deduct  this  weight  from  the  weight  of  the  dish  and  the 
residue  dried  on  the  water-bath  to  obtain  the  weight  of  the  ammonium 
chloride. 

Calculate  the  weight  of  hydrochoric  acid  present  in  50  c.c.  by  the 
proportion, 

Mol.  wt.  NH4C1  :  Mol.  wt.  HC1  :  :  weight  NH4C1  :  weight  HC1. 

*  The  2Hiter  glass-stoppered  bottles  in  which  the  concentrated  C.  P.  acids 
are  purchased  are  very  convenient  for  this  purpose. 


264  VOLUMETRIC  METHODS. 

363.  Second  Method.  By  Titration  with  Sodium  Carbonate. — Place  about 
4J  grams  of  pure  sodium  bicarbonate  in  a  weighed  platinum  crucible. 
Place  it  in  a  sand-bath  so  that  the  crucible  is  nearly  immersed  in  the  sand. 
Insert  a  thermometer  in  the  bicarbonate  and  heat  the  sand-bath  until  the 
thermometer  registers  270°  to  300°  for  one-half  to  one  hour.  If  an  air-bath 
is  at  hand  which  can  be  raised  to  this  temperature,  the  bicarbonate  may 
be  heated  in  it  for  the  same  length  of  time.  It  may  then  be  placed  on  a 
weighed  watch-crystal.  The  carbonate  is  cooled  in  the  desiccator  and 
weighed.  The  heating  is  repeated  for  half-hour  periods  until  constant 
weight  is  obtained.  The  weight  of  the  carbonate  is  then  brought  to  exactly 
2.650  grams  by  taking  out  small  portions  with  a  spatula.  Transfer  the 
carbonate  to  a  beaker,  rinse  the  crucible  with  water,  dissolve  the  material 
in  water,  and  transfer  to  a  250-c.c.  flask.  After  diluting  to  the  mark  and 
shaking  thoroughly,  fill  a  burette  with  the  solution  and  titrate  against  the 
hydrochloric  acid  solution,  using  methyl  orange  as  the  indicator.  The  titra- 
tion  should  be  made  the  same  day  the  carbonate  is  dissolved,  as  the  solution  is 
not  entirely  stable  because  of  action  on  the  glass. 

Measure  out  25  c.c.  of  the  alkaline  solution  into  a  100-c.c.  beaker,  and 
add  one  drop  of  the  indicator.  50  c.c.  of  distilled  water  are  placed  in  a  similar 
beaker,  one  drop  of  methyl  orange  added,  and  the  solution  saturated  with 
carbon  dioxide  which  has  been  passed  through  a  solution  of  sodium  bicar- 
bonate. To  the  beaker  containing  the  alkali  hydrochloric  acid  is  added 
until,  when  compared  with  the  neutral  solution,  a  pink  tinge  is  noticeable. 
A  drop  of  alkali  is  then  added,  and  if  the  neutral  tint  is  not  restored,  the 
solution  is  made  alkaline  and  again  neutralized  by  cautious  additions  of  the 
acid.  This  process  is  repeated  until  no  difficulty  is  experienced  in  so  closely 
judging  the  acid  tint  that  one  drop  of  the  alkali  will  restore  the  neutral 
tint.  When  a  satisfactory  end-point  has  been  obtained  both  burettes  are 
carefully  read.  The  titration  is  repeated  until  duplicates  agreeing  within 
less  than  0.1  c.c.  are  obtained. 

When  skill  has  been  attained  in  making  the  titration,  more  accurate 
results  can  be  obtained  by  weighing  out  portions  of  the  carbonate  just  suffi- 
cient for  one  titration.  A  considerable  amount  of  the  bicarbonate  is  con- 
verted into  carbonate  and  brought  to  constant  weight.  Portions  weighing 
from  0.4  gram  to  not  over  .5265  gram  are  weighed  out  and  transferred  to 
beakers.  After  dissolving  in  about  50  c.c.  of  distilled  water  and  adding  a 
drop  of  methyl  orange  the  solutions  are  titrated  with  the  hydrochloric  acid. 

Calculation. — By  dividing  the  weight  of  sodium  carbonate  by  the  num- 
ber of  cubic  centimeters  of  acid  used  in  each  case  the  weight  of  sodium 
carbonate  per  cubic  centimeter  will  be  obtained,  which  for  an  exactly  N/5 
normal  solution  is  .01061  gram.  The  amount  found  per  cubic  centimeter 
should  be  divided  into  .01061,  and  the  quotient  multiplied  by  1000  to 
find  the  number  of  cubic  centimeters  to  be  diluted  to  one  liter  to  make 
the  acid  exactly  fifth-normal. 


STANDARD  ACIDS.  265 

364.  Third  Method.  By  Determination  of  Chlorine  as  Silver  Chloride.  — 
Prepare  two  Gooch  crucibles  and  dry  on  the  hot  plate.  Measure  out  two 
25-c.c.  portions  of  the  acid  into  Erlenmeyer  flasks  and  dilute  to  200  c.c. 
Add  silver  nitrate  solution  with  constant  agitation  until  all  the  chlorine 
is  precipitated.  Heat  nearly  to  boiling,  and  shake  vigorously  until  the 
supernatant  liquid  is  clear.  Decant  the  clear  liquid  through  the  Gooch 
crucibles,  wash  two  or  three  times  by  decantation  with  hot  water,  transfer 
the  precipitates  to  the  crucibles,  and  wash  with  hot  water  until  free  from 
silver.  Dry  on  the  hot  plate  and  weigh.  If  the  duplicates  differ  from 
each  other  by  more  than  J  mg.,  repeat  the  determination,  using  the  same 
crucibles  without  removing  the  precipitates. 

Calculation  and  Dilution  of  Acid.  —  From  the  average  weight  of  silver 
chloride  obtained  from  25  c.c.  ol  arid  the  strength  of  the  acid  is  obtained 
by  the  following  proportion  : 

Mol.  wt.  AgCl  :  mol.  wt.  HCl  :    weight  AgCl  :  X, 

where  X  is  equal  to  the  number  ol  grams  of  hydrochloric  acid  in  25  c.c. 
This    number  divided    into    7.292    and    the    quotient    multiplied   by   25 


l~7  292 


(1000)     gives  the  number  of  cubic  centimeters  to  be  diluted  to  1000 


[_  40A 

to  give  a  fifth-normal  solution.  This  number  subtracted  from  1000  gives 
the  number  of  cubic  centimeters  of  water  required.  If  the  liter-flask  is 
wet,  it  should  be  rinsed  two  or  three  times  with  a  little  of  the  acid  to  be 
diluted.  The  necessary  amount  of  water  is  carefully  measured  out  with  a 
burette  and  allowed  to  flow  into  the  flask,  which  is  then  filled  to  the  mark 
with  the  acid  and  thoroughly  shaken.  The  diluted  acid  is  emptied  into  a 
bottle  capable  of  holding  all  of  the  acid  to  be  diluted.  If  it  is  not  dry,  it  is 
rinsed  with  a  little  of  the  diluted  acid.  The  remainder  of  the  acid  is  diluted 
in  the  same  manner  and  poured  into  the  large  bottle.  The  strength  of  the 
diluted  acid  must  be  verified  by  one  of  the  methods  already  used.  This 
is  especially  necessary  if  the  amount  of  water  added  is  large. 

DETERMINATION  OF    SODIUM  HYDROXIDE,  CARBON- 
ATE, AND  BICARBONATE. 

These  three  compounds  of  sodium  evidently  cannot  occur 
together,  because  the  hydroxide  and  bicarbonate  would  react  to 
form  the  normal  carbonate.  We  have  therefore  to  consider  the 
determination  of  sodium  hydroxide  and  carbonate  when  occurring 
together,  as  well  as  the  analysis  of  mixtures  of  the  bicarbonate  and 
the  carbonate. 

365.  Titration  of  Carbonic  Acid.  —  One  method  of  analyzing 
the  first  combination  has  already  been  outlined  in  the  discussion 


266  VOLUMETRIC  METHODS. 

of  the  properties  of  indicators.  Using  phenolphthalein  as  the 
indicator,  all  of  the  hydroxide  will  be  neutralized  by  standard 
acid,  while  the  carbonate  will  be  converted  into  bicarbonate. 
On  adding  methyl  orange  and  continuing  the  addition  of  acid 
until  the  solution  is  pink  the  bicarbonate  formed  will  be  decom- 
posed. The  amount  of  acid  added  during  the  second  part  of  the 
titration  will  be  half  that  required  to  completely  decompose  the 
carbonate.  The  difference  between  the  two  amounts  of  acid 
used  will  be  the  amount  necessary  to  neutralize  the  sodium  hydrox- 
ide. This  method  will  give  correct  results  only  if  the  amount  of 
carbonate  is  small  and  the  solution  is  dilute  and  cold.  The  escape 
of  carbon  dioxide  because  of  a  local  excess  of  acid  must  be  abso- 
lutely prevented  by  vigorous  stirring.  The  method  is  well  adapted 
to  the  analysis  of  commercial  caustic  soda. 

366.  Precipitation  of  Carbonic  Acid. — If  a  large  amount  of 
sodium  carbonate  is  present,  the  total  amount  of  alkali  present  is 
determined  by  titration  with  standard  acid,  using  methyl  orange 
as  the  indicator.  From  another  portion  the  carbon  dioxide  is 
precipitated  by  the  addition  of  BARIUM  CHLORIDE  solution.  The 
alkali  is  in  this  manner  converted  into  chloride,  and  on  titrating 
the  solution  only  the  alkali  hydroxide  remains  to  react  with  the 
acid.  Any  excess  of  barium  chloride  will  remain  as  such  or  react 
with  the  sodium  hydroxide,  giving  sodium  chloride  and  barium 
hydroxide.  It  has  been  shown,  however,  that  the  excess  of 
barium  chloride  must  be  small,  as  otherwise  the  percentage  of 
caustic  soda  found  will  be  low.  Using  phenolphthalein  as  the 
indicator,  the  alkali  hydroxide  may  be  titrated  in  the  presence  of 
the  barium  carbonate,  if  care  is  exercised  to  stir  the  solution 
thoroughly  so  as  not  to  have,  at  any  time,  an  excess  of  acid  pres- 
ent in  any  part  of  the  solution.  The  acid  reaction  of  the  solution 
is  not  permanent  because  of  the  slight  solubility  of  the  barium 
carbonate.  The  titration  is  therefore  conducted  quite  rapidly 
and  a  momentary  disappearance  of  the  red  color  is  taken  as  the 
end-point. 

The  solution,  after  the  addition  of  the  barium  chloride,  may 
also  be  diluted  to  a  definite  volume,  and  after  allowing  the  pre- 
cipitate to  settle,  a  measured  portion  of  the  clear  liquid  may  be 
withdrawn  and  titrated.  The  amount  of  acid  necessary  to  neu- 


TIT  RAT  ION  OF    THE  ALKALIES.  267 

tralize  the  carbonate  present  is  found  by  subtracting  the  amount 
of  acid  necessary  to  neutralize  the  sodium  hydroxide  from  the 
amount  used  in  titrating  the  total  alkali. 

367.  Determination  of  Bicarbonates. — If  a  mixture  of  sodium 
carbonate  and  bicarbonate  is  to  be  analyzed,  the  total  alkali  may 
be  found  by  tit  ration  with  acid,  using  methyl  orange  as  the  indi- 
cator. As  the  precipitation  of  the  carbon  dioxide  by  barium 
chloride  would  tend  to  produce  free  acid  according  to  the  equation 

2NaHC03  +2BaCl2  =2BaC03  +2NaCl  +2HC1, 

thus  preventing  the  completion  of  the  reaction,  a  measured 
excess  of  standard  alkali  solution  must  first  be  added.  After 
the  precipitation  of  the  carbon  dioxide  by  the  addition  of 
barium  chloride  the  alkali  present  is  titrated  with  standard  acid. 
The  difference  between  the  amount  of  alkali  added  and  that 
found  after  the  precipitation  of  the  carbon  dioxide  is  that  neces- 
sary to  convert  the  bicarbonate  present  into  carbonate  or  to  neu- 
tralize the  equivalent  amount  of  hydrochloric  acid  liberated  from 
the  barium  chloride.  The  standard  alkali  used  in  this  case  may 
be  either  ammonia  or  caustic  soda.  The  former  is  disadvanta- 
geous because  of  its  disturbing  effect  on  phenolphthalein.  while 
it  is  difficult  to  obtain  either  alkaline  solution  free  from  carbon 
dioxide.  Perhaps  the  simplest  way  of  overcoming  this  difficulty 
is  to  conduct  a  blank  determination  by  precipitating  the  carbonate 
from  a  measured  volume  of  the  alkaline  solution  and  titrating  as 
in  the  actual  determination. 

EXERCISE  49. 

Determination  of  Sodium  Hydroxide,  Sodium  Carbonate,  and  Water  in 

Caustic  Soda. 

Weigh  out  in  a  weighing-bottle  from  3  to  5  grams  of  caustic  soda.  Do 
not  take  the  sample  from  the  top  of  the  bottle,  where  it  has  been  exposed 
to  the  moisture  and  carbon  dioxide  of  the  air.  Transfer  to  the  weighing- 
bottle  with  as  little  exposure  as  possible  and  stopper  quickly.  When 
weighed,  transfer  to  a  beaker,  rinse  the  bottle  with  water,  and  dissolve  the 
soda.  Transfer  to  a  250-c.c.  flask  and  dilute  to  the  mark  with  boiled  water. 

368.  First  Method.  Titration  of  the  Carbonic  Acid. — Take  out  23  c.c. 
with  a  pipette  and  dilute  to  about  100  c.c.  with  boiled  water  which  has  been 
thoroughly  cooled.  Titrate,  while  stirring  vigorously,  with  N  /5  acid  after  add- 
ing 2  or  3  drops  of  phenolphthalein  until  the  pink  color  has  just  disappeared. 


268  VOLUMETRIC  METHODS. 

Add  I  or  2  drops  of  methyl  orange  and  continue  the  titration  until  a  faint 
pink  appears.  Repeat  until  duplicates  are  obtained.  Twice  the  amount 
of  acid  used  in  the  methyl-orange  tritation  is  the  amount  necessary  to 
neutralize  the  sodium  carbonate.  1  c.c.  of  N/5  acid  equals  .01061  gram 
Na2C03.  The  difference  between  the  amounts  of  acid  used  in  the  phenol- 
phthalein  and  the  methyl  orange  titrations  is  the  amount  necessary  to  neu- 
tralize the  sodium  hydroxide.  1  c.c.  of  N/5  acid  equals  .008012  gram  NaOH. 
Calculate  the  amount  of  Na2CO3  and  of  NaOH  present  in  250  c.c.  The 
difference  between  the  sum  of  these  weights  and  the  weight  of  the  caustic 
soda  gives  the  weight  of  water  present.  Calculate  the  per  cent  of  each 
constituent. 

369.  Second  Method.  Precipitation  of  the  Carbonic  Acid. — Measure  out 
25  c.c.  of  the  soda  solution  with  a  pipette  into  a  beaker,  add  1  or  2  drops  of 
methyl  orange,  and  titrate  to  faint  pink  with  N/5  acid. 

Measure  out  50  c.c.  into  a  100-c.c.  flask  and  add  with  shaking  a  little 
dilute  solution  of  barium  chloride  drop  by  drop  as  long  as  a  white  precipi- 
tate forms.  Dilute  to  the  mark  with  boiled  and  cooled  .water,  shake  thor- 
oughly, and  allow  to  settle.  Measure  out  50  c.c.  'and  titrate  with  the  N/5 
acid,  using  phenolphthalein  as  the  indicator.  Repeat  both  titrations  until 
duplicates  are  obtained.  The  second  titration  gives  the  amount  of  acid 
necessary  to  neutralize  the  caustic  soda,  while  the  difference  between  the 
first  and  second  gives  the  amount  of  acid  necessary  to  neutralize  the  sodium 
carbonate  present  in  50  c.c.  of  the  solution.  Calculate  the  percentage  of 
NaOH,  Na2C03,  and  H2O  as  directed  under  the  first  method. 

DETERMINATION  OF  HARDNESS  OF  WATER. 

370.  Temporary  Hardness  or  Alkalinity.— The  hardness  of 
water  is  caused  by  the  presence  of  salts  of  calcium  and  magnesium 
in  solution.  If  these  metals  are  in  solution  as  bicarbonates,  they 
may  be  precipitated  as  carbonates  by  boiling  the  water,  the  excess 
of  the  carbon  dioxide  being  expelled.  The  hardness  of  water 
which  may  be  removed  in  this  manner  is  called  temporary  hard- 
ness. The  amount  of  calcium  and  magnesium  held  in  solution  in 
this  manner  may  be  ascertained  by  titrating  the  solution  with  a 
standard  acid,  using  methyl  orange  or  erythrosene  and  chloroform 
as  the  indicator.  The  result  is  given  in  parts- of  calcium  carbon- 
ate per  100,000,  giving  the  so-called  degrees  of  hardness.  As  the 
bicarbonates  of  calcium  and  magnesium  react  alkaline  to  methyl 
orange  and  some  other  indicators,  this  titration  also  gives  the 
alkalinity  of  the  water.  If  sodium  or  potassium  carbonates  are 
present,  they  will  also  react  alkaline,  but  a  correction  for  this 
error  will  be  obtained  in  determining  permanent  hardness. 


I 

DETERMINATION   OF  HARDNESS  OF  WATER.  269 

371.  Permanent  Hardness  or  Incrustants. — The  presence  of 
chlorides  and  sulphates  of  calcium  and  magnesium  produce  per- 
manent hardness,  so  called  because  these  salts  cannot  be  removed 
by  boiling  the  water.  On  adding  excess  of  sodium  carbonate,  and 
Waporating  to  dryness,  the  sulphates  and  chlorides  of  calcium  and 
magnesium  are  converted  into  the  carbonates  of  these  two  metals 
and  the  sulphate  and  chloride  of  sodium.  The  bicarbonates  of 
calcium  and  magnesium  are  also  decomposed.  On  filtering  and 
washing,  the  carbonates  of  calcium  and  magnesium,  being  insol- 
uble, remain  on  the  paper,  while  the  excess  of  the  sodium  car- 
bonate as  well  as  the  sodium  chloride  and  sulphate  pass  into  solu- 
tion. The  excess  of  sodium  carbonate  having  been  ascertained  by 
titration,  the  amount  needed  to  decompose  the  chlorides  and  sul- 
phates of  calcium  and  magnesium  is  found  ly  difference.  If  the 
so-called  soda  reagent  is  used,  the  magnesium  will  be  precipitated 
as  hydroxide.  The  amount  of  permanent  hardness  is  also  calcu- 
lated as  parts  of  calcium  carbonate  per  100,000  or  per  million. 
As -the  principal  salt  producing  permanent  hardness  is  calcium 
sulphate,  which  tends  to  produce  a  very  hard  scale  in  boilers,  the 
term  incrustants  is  also  used  to  designate  these  constituents  of  the 
water.  If  the  alkalinity  of  the  filtrate  is  greater  than  that  due  to 
the  sodium  carbonate  added,  alkali  carbonates  must  have  been 
present  in  the  water.  Permanent  hardness  is 'then  absent,  and 
the  amount  of  alkali  carbonates  present  computed  as  calcium  car- 
bonate must  be  subtracted  from  the  temporary  hardness  already 
found. 

EXERCISE  50. 
Determination  GJ.  flardness  of  Water,  Temporary  and  Permanent. 

Make  an  N/50  solution  of  hydrochloric  acid  by  measuring  out  100  c.c. 
of  the  IS  5  acid  into  a  liter  flask  and  diluting  to  the  mark.  Make  an  N/50 
solution  of  sodium  carbonate  by  diluting  100  c.c.  of  the  N/5  alkali  to  a  liter, 
or  weigh  out  1 .061  grams  of  the  pure  dry  salt,  dissolve  in  water,  and  make 
up  to  a  liter. 

372.  Temporary  Hardness  or  Alkalinity. — Measure  out  100  c.c.  of  the 
water  to  be  tested  into  a  porcelain  dish  and  add  one  drop  of  methyl  orange. 
Measure  out  100  c.c.  of  distilled  water  into  a  similar  porcelain  dish  and  add 
one  drop  of  the  indicator.  Add  the  N/50  acid  drop  by  drop  untj]  a  faint 
pink  color  is  obtained.  Titrate  the  water  in  the  other  dish  with  the  acid 
until  the  color  matches  that  in  the  dish  containing  the  distilled  water.  From 
tae  amount  of  acid  used,  less  the  amount  added  to  the  distilled  water  (.1  to 


270  VOLUMETRIC  METHODS. 

.3  c.c.),  the  alkalinity  or  hardness  is  calculated  in  parts  per  100,000  or  parts 
per  million. 

Erythrosene  and  chloroform  may  also  be  used  as  the  indicator  in  this 
titration.  The  end-point  with  this  indicator  is  sharper  and  its  use  is  espe- 
cially to  be  advised  when  alum  is  likely  to  be  present  in  the  water.  O.I 
gram  of  the  erythrosene  is  dissolved  in  1000  c.c.  of  water.  100  c.c.  of  the 
water  to  be  tested  is  placed  in  a  250-c.c.  glass-stoppered  bottle;  2.5  c.c.  of 
the  indicator*  and  5  c.c.  of  chloroform  are  added.  The  bottle  is  shaken 
thoroughly,  so  that  the  chloroform  forms  an  emulsion.  The  standard  acid 
is  added  and  the  shaking  continued  until  the  color  is  discharged.  The 
alkalinity  is  calculated  from  the  amount  of  acid  used. 

373.  Permanent  Hardness  or  Incrustants. — 100  c.c.  of  the  water  is  meas- 
ured out  into  a  porcelain  evaporating-dish.  Add  a  measured  amount  of 
the  N/50  sodium  carbonate  solution  sufficient  to  make  the  water  strongly 
alkaline.  Evaporate  to  dryness,  take  up  with  a  small  amount  of  water, 
filter,  and  wash  the  dish  and  filter-paper  until  free  from  alkali.  Titrate 
the  excess  of  the  alkali  with  N/50  acid,  using  methyl  orange  as  the  indicator- 
The  difference  between  the  amount  of  alkali  added  and  that  titrated  back 
is  the  amount  used  in  precipitating  the  calcium  and  magnesium,  which 
produced  the  permanent  hardness.  The  calculation  is  made  in  the  same 
manner  as  that  of  temporary  hardness.  If  the  amount  of  alkali  titrated 
back  should  be  greater  than  the  amount  added,  the  water  contains  alkali 
carbonates  which  have  been  reckoned  as  temporary  hardness.  The  amount 
of  excess  should  therefore  be  deducted  from  the  temporary  hardness.  Per- 
manent hardness  is  absent. 

The  so-called  soda  reagent  gives  somewhat  more  accurate  results  because 
the  salts  of  calcium  and  magnesium  are  rendered  less  soluble  by  this  reagent. 
It  is  composed  of  equal  parts  of  N/10  sodium  carbonate  and  sodium  hydrox- 
ide. It  may  be  prepared  by  weighing  out  2.653  grams  of  pure  sodium 
carbonate  or  measuring  out  250  c.c.  of  N/5  sodium  carbonate  solution  and 
250  c.c.  of  N/5  sodium  hydroxide  solution  (page  282) ,  and  diluting  to  one 
liter.  200  c.c.  of  the  water  to  be  tested  is  measured  into  a  Jena  Erlen- 
meyer  flask.  After  boiling  10  minutes  to  expel  free  carbonic  acid,  25  c.c. 
of  the  soda  reagent  is  added.  It  is  again  boiled  down  to  a  volume  of  100 
c.c.,  cooled  and  rinsed  into  a  200-c.c.  graduated  flask  and  the  volume 
made  up  to  200  c.c.  with  boiled  distilled  water  and  thoroughly  mixed  by 
shaking  the  flask.  The  solution  is  filtered  through  a  dry  paper,  and  after 
rejecting  the  first  50  c.c.,  100  c.c.  is  titrated  with  the  N/50  acid,  using  ery- 
throsine  as  the  indicator.  The  calculation  is  made  in  the  same  manner 
as  before  except  that  the  25  c.c.  of  tenth,  normal  soda  reagent  must  be 
considered  equal  to  125  c.c.  of  the  N/50  acid. 

When'  the  permanent  hardness  is  small,  better  results  are  obtained  by 
determining  total  hardness  by  means  of  the  soda  reagent,  and  getting  per- 

*When  erythrosene  is  referred  to  as  the  indicator,  the  .01%  solution  of  the 
sodium  salt  of  erythrosene  is  implied. 


STANDARD  SULPHURIC  ACID. 


271 


manent  hardness  by  difference.  For  this  purpose  the  exact  amount  of 
acid  used  in  titrating  the  alkalinity  is  added  to  100  c.c.  of  the  water  being 
tested,  thus  converting  the  temporary  hardness  to  permanent  hardness, 
which  is  determined  by  the  method  already  described. 

EXERCISE  51. 
Standard  N/s  Sulphuric  Acid. 

374.  Specific  Gravity.— To  100  c.c.  distilled  water  add  40  grams  con- 
centrated sulphuric  acid.  Cool  to  15°  C.  and  take  the  specific  gravity  with 
the  Westphal  balance.  In  setting  up  this  balance,  the  screw  A  is  adjusted 
so  that  the  two  points  at  C  are  exactly  on  a  level  when  the  thermometer  D 


FIG.  45. 

is  suspended  in  the  air.  By  the  thumb-screw  B  the  height  of  the  beam  is 
regulated  so  that  the  thermometer  does  net  touch  the  bottom  of  the  small 
cylinder.  The  weights  are  of  four  sizes.  The  largest  size  hung  on  the  hook 
on  the  end  of  the  beam  gives  units,  while  on  any  other  space  gives  tenths, 
the  next  size  gives  hundredths,  the  next  size  thousandths,  and  the  smallest 
ten-thousandths  of  specific  gravity.  The  instrument  in  Fig.  45  reads  0.0885, 
the  largest  size  weight  being  absent.  The  sulphuric  acid  is  poured  into  the 
cylinder  and  the  weights  placed  on  the  various  divisions  until  the  two  points 
at  C  are  again  exactly  on  a  level.  If  the  thermometer  now  reads  15°,  the 


272  VOLUMETRIC  METHODS. 

specific  gravity  is  given  by  the  weights  on  the  beam.  The  percentage  of 
HjjSO^  corresponding  to  the  specific  gravity  found,  is  obtained  from  the 
table  of  specific  gravity  of  sulphuric  acid  given  on  p.  523.  The  specific 
gravity  times  the  per  cent  of  H2SO4  present  gives  the  number  of  grams  of 
HsSC^  per  cubic  centimeter.  The  number  of  cubic  centimeters  of  acid 
to  be  measured  out  to  give  9808  grams  of  H2SO4  is  now  computed. 
The  acid,  still  being  at  15°,  is  transferred  to  a  burette,  measured  out  into  a 
liter  flask,  diluted  to  the  mark  and  thoroughly  mixed.  Another  liter  is 
prepared  in  the  same  manner  and  mixed  with  the  first  in  a  2-liter  bottle.^ 
Instead  of  measuring  out  the  concentrated  'acid,  it  may  be  weighed  out.' 
For  this  purpose  it  need  not  be  kept  at  15°.  The  amount  to  be  weighed 
out  for  one  liter  is  found  by  dividing  9.808  by  the  per  cent  found  in  the 
table  and  multiplying  the  quotient  by  100.  If  this  solution  has  been  care- 
fully prepared,  it  will  be  very  near  the  calculated  strength, 

STANDARDIZATION. 

First  Method. — Same  as  first  method  in  Exercise  48. 
Second  Method. — Same  as  second  method  in  Exercise  48. 

375.  Third  Method.     Precipitation  as  Barium  Sulphate. — Measure  out  by 
a  burette  50  c.c.  of  the  sulphuric  acid  into  a  beaker.     Dilute  to  400  c.c., 
heat  to  boiling,  and  add  barium  chloride  solution  drop  by  drop  while  stirring 
the  solution  vigorously  until  all  of  the  sulphuric  acid  is  precipitated.     Digest 
until  the  solution  is  clear.     Wash  by  decantation,  dry,  ignite,  and  weigh. 
Repeat  the  determination  until  duplicates  are  obtained  agreeing  within 
.2%.     Compute  the  weight  of  H2SO4,  multiply  by  20,  and  divide  9.808  by 
the  product.     This  gives  the  number  of  cubic  centimeters  of  the  acid  to  be 
diluted  to  1000  c.c.    The  acid  is  diluted  as  directed  in  Exercise  48. 

DETERMINATION  OF  SPECIFIC  GRAVITY. 

The  strength  of  acids  as  well  as  of  a  great  variety  of  solutions 
is  commonly  ascertained  by  a  determination  of  the  specific  grav- 
ity. This  method  is  especially  advantageous  when  dealing  with 
solutions  of  pure  substances. 

376.  Specific  Gravity  may  be  defined  as  the  ratio  between  the 
weight  of  a  given  volume  of  a  substance  and  the  weight  of  an  equal 
volume  of  some  standard  substance.    When  dealing  with  liquids  or 
solids,  water  is  almost  invariably  used   as  the  standard  of  com- 
parison.    In  terms  of  the  metric  system  of  weights  and  measures, 
specific  gravity  may  be  defined  as  the  ratio  of  the  weight  of  a 
cubic  centimeter  of  a  substance  to  the  weight  of  a  cubic  centi- 
meter of  water.    As  the  weight  of  the  latter  is  1  gram,  specific 


DETERMINATION  OF  SPECIFIC  GRAVITY.  273 

gravity  may  be  defined  as  the  weight  in  grams  of  1  cubic  centi- 
meter of  a  given  substance.  When  using  the  metric  system  of 
weights  and  measures,  specific  gravity  is  therefore  identical  with 
density,  which  is  defined  as  the  weight  of  unit  volume  of  a  sub- 
stance. 

377.  Standard  Temperatures. — With  the  English  units  of  weights 
and  measures,  specific  gravity  and  density  are  not  identical.  The 
identity  is  not  exact  even  with  the  metric  system,  except  when 
the  water  is  used  at  its  maximum  density  (4°  C.)  and  is  weighed 
in  vacuo.  Considerable  confusion  has  arisen  because  in  deter- 
mi:  ing  specific  gravity  the  weight  of  water  in  the  air  and  at 
other  temperatures  than  4°  C.  has  been  assumed  to  be  1  gram 
per  cubic  centimeter.  Even  when  the  exact  weight  of  the  water 
has  been  ascertained,  confusion  may  arise  if  the  conditions  used 
are  not  given.  The  temperature  at  which  both  the  substance 
and  the  water  are  weighed  should  always  be  given.  This  is  ordi- 
narily done  by  placing  a  fraction  after  the  figure  giving  the  specific 
gravity.  The  numerator  of  the  fraction  gives  the  temperature  of 
the  substance,  while  the  denominator  gives  the  temperature  of 
the  water.  1.563^,  for  instance,  means  that  a  given  volume  of 
the  substance  at  15°  weighs  1.563  times  as  much  as  the  same 
volume  of  water  at  4°  C.  If  these  weights  have  been  reduced  to 
vacuo,  we  may  also  state  that  the  density  of  the  substance  at  15° 
is  1.563;  that  is,  the  weight  of  1  cubic  centimeter  is  1.563  grams. 
For  substances  whose  specific  gravity  is  small,  the  correction 
obtained  by  reducing  the  weight  to  vacuo  is  very  small,  being 
about  one-tenth  per  cent.  The  error  introduced  by  considering 
the  weight  of  1  c.c.  of  water  at  15°  to  be  1  gram  is  .08%,  while 
at  20°  the  error  is  .17%,  and  at  25°  it  is  .29%.  As  both  of  these 
corrections  are  positive,  the  error  introduced  by  neglecting  both 
of  them  is  the  sum  of  the  two.  The  simplest  and  most  conve- 
nient method  of  determining  specific  gravity  consists  in  weighing 
both  the  substance  and  the  water  in  the  air  at  ordinary  temper- 
tures,  such  as  15°  or  20°.  When  the  specific-gravity  tables  have 
been  calculated  from  determinations  made  in  the  same  manner, 
no  error  is  introduced  unless  the  volume  of  solutions  are  calcu- 
lated from  their  weight  or  weights  from  volumes.  Such  calcula- 
tions require  true  densities. 


274  VOLUMETRIC  METHODS. 

378.  Pycnometers.— The  most  reliable  method  of  determining 
specific  gravity  involves  the  use  of  some  form  of  pycnometer.  The 
simplest  form  of  this  instrument  consists  of  a  small  bottle  closed 
with  a  glass  stopper  containing  a  capillary  opening  through  which 
the  excess  of  liquid  may  escape  when  inserting  the  stopper  after 
filling  the  bottle.  It  is  first  weighed  empty  and  dry,  again  weighed 
after  filling  with  water,  and  finally  the  weight  of  the  bottle  filled 
with  the  substance  being  tested  is  taken.  The  latter  as  well  as 
the  water  must  be  brought  to  the  standard  temperature  before 
filling  the  bottle.  As  most  liquids  transmit  heat  quite  readily 
and  have  a  fairly  high  coefficient  of  expansion,  there  is  consid- 
erable difficulty  in  getting  the  pycnometer  filled  at  the  desired 
temperature.  After  being  filled  the  bottle  may  be  placed  in  a 
vessel  of  water  which  is  maintained  at  the  standard  temperature. 
If  allowed  to  remain  sufficiently  long  in  the  water,  the  bottle  and 
contents  ultimately  attain  the  same  temperature  as  the  water. 
The  time  necessary  for  this  purpose  varies  with  the  viscosity  and 
other  properties  of  the  liquid.  To  obviate  uncertainty  in  this 
respect,  pycnometers  are  made  with  a  thermometer  ground  so  as 
to  serve  as  a  stopper,  the  bulb  being  in  the  liquid  while  the  stem 
projects  out  of  the  bottle.  A  capillary  tube  is  fused  into  the 
side  of  the  pycnometer  and  is  provided  with  a  hollow  glass  ground 
cap.  The  liquid  is  cooled  down  below  the  standard  temperature 
before  filling  the  bottle,  which  is  then  placed  in  water  maintained 
at  this  temperature.  As  the  liquid  expands  the  excess  escapes 
through  the  capillary  tube.  When  the  thermometer  in  the  pyc- 
nometer indicates  the  desired  temperature,  the  excess  of  liquid  on 
the  end  of  the  capillary  tube  is  removed  and  the  cap  put  on.  The 
bottle  is  removed  from  the  water  and  wiped  clean  and  set  aside 
until  the  contents  have  reached  room  temperature,  when  it  is 
weighed.  The  weight  of  distilled  water  which  just  fills  the  bottle 
at  the  standard  temperature  is  obtained  in  the  same  manner.  If 
the  true  density  is  desired,  the  weight  of  the  water  is  corrected 
for  air  displacement,  as  explained  on  page  11,  and  the  volume  in 
cubic  centimeters  calculated  from  the  density  of  the  water  at  the 
temperature  used.  The  weight  of  an  equal  volume  of  water  in 
grams  at  4°  C.  in  vacuo  is  numerically  equal  to  the  number  of 
cubic  centimeters  found  by  the  calculation  given.  This  weight, 


DETERMINATION  OF  SPECIFIC  GRAVITY.  275 

divided  into  the  weight  of  the  bottle  full  of  the  liquid  being 
tested,  gives  the  density  or  the  specific  gravity  compared  with 
water  at  4°  C. 

379.  The  Westphal  Balance. — The  manipulation  of  this  instru- 
ment is  very  simple,  and  the  results  are  almost  as  accurate  and 
reliable  as  those  obtained  with  the  pycnometer.     It  consists  of  a 
beam  suspended  on  a  knife-edge  and  so  adjusted  that  when  a  glass 
plummet  is  suspended  from  a  hook  at  one  end,  the  weight  on  the 
other  end  is  just  counterbalanced,  as  indicated  by  the  two  points 
at  C,  Fig.  45,  which  are  then  exactly  on  a  level.     If  they  are  not, 
the  balance  may  be  adjusted  by  means  of  the  thumb -screw  A. 
As  commonly  made,  the  plummet  weighs  15  grams  and  displaces 
5  grams  of  water  at  the  standard  temperature.     The  arm  from 
which  the  plummet  is  suspended  is  divided  into  10  equal  divi- 
sions, and  a  set  of  weights  in  the  form  of  riders  is  provided,  such 
that  the  largest  is  just  equal  to  the  weight  of  the  water  displaced 
by  the  plummet,  generally  5  grams.     If  the  plummet  is  suspended 
in  water,  it  will  lose  5  grams  in  weight,  which  will  te  exactly 
counterbalanced  by  the  large  weight  when  hung  from  the  hook  at 
the  end  of  the  beam.     This  indicates  a  specific  gravity  of  one. 
When  the  plummet  is  suspended  in  other  liquids,  the  large  weight 
indicates  tenths  when  placed  on  other  divisions  of  the  beam. 
Similarly,  riders  weighing  0.5,  0.05,  and  0.005  gram  indicate  hun- 
dredths,  thousandths,  and  ten  thousandths    of    specific  gravity 
when   they  are  hung  on  the  beam  so  as  to  counterbalance  the 
buoyant  force  of  the  liquid  being  tested  on  the  glass  plummet. 
The  temperature  of  the  liquid  is  taken  by  means  of  a  thermometer 
within  the  plummet. 

380.  Hydrometers,  or  specific-gravity  spindles,  are  instruments 
the  construction  of  which  is  based  on  the  principle  that  the  buoyant 
force  of  a  liquid  is  directly  proportional  to  its  density  or  specific 
gravity.     The  hydrometer  is  essentially  a  long,  closed,  glass  tube, 
more  or  less  enlarged  at  the  lower  end  and  so  weighted  with  shot 
or  mercury  that  it  will  assume  a  perpendicular  position  in  a  liquid 
in  which  it  does  not  entirely  sink.     As  the  weight  of  the  instru- 
ment is  constant,  the  depth  to  which  the  hydrometer  sinks  in  a 
given  liquid  may  be  used  as  an  index  of  the  specific  gravity  of 
the  liquid.    A  paper  scale  is  inserted  in  the  stem  of  the  instru- 


276  VOLUMETRIC  METHODS. 

merit  and  fixed  in  such  a  position  that  the  specific  gravity  may 
be  read  directly  on  the  scale  at  the  surface  of  the  liquid.  A 
thermometer  is  sometimes  made  a  part  of  the  instrument,  so  that 
the  temperature  at  which  the  readings  are  taken  may  be  noted. 
These  instruments  are  very  convenient  and  rapid  in  use,  but  are 
not  well  adapted  for  accurate  work.  Specific  gravity  is  often 
designated  by  degrees  of  one  of  the  several  scales  which  have 
been  devised  and  which  are  almost  invariably  made  use  of  in  the 
construction  and  graduation  of  hydrometers.  The  degrees 
Twaddell  are  the  simplest  and  most  scientific  of  these  scales. 
On  this  scale  a  degree  corresponds  to  .005  specific  gravity  above 
unity.  7°  Twaddell  is  therefore  equal  to  1.035  sp.  gr.;  30°  is 
equal  to  1.150,  etc.  The  spindle  is  usually  made  quite  large,  with 
a  very  small  stem,  so  that  determinations  of  specific  gravity  may 
be  made  with  very  great  accuracy.  The  entire  scale  of  150°  is 
usually  divided  equally  among  6  spindles.  This  instrument  is 
very  generally  used  in  Great  Britain. 

381.  The  Baume  Scale  is  generally  used  in  other  countries.    This 
instrument  was  originally  graduated  as  follows:    The  point  to 
which  the  spindle  sank  in  water  at  17J0  C.  was  marked  0,  and 
the  point  to  which  it  sank  in  a  10  per  cent  solution  of  common 
salt  was  marked  10°.     This  space  was  divided  into  10  equal  divi- 
sions, and  the  remainder  of  the  stem  marked  off  in  spaces  of  the 
same  length.     This  gave  the  scale  for  liquids  heavier  than  water. 
For  liquids  lighter  than  water,  the  point  to  which  the  spindle  sank 
in  a  solution  of  1  part  of  salt  in  9  parts  of  water  was  marked  0, 
and  the  point  to  which  it  sank  in  water  was  marked  10.     One- 
tenth  of  this  space  was  taken  as  a  degree,  and  the  remainder  of 
the  scale  divided  into  degrees  of  this  size.     This  method  gave 
degrees  of  unequal  value  in  terms  of  specific  gravity.     On  account 
of  the  varying  purity  of   the  salt  used,  the  degrees  on  different 
instruments  varied  greatly  in  value,  so  that  66°  Baume  might 
indicate  in  the  case  of  sulphuric  acid  a  minimum  strength  of 
89.5%  H2S04,  or  a  maximum  of  95%  H2S04.     As  a  remedy  for 
this  confusion,  several  formulas  have  been  proposed  by  which  the 
degree  Baume  may  be  calculated  from  the  specific  gravity,  or  the 
latter  calculated  from  the  former.     The  so-called  rational  formula 

144  3 

is  d=  '     0,  in  which  d  =  specific  gravity  and  B°  =  the  degree 

JL44.O  —  Jt5 


DETERMINATION  OF  SPECIFIC  GRAVITY.  277 

144 
Baume.     In  Holland  the  formula  d  =  —  —  —  —  0  and  in  the  United 

—  JtS 


145 

States  the  formula  d  =  ——  —  —  0  has  been  adopted  as  the  standard 
14o  —  13 

Baume  scale  for  liquids  heavier  than  water,  and  for  liquids  lighter 

140 
than  water,  the  formula  d  =  ~—  —  —   has  been  adopted  as   the 

J.OU  ~p  _D 

standard  American  scale.  Under  these  circumstances  it  is  evident 
that  the  Baume  scale  is  the  worst  which  could  have  been  adopted, 
and  that  degrees  Baume  is  by  no  means  a  definite  statement  of 
specific  gravity  unless  it  is  known  what  particular  scale  has  been 
used.  The  formula  giving  the  relation  between  specific  gravity 
and  degrees  Baume  should  be  printed  on  the  scale,  and  only  the 
standard  American  scale  should  be  used  in  the  United  States. 


PROBLEMS. 

Problem  5.  How  many  cubic  centimeters  of  sulphuric  acid  having  a 
specific  gravity  of  1.3567  at  15°  must  be  diluted  to  a  liter  to  make  N/5 
acid?  How  many  grams  of  the  same  acid  must  be  used  for  making  a  liter 
of  N/5  acid? 

Problem  6.  Four  grams  of  caustic  soda  were  weighed  out,  dissolved  in 
water,  and  diluted  to  500  c.c.  Fifty  c.  c.  of  this  solution  required, 
when  titrated  with  N/5  acid  and  phenolphthalein,  46.36  c.c.  of  acid,  and  when 
methyl  orange  was  added,  0.94  c.c.  was  required  to  give  the  end-point  with 
this  indicator.  Calculate  the  percentage  of  sodium  hydroxide,  sodium  car- 
bonate, and  water  in  the  caustic  soda. 

Problem  7.  One  gram  of  sodium  phosphate  was  dissolved  in  water  and 
titrated  with  N/5  acid,  using  phenolphthalein  as  the  indicator,  0.67  c.c.  of 
the  acid  being  used.  Methyl  orange  was  then  introduced  and  acid  again 
added  until  the  end-point  was  reached,  13.92  c.c.  of  the  N/5  acid  being 
used  in  the  second  titration.  Calculate  the  percentage  of  crystallized  tri- 
sodium  and  disodium  phosphate  present. 

Problem  8.  Ten  grams  of  a  mixture  of  sodium  carbonate  and  sodium 
bicarbonate  was  dissolved  in  water  and  diluted  to  a  liter.  To  50  c.c.  of 
this  solution,  25  c.c.  of  N/5  sodium  hydroxide  was  added,  and  then  enough 
barium  chloride  to  precipitate  the  carbonate.  On  titrating  with  N/5  hydro- 
chloric acid,  15  c.c.  was  used.  Another  50-c.c.  portion  required  40  c.c. 
when  titrated  with  the  N/5  acid  and  methyl  orange.  Calculate  the  per- 
centage of  sodium  carbonate  and  bicarbonate  present. 

Problem  9.  In  titrating  100  c.c.  of  a  sample  of  water,  5  c.c.  of  N/20 
sodium  hydroxide  was  required  with  phenolphthalein.  Another  100-c.c. 


278  VOLUMETRIC  METHODS. 

portion  required  15  c.c.  of  N/20  hydrochloric  acid  with  methyl  orange. 
Calculate  (a)  the  amount  of  carbon  dioxide  present  and  the  alkalinity  in 
parts  per  million;  (6)  calculate  the  amount  of  calcium  oxide  necessary 
to  soften  1000  gallons  of  the  water;  (c)  calculate  the  amount  of  lime- 
water  necessary  to  soften  the  same  quantity  of  water. 

Problem  10.  To  100  c.c.  of  the  same  water,  50  c.c.  of  N/50  sodium-car- 
bonate solution  was  added,  and  after  evaporation,  35  c.c.  of  N/50  hydro- 
chloric acid  was  required  to  titrate  the  excess.  Calculate  (a)  the  permanent 
hardness  in  parts  per  million ;  (b)  the  amount  of  sodium  carbonate  neces- 
sary to  soften  1000  gallons  of  the  water. 

After  adding  the  softening  agent  and  allowing  the  water  to  settle,  or 
filtering  it,  how  could  (a)  the  presence  of  an  excess  of  lime  be  found  by 
titration,  (6)  a  deficiency  of  lime,  (c)  an  excess  or  deficiency  of  the  soda- 
ash? 


CHAPTER  XXII. 
STANDARD  ALKALIES. 

382.  Standard  Alkaline  Solutions  not  Permanent. — While 
standard  solutions  of  hydrochloric  and  sulphuric  acids  may  be 
kept  for  months  and  even  years  without  change  in  strength,  no 
suitable  alkaline  solution  has  been  found  which  in  practice  may 
be  preserved  without  deterioration.  This  arises  mainly  from  two 
causes,  the  absorption  of  carbon  dioxide  from  the  air  and  the  action 
of  the  alkaline  solutions  on  the  glass  of  the  containing  bottles.  In 
the  case  of  AMMONIA  neither  of  these  causes  of  deterioration  is 
serious,  but  another  difficulty  is  met  with  in  the  volatility  of  the 
base.  Moreover,  it  is  a  weak  base  and  cannot  be  used  with 
phenol phthaiein.  The  strength  of  all  basic  solutions  must  there- 
fore be  frequently  checked  by  titration  with  standard  acid.  The 
presence  of  carbon  dioxide  may  in  many  cases  be  disregarded 
by  using  methyl  orange  as  the  indicator  in  titrations.  As  this  is 
not  always  possible  on  account  of  the  nature  of  the  acid,  standard 
alkaline  solutions  must  frequently  be  prepared  and  kept  free 
from  carbon  dioxide  by  methods  described  later. 

383.  Caustic  Soda  has  been  found  to  be  the  alkali  best  suited 
for  general  work.  It  is  a  strong  base,  forms  soluble  salts  with 
all  acids,  and  may  be  prepared  of  any  desired  strength,  while 
the  pure  commercial  article  is  cheaper  than  caustic  potash.  A 
solution  of  the  highest  purity  may  be  made  by  dissolving  metallic 
sodium  in  pure  water.  This  metal  may  now  be  readily  obtained 
in  pure  condition.  Caustic  soda  made  from  the  metal  may  also 
be  purchased.  The  pure  commercial  caustic  soda  almost  invari- 
ably contains  small  amounts  of  chlorides,  sulphates,  silica,  and 
alumina,  besides  considerable  amounts  of  carbon  dioxide  and 
water.  About  20%  more  than  the  calculated  amount  of  NaOH 
should  therefore  be  weighed  out,  and  dissolved  in  the  required 

279 


280 


VOLUMETRIC  METHODS. 


amount  of  water.  After  titrating  this  solution  against  standard 
acid  it  is  diluted  to  the  required  strength.  On  account  of  the 
presence  of  carbon  dioxide,  methyl  orange  must  be  used  as  the 
indicator,  or  litmus  accompanied  by  boiling  to  expel  the  carbon 
dioxide. 

384.  Removal  of  Carbon  Dioxide.  —  The  alkaline  solution 
must  be  kept  in  a  bottle  closed  with  a  rubber  stopper  or  a  glass 
stopper  coated  with  paraffine  or  vaseline.  If  the  solution  is  to 
be  used  for  titrating  organic  acids  with  phenolphthalein  as  the 
indicator,  the  carbon  dioxide  must  be  removed  by  means  of  barium 
chloride  or  hydroxide,  and  the  solution  subsequently  protected  from 
the  carbon  dioxide  of  the  air.  This  may  be  accomplished  either 
by  pouring  a  layer  of  kerosene  over  the  solution  or  by  passing  the 
air  which  enters  the  bottle  through  a  soda-lime  tube.  In  either 
case  the  soda  solution  must  be  withdrawn  by  means  of  a  siphon. 
The  arrangement  shown  in  Fig.  46  is  very  convenient.  The  caus- 


FIG.  46. 

tic-soda  solution  is  made  up  in  a  bottle  of  suitable  size  and,  after 
the  addition  of  enough  barium  chloride  or  barium  hydroxide  solu- 
tion to  precipitate  all  of  the  carbon  dioxide,  is  allowed  to  stand 
until  the  precipitate  has  settled.  The  clear  solution  is  then 
siphoned  off  into  the  bottle  A,  and  if  a  layer  of  petroleum-oil  is  not 


STANDARD  ALKALIES.  281 

placed  on  top,  the  tube  B  is  filled  with  soda-lime  and  inserted  into 
the  stopper.  The  long  arm  of  the  siphon  C  is  pushed  through 
the  second  hole  of  the  stopper  so  that  a  space  of  about  one  inch 
is  left  between  the  end  and  the  bottom  of  the  bottle.  The  alkali 
may  then  be  withdrawn,  leaving  undisturbed  any  barium  carbon- 
ate which  settles  to  the  bottom.  The  siphon  is  filled  by  apply- 
ing suction  to  the  tube  D,  which  is  filled  with  soda-lime,  or  pressure 
may  be  applied  at  B.  The  long  arm  of  the  siphon  may  be  adjusted 
so  that  the  end  is  opposite  the  zero-mark  on  the  burette,  and  any 
excess  of  alkali  drawn  over  will  flow  back  into  the  stock-bottle. 

385.  Alcoholic   Caustic-potash  Solution  is  frequently  used  foi 
the  titration  of  ORGANIC  ACIDS  with  phenolphthalein  as  the  indi- 
cator.     Such  a  solution  is  of  twofold  advantage.     Many  of  the 
organic  acids  are  insoluble  in  water,  but  dissolve  readily  in  alco- 
hol.    Alcoholic  solutions  of  caustic  potash  are  free  from  carbon 
dioxide,  because  potassium  carbonate  is  almost  absolutely  insolu- 
ble in  alcohol.     On  dissolving  the  caustic  potash  in  the  alcohol, 
the   potassium   carbonate   remains   undissolved.     After   standing 
twenty-four  hours  the  clear  solution  may  readily  be  siphoned 
off  from  the  potassium  carbonate,  which  adheres  quite  firmly  to 
the  bottle.     The  alcohol  used  must  first  be  freed  from  aldehyde 
or  organic  material  extracted  from  the  oak-barrel,  which  turns 
yellow  on  the  addition  of  alkali  and  interferes  with  the  titration. 
For  this  purpose  a  stick  of  caustic  potash  is  added,  and  after  stand- 
ing for  some  time  the  alcohol  is  distilled  off.     The  caustic-potash 
solution  must  be  protected  from  the  carbon  dioxide  of  the  air 
by  the  method  suggested  for  caustic  soda  or  some  similar  device. 

386.  Water  Solutions  of  Barium  Hydroxide  have  been  largely 
used  when  the  presence  of  carbon  dioxide  would  be  objectionable. 
Any  carbon  dioxide   which  enters  the  solution  is  immediately 
precipitated  as  the  insoluble  barium  salt.     A  solution  saturated 
with  barium  hydroxide  at  the  ordinary  temperature  is  about  two- 
fifths  normal.     After  shaking  the  solution  with  excess  of  the  com- 
mercial barium  hydroxide  until  no  more  dissolves,  the  clear  liquid 
should  be  siphoned  off  and  diluted  a  little  before  standardizing 
so  as  to  prevent  the  crystallizing  out  of  the  barium  hydroxide 
during  occasional  periods  of  low  temperature.     This  alkali  gives 
a  very  sharp  end -reaction  with  phenolphthalein. 


282  VOLUMETRIC  METHODS. 

387.  Ammonia. — On  account  of  its  volatility  strong  solutions 
of  ammonia  cannot  be  used.  Even  with  tenth-  or  fifth-normal 
solutions  the  loss  of  alkali  is  considerable  if  the  solution  is 
kept  in  a  half-filled  bottle  and  is  poured  out  into  the  burette. 
During  this  process  the  air  above  the  liquid  is  largely  replaced, 
and  as  the  pressure  of  the  ammonia  vapor  is  proportional  to  the 
concentration  of  the  ammonia  in  solution,  the  loss  in  any  case  is 
considerable.  The  action  of  the  ammonia  on  the  glass  is  com- 
paratively slight,  especially  if  the  same  bottle  is  used  continually. 
The  amount  of  carbon  dioxide  absorbed  is  also  inconsiderable. 
If  therefore  the  ammonia  solution  is  siphoned  out  of  the  bottle, 
a  fifth-  or  tenth-normal  solution  will  show  only  inappreciable 
changes  in  strength  from  day  to  day,  and  if  kept  in  an  old  bottle 
may  be  used  for  considerable  periods  without  change  of  titre. 


EXERCISE  52. 

Preparation  of  N/5  Caustic  Soda  and  Determination  of  Ammonia  in  Ammonium 

Chloride. 

Weigh  out  roughly  about  10  grams  of  pure  caustic  soda,  dissolve  in 
water,  and  dilute  to  1  liter.  Place  in  a  bottle  provided  with  a  rubber  stopper. 
Standardize  by  titrating  against  standard  hydrochloric  or  sulphuric  acid, 
using  methyl  orange  as  the  indicator.  Use  at  least  25  c.c.  of  alkali  for 
each  titration.  If  the  acid  is  exactly  N/5  normal,  divide  the  number  of 
cubic  centimeters  of  alkali  by  the  number  of  cubic  centimeters  of  acid  used. 
The  quotient  multiplied  by  1000  gives  the  number  of  cubic  centimeters  of  the 
alkali  to  be  diluted  to  1000  to  make  a  N/5  solution.  Verify  the  strength 
of  the  diluted  solution  by  titrating  against  the  standard  acid. 

Weigh  out  250  mg.  of  ammonium  chloride  and  transfer  to  a  300-c.c. 
Jena  beaker.  Dissolve  in  50  c.c.  of  water  and  add  50  c.c.  of  the  caustic- 
soda  solution.  In  a  similar  beaker  place  50  c.c.  of  water  and  50  c.c.  of  the 
caustic-soda  solution.  Place  both  beakers  on  the  hot  plate  and  warm, 
finally  to  boiling,  until  no  more  ammonia  is  evolved.  Cool  both  beakers, 
and  titrate  the  alkali  remaining  in  each  with  the  standard  acid,  using  methyl 
orange  as  the  indicator.  The  difference  between  the  amounts  of  acid  used 
to  neutralize  the  alkali  in  the  beakers  is  the  amount  necessary  to  neu- 
tralize the  ammonia  in  the  ammonium  chloride.  1  c.c.  of  N/5  acid  =.003407 
gram  of  ammonia  or  .0107  gram  of  ammonium  chloride.  Compute  the 
percentage  of  ammonia  as  well  as  of  ammonium  chloride  in  the  sample 
analyzed. 


KJELDAHL  METHOD   FOR  NITROGEN.  283 


THE  KJELDAHL  METHOD  OF  DETERMINING  NITROGEN. 

In  1883  Kjeldahl  described  a  method  for  determining  nitro- 
gen in  organic  substances  which,  with  the  numerous  modifications 
which  have  since  been  suggested,  is  now  the  method  most  largely 
used  for  the  determination  of  nitrogen,  not  only  in  organic  com- 
pounds of  the  greatest  diversity  of  composition,  but  also  in  nitrates 
and  other  inorganic  compounds.  There  are  very  few  nitrogen 
compounds  in  which  by  one  or  the  other  modification  the  nitro- 
gen cannot  be  determined  with  a  degree  of  accuracy  and  with  a 
rapidity  almost  unequalled  by  any  other  quantitative  method. 

388.  Oxidation  of  Organic  Matter  and  Conversion  of  Nitrogen 
into  Ammonium   Sulphate. — By  the  original  method  the  organic 
material  was  digested  hot  with  concentrated  sulphuric  acid,  by 
which  the  carbon  was  oxidized  to  carbon  dioxide,  while  the  nitrogen 
remained  as  ammonium  sulphate.     The  oxidation  was  completed 
by    the    addition    of   powdered    potassium    permanganate.     The 
concentrated  acid  was  then  diluted  somewhat,  and  neutralized 
with  caustic  soda.     The  ammonia  liberated  was  distilled  off,  and 
determined  by  means  of  standard  sulphuric  acid.     In  many  cases 
the  combustion  of  the  carbon  was  very  slow.     Various  modifica- 
tions have  been  introduced  to  overcome  this  difficulty.     The  addi- 
'tion  of  mercury,  mercuric  oxide,  or  copper  oxide  or  sulphate  greatly 
shortens  the  time  of  combustion,  probably  by  acting  as  carriers 
of  oxygen,  the  metal  being  alternately  oxidized  by  the  sulphuric 
acid  and  reduced  by  the  carbon.     The  addition  of  phosphorus 
pentoxide  or  potassium  sulphate  serves  the  same  purpose  by  raising 
the  temperature  to  which  the  solution  may  be  heated.     When 
mercury  has  been  added  to  the  sulphuric  acid  it  must  be  re- 
moved by  means  of  sodium  sulphide  before  the  ammonia  is  dis- 
tilled   off,  otherwise   some    of   the  ammonia  combines  with  the 
mercuric  oxide  and  cannot  be  expelled  by  distillation. 

389.  Reduction  of  Nitrates. — By  the  original  Kjeldahl  method 
the  conversion  of  nitrates  to  ammonia  was  very  uncertain.     To 
overcome  this  difficulty,  salicylic  acid  or  phenol  was  dissolved  in 
the  sulphuric  acid.     This  solution  first  converts  the  nitrate  into 
nitro-phenol.     This  compound  is  readily  reduced  to  amido-phenol, 


284 


VOLUMETRIC  METHODS. 


which  is  completely  hydrolyzed  by  caustic  soda  with  liberation 
of  ammonia.  These  reactions  take  place  according  to  the  follow- 
ing equations:  * 

C6H5.OH  +HN03  =C6H4.OH.N02  +H20  ; 
C6H4.OH.N02  +3H2      =C6H4.OH.NH2  +2H20; 
C6H4.OH.NH2  +NaOH  =C6H4.OH.ONa+  NH3. 

The  reduction  is  greatly  assisted  by  the  addition  of  sodium  thio- 
sulphate  or  zinc.  The  addition  of  pure  cane-sugar  is  also  advan- 
tageous when  substances  of  high  nitrogen  content  are  oxidized. 

390.  Kjeldahl  Digestion-flasks. — The  digestion  of  the  nitrog- 
enous substance  with  sulphuric  acid  is  carried  out  in  pear- 
shaped,  long-necked  flasks  of  Bohemian  or  Jena  glass  which  are 
made  for  this  purpose.  The  flasks  are  made  of  various  sizes, 
since  some  material  foams  considerably  on  being  heated  with  the 
sulphuric  acid,  and  therefore  requires  the  use  of  a  larger  flask. 
Some  workers  also  desire  to  obviate  the  transference  of  the  sul- 
phuric-acid solution  from  the  digestion-flask  to  a  special  distilla- 
tion-flask. If  this  method  is  pursued,  a  digestion-flask  capable 


tf-2 


FIG.  47. 

of  holding  about  500  c.c.  must  be  used.  If  a  separate  distillation- 
flask  is  employed,  the  digestion-flask  need  not  ordinarily  have  a 
capacity  of  more  than  200  c.c. 

391.  Digestion. — During  the  digestion  the  flask  is  placed  so 
as  to  be  inclined  at  an  angle  of  45°,  in  order  that  there  shall  be  no 
loss  of  material  by  spattering,  nor  any  contamination  by  the  falling 

*  These  reactions  are  in  reality  more  complicated  than  represented,  as  several 
sulphuric  acid  molecules  are  combined  with  the  benzene  radicle.  The  reactions 
as  given  simply  show  the  transformation  of  the  nitric-acid  radicle  into  ammonia. ' 


KJELDAHL  METHOD  FOR  NITROGEN.  285 

in  of  foreign  material.  As  an  additional  precaution  some 
workers  close  the  mouth  of  the  flask  by  means  of  a  stopper 
made  by  drawing  out  a  glass  tube  which  is  somewhat  larger  than 
the  neck  of  the  digestion-flask.  The  drawn-out  end  is  sealed, 
and  after  cutting  off  about  an  inch  of  the  glass  tube  the  stopper 
is  inserted  with  the  drawn-out  end  in  the  neck  of  the  flask. 
Racks  for  holding  six  or  eight  flasks  in  an  inclined  position 
are  used  where  much  work  must  be  done.  These  racks  are  pro- 
vided with  a  Bunsen  burner  under  each  flask.  If  the  material 
froths  much,  the  flask  is  at  first  heated  with  a  small  flame  and  over 
a  wire  gauze.  The  heat  is  gradually  increased  until  finally  the 
flask  is  heated  with  the  bare  flame,  until  the  acid  is  colorless  or  a 
light  yellow.  The  digestion  requires  from  one-half  to  several 
hours.  As  fumes  of  sulphuric  acid  and  sulphur  dioxide  are  copi- 
ously evolved,  the  digestion  must  be  conducted  under  a  hood. 

392.  Distillation. — After  cooling,  the  acid  is  diluted  somewhat 
and  transferred  to  the  distillation-flask,  if  a  separate  flask  is  used 
for  this  purpose.  The  distillation-flasks  should  have 
a  capacity  of  500  to  600  c.c.  and  should  be  made  of 
hard  Bohemian  or  Jena  glass.  They  may  be  round- 
bottomed  or  of  the  Erlenmeyer  form.  To  '  prevent 
the  carrying  over  of  the  caustic  alkali  in  the  form  of 
spray,  a  trap  of  some  kind  must  be  interposed  between 
the  distillation-flask  and  the  condenser.  The  simplest 
form  consists  of  a  glass  bulb  4  to  5  cm.  in  diameter 
filled  with  glass  wool.  In  another  trap  shown  in  Fig.  48, 
which  is  very  much  used,  the  upper  exit  tube  extends 
into  and  is  somewhat  curved  towards  the  side  of  the 
bulb.  The  rate  of  distillation  is  much  increased  by 
covering  the  bulb  with  thin  asbestos  paper,  which  may 
be  made  to  adhere  by  simply  moistening  with  water.  It  is  then 
pressed  with  the  hand  around  the  bulb. 

The  trap  is  connected  to  the  distillation-flask  by  means  of  a 
rubber  stopper  and  to  the  condenser-tube  by  means  of  a  piece  of 
thick- walled  rubber  tubing.  The  tube  of  the  condenser  must  be  of 
block  tin.  Glass  cannot  be  used,  as  it  is  dissolved  by  the  steam 
and  ammonia  vapor,  increasing  the  alkalinity  of  the  distillate. 
The  condenser-tube  is  curved  downwards,  where  it  is  joined  to 


286 


VOLUMETRIC  METHODS. 


the  trap.  A  number  of  these  block-tin  tubes  are  frequently 
passed  through  the  same  condenser,  which  consists  of  a  copper  or 
iron  tank  of  convenient  size,  as  shown  in  Fig.  49. 

The  DISTILLATE  is  received  in  an  Erlenmeyer  flask  of  300-  to 
400-c.c.  capacity,  in  which  a  measured  amount  of  standard  sul- 
phuric acid  has  been  placed.  If  the  condenser  is  supplied  with  a 
sufficient  amount  of  cold  water,  it  appears  to  be  unnecessary  for 
the  end  of  the  condenser-tube  to  dip  into  the  standard  acid.  It 
is  customary,  however,  to  attach  a  large-sized  glass  tube  to  the  exit 
of  the  tin  condenser-tube  by  means  of  a  piece  of  rubber  tubing. 


FIG.  49. 

The  end  of  this  glass  tube  should  touch  the  surface  of  the  acid. 
All  of  the  ammonia  is  usually  carried  over  when  150  c.c.  of  liquid 
has  distilled  over.  Until  experience  is  gained  it  is  advisable  to 
continue  the  distillation  after  the  Erlenmeyer  flask  containing 
about  150  c.c.  of  distillate  has  been  removed.  The  distillate  is 
then  allowed  to  flow  into  a  small  beaker  containing  a  drop  of  the 
indicator.  If  the  distillate  is  still  alkaline,  the  contents  of  the 
beaker  are  poured  into  the  Erlenmeyer  flask.  The  bumping 
which  so  frequently  accompanies  the  boiling  of  an  alkaline  solu- 


KJELDAHL  METHOD  FOR  NITROGEN.  287 

tion  may  generally  be  prevented  by  the  addition  of  a  few  grams 
of  granulated  zinc.  Bumping  is  also  very  much  reduced  if  caus- 
tic soda  free  from  carbonate  is  used.  A  small  piece  of  paraffine 
will  prevent  foaming  of  the  liquid. 

393-  Titration  of  Ammonia. — The  excess  of  acid  in  the  distil- 
late is  titrated  back  with  standard  alkali,  which  may  be  caustic 
soda,  potash,  or  ammonia.  The  indicator  used  may  be  methyl 
orange,  litmus,  or  cochineal.  As  the  various  reagents  used  may 
contain  more  or  less  nitrogen,  a  blank  must  be  conducted  when- 
ever one  or  more  of  the  reagents  is  used  for  the  first  time.  For 
the  blank  determination  pure  cane-sugar  is  digested,  and  care 
must  be  taken  to  add  each  of  the  reagents  in  the  amount  used  hi 
the  actual  analysis.  The  amount  of  nitrogen  found  hi  this  deter- 
mination is  deducted  from  each  result  subsequently  obtained 
with  the  reagents  tested. 

WILFARTH'S  MODIFICATION 

APPLICABLE   TO   SUBSTANCES   FREE  FROM   NITRATES. 

One  gram  of  the  finely  powdered  substance  is  placed  in  the 
digestion-flask,  1  gram  of  metallic  mercury  or  about  0.7  gram  of 
mercuric  oxide  is  added,  and  25  c.c.  of  a  solution  of  200  grams  of 
phosphorus  pentoxide  in  1  liter  of  concentrated  sulphuric  acid 
(sp.  gr.  1.84).  After  digesting  and  transferring  the  acid  to  the 
distillation-flask  with  about  200  c.c.  of  water  the  mercury  is  pre- 
cipitated by  the  addition  of  25  c.c.  of  a  solution  of  40  grams  of 
potassium  sulphide  in  1  liter  of  water.  The  acid  is  then  neutral- 
ized by  the  addition  of  a  saturated  solution  of  caustic  soda,  of 
which  about  50  c.c.  will  be  required.  After  the  addition  of  a  few 
grams  of  granulated  zinc  the  ammonia  is  distilled  into  the  sul- 
phuric acid,  of  which  50  c.c.  of  N/5  acid  is  usually  ample.  The 
excess  of  acid  is  titrated  back  with  N/5  or  N/10  alkali. 

GUNNING'S  MODIFICATION 

APPLICABLE   TO   SUBSTANCES   FREE    FROM    NITRATES. 

Gunning's  improvement  consisted  in  the  addition  of  potassium 
sulphate  to  the  sulphuric  acid,  by  which  the  time  of  digestion  is 
considerably  reduced.  Sodium  sulphate  may  also  be  used  as 


288  VOLUMETRIC  METHODS. 

well  as  sodium  pyrophosphate.  A  very  excellent  digestion  mix- 
ture consists  of  20  c.c.  of  the  mixture  of  phosphorus  pentoxide 
and  sulphuric  acid,  given  in  Wilfarth's  modification,  to  which  10  to 
15  grams  of  potassium  sulphate  are  added,  either  in  the  beginning 
or  after  the  first  violent  oxidation  is  ended.  Excellent  results 
may  be  obtained  with  this  mixture  without  the  addition  of 
mercury,  though  the  oxidation  is  considerably  hastened  by  the 
addition  of  the  usual  amount  of  the  latter.  The  decomposition 
is  frequently  complete  in  thirty  minutes  and  rarely  requires  more 
than  an  hour.  The  precipitation  of  the  mercury,  neutraliza- 
tion, distillation,  etc.,  are  carried  out  as  given  in  the  Wilfarth 
modification. 

GUNNING-JODLBAUER  MODIFICATION 

APPLICABLE    TO    NITRATES. 

In  this  modification  the  digestion  is  carried  out  with  sulphuric 
acid  which  contains  40  grams  of  phenol  or  salicylic  acid  per  liter. 
1  gram  of  the  nitrogenous  substance  is  placed  in  the  digestion- 
flask  and  dissolved  in  30  c.c.  of  the  phenol-sulphuric  acid.  The 
solution  should  be  kept  cold  and  thoroughly  shaken  to  hasten 
the  solution.  2  to  3  grams  of  zinc-dust  and  1  gram  of  mercury 
are  then  added  while  the  flask  is  shaken  and  cooled.  After  stand- 
ing one  to  two  hours  the  flask  is  heated,  at  first  gently,  then  more 
strongly.  10  to  15  grams  of  potassium  sulphate  may  then  be 
added,  and  the  flask  heated  until  the  solution  is  colorless  or  nearly 
so.  The  remainder  of  the  process  is  conducted  as  directed  for 
the  other  modifications. 

FORSTER'S  MODIFICATION 

APPLICABLE    TO   NITRATES. 

Having  failed  to  obtain  all  of  the  nitrogen  from  nitrates  by 
the  previous  modification,  Forster  introduced  the  use  of  sodium 
thiosulphate  as  a  reducing  agent.  He  treated  J  gram  of  the 
nitrate  in  the  digestion-flask  with  15  c.c.  of  a  6%  phenol-sulphuric 
acid,  and  after  the  nitrate  was  completely  dissolved  1  to  2  grams 
of  sodium  thiosulphate  were  added.  When  the  decomposition  of 
the  thiosulphate  is  complete  10  c.c.  of  concentrated  sulphuric 


KJELDAHL  METHOD  FOR  NITROGEN.  289 

acid  and  1  gram  of  mercury  are  added.  The  decomposition  is 
completed  in  the  usual  manner.  The  thiosulphate  must  not  be 
added  before  the  addition  of  the  phenol-sulphuric  acid,  or  nitrogen 
will  be  lost.  The  solution  need  not  be  cooled  as  hi  the 
Jodlbauer  modification.  The  oxidation  is  generally  complete  hi 
1 J  hours. 

394.  Substances  Decomposed  with  Difficulty. — In  regard  to 
the  substances  which  cannot  be  determined  by  the  Kjeldahl 
method  and  its  modifications  Dyer*  states  that  the  nitrogen  in 
aromatic  nitro-compounds  cannot  be  determined  by  simple  reduc- 
tion with  zinc.  Phenol  or  salicylic  acid  must  be  present.  The 
same  thing  is  true  of  many  of  the  azo  compounds.  Hydroxyl- 
amine  and  acetaldoxime  required  the  addition  of  sugar  before 
all  of  the  nitrogen  was  converted  into  ammonia  by  the  Jodlbauer 
method.  The  cyanide  and  ferrocyanide  of  potassium  could  be 
analyzed  by  the  Gunning  modification,  while  the  ferricyanide 
required  the  Forster  modification.  Phenylhydrazine  could  not 
be  correctly  analyzed  by  any  modification.  H.  C.  Sherman  f 
states  that  where  large  proportions  of  chlorides  and  nitrates 
exist  together  all  modifications  fail. 


EXERCISE  53. 

Determination  of  Nitrogen  in  Milk  by  the  Kjeldahl-Gunning  Method. 

395.  Standard  Solutions. — For  this  determination  a  fifth  normal  acid 
and  alkali  will  be  needed.  The  fifth  normal  sulphuric-  or  hydrochloric-acid 
solution  already  prepared  may  be  used.  The  fifth-normal  solution  of  caustic 
soda  may  also  be  used,  or  a  fifth-normal  solution  of  ammonia  may  be  pre- 
pared as  follows:  About  15  c.c.  of  concentrated  ammonia  (sp.  gr.  0.90)  are 
diluted  to  1  liter.  A  second  liter  is  made  in  the  same  manner  as  the  first. 
The  entire  solution  is  well  mixed  in  a  large  bottle,  and  then  titrated  against 
the  N/5  standard  acid,  using  cochineal  or  methyl  orange  as  the  indicator. 
The  solution  of  ammonia,  which  is  somewhat  stronger  than  fifth-normal, 
is  then  diluted  to  the  exact  strength,  and  placed  in  a  bottle  connected 
with  the  burette  by  means  of  a  siphon,  as  described  on  p.  280.  The 
soda-lime  tube  may  be  omitted.  A  simpler  arrangement,  which  answers 
fully  as  well  for  this  purpose,  consists  of  a  suitable-sized  bottle,  closed 

*  Chem.  Centr.,  Ill,  XVII,  433. 
t  J.  Am.  Chem.  Soc..  XVII.  567. 


290 


VOLUMETRIC  METHODS. 


by  a  two-holed  rubber  stopper  through  which  a  siphon  passes,  the  long 
arm  of  which  is  outside  the  bottle.  To  the  end  of  the  long  arm  is 
attached,  by  means  of  a  short  piece  of  rubber  tubing,  a  piece  of  glass 
tubing.  A  pinch-cock  is  placed  on  the  rubber 
tube.  By  opening  this  pinch-cock  and  forcing 
air  through  the  second  opening  in  the  stopper, 
the  siphon  may  be  filled.  The  burette  is  filled 
by  opening  the  pinch-cock  and  allowing  the  so- 
lution to  flow  out. 

3Qb.  Weighing  the  Sample. — The  sample  of  milk 
to  be  analyzed  must  be  thoroughly  mixed  by  shak- 
ing the  bottle.  10  c.c.  is  withdrawn  with  a  pipette 
and  allowed  to  flow  over  the  asbestos  contained  in 
a  Hofmeister  capsule.  The  asbestos  must  be  first 
ignited,  and  after  placing  in  the  capsule,  the 
whole  is  weighed.  After  the  milk  has  been  added 
the  whole  is  again  weighed.  Neither  of  these 
weights  need  be  taken  closer  than  several  milli- 
grams. A  second  or  even  a  third  sample  of  milk 
is  weighed  out  in  the  same  manner.  The  milk 
FIG.  50.  js  evaporated  to  dryness  on  the  water-bath, 

the  capsule  crushed  in  a  clean  porcelain  mortar  and  transferred  to  the 
digestion-flask. 

The  milk  may  also  be  evaporated  in  the  flasks,  the  weight  then  being 
obtained  from  the  specific  gravity,  or  the  average  weight  of  10  c.c.  may  be 
obtained  by  first  weighing  several  portions  in  small  beakers.  The  evapora- 
tion in  the  flasks  is  slow  unless  the  steam  is  displaced  by  a  stream  of  air. 
The  water  may  also  be  boiled  out  after  the  addition  of  the  digestion- 
mixture. 

397.  Digestion. — Weigh  out  20  grams  of  phosphorous  pentoxide  in  a 
beaker,  and  add  500  c.c.  of  concentrated  C.  P.  sulphuric  acid.  Stir  until 
solution  is  complete,  and  transfer  to  a  glass-stoppered  bottle.  Transfer 
20  c.c.  portions  of  this  solution  to  the  digestion-flasks  containing  the  milk, 
and  also  to  two  other  flasks  in  each  of  which  about  a  gram  of  granulated 
sugar  has  been  placed.  Add  10  grams  of  potassium  sulphate  to  each  flask, 
and  heat  gently  over  the  wire  gauze  with  the  Bunsen  burner.  The  heating 
is  conducted  under  the  hood.  Increase  the  heat  as  the  frothing  ceases, 
finally  remove  the  wire  gauze,  and  heat  strongly  until  the  solution  is  color- 
less or  only  a  light  yellow. 

While  the  solution  is  digesting  a  saturated  solution  of  caustic  soda  is 
made  by  dissolving  200  grams  in  twice  this  amount  of  water.  40  or  50  c.c. 
of  the  standard  acid  is  measured  out  into  each  of  the  Erlenmeyer  flasks 
to  be  used  as  receivers.  If  the  condensers  of  the  distilling  apparatus  have 
not  been  recently  used,  they  are  cleaned  by  passing  through  them  the 
steam  from  a  flask  of  water  strongly  acidified  with  sulphuric  acid  until 


KJELDAHL  METHOD  FOR  NITROGEN.         291 

the  distillate  is  no  longer  alkaline  to  cochineal  solution.  If  100  c.c.  of  the 
laboratory  supply  of  distilled  water  reacts  alkaline  with  the  cochineal 
solution,  ammonia-free  water  should  be  made  by  condensing  the  steam 
from  a  flask  of  water  acidified  with  sulphuric  acid.  This  steam  should  be 
condensed  by  passing  it  through  one  of  the  block-tin  tubes  of  the  Kjeldahl 
condenser.  This  distilled  water  should  then  be  used  for  diluting  the  con- 
centrated acid  and  whenever  needed  during  the  titrations. 

398.  Distillation. — When  the  digestion-flasks  have  cooled,  about  50  c.c.  of 
water  are  added  to  each,  and  if  the  small-sized  digestion-flasks  have  been 
employed,  the  solutions  are  transferred  to  the  distillation-flasks,  using  about 
150  c.c.  of  the  ammonia-free  water  to  rinse  out  the  digestion-flasks.  The 
acid  is  neutralized  with  the  concentrated  caustic-soda  solution,  of  which 
about  50  c.c.  will  be  required.  2  or  3  grams  of  granulated  zinc  are  added 
and  the  flask  is  immediately  connected  with  the  condenser  to  prevent  loss 
of  ammonia  from  the  hot  alkaline  solution.  The  distillation  of  the  ammonia 
in  the  first  flask  is  begun  immediately,  and  the  acid  in  the  other  flasks  is 
diluted,  transferred,  and  neutralized  in  rotation. 

399-  Titration. — When  about  150  c.c.  of  distillate  has  passed  into  the 
first  Erlenmeyer  flask,  it  is  replaced  by  a  small  beaker  containing  a  little  of  the 
indicator.  If  10  or  15  c.c.  of  the  distillate  does  not  react  alkaline  with 
the  indicator,  2  or  3  drops  of  the  cochineal  are  added  to  the  solution  in  the 
Erlenmeyer  flask  and  the  excess  of  acid  titrated  with  the  standard  alkaline 
solution.  If  the  latter  has  not  been  compared  within  a  day  or  two  with 
the  standard  acid,  a  measured  volume  must  be  titrated  against  the 
acid.  The  other  Erlenmeyer  flasks  should  be  removed  and  the  solu- 
tions titrated  in  the  order  in  which  the  acid  solutions  were  transferred  to 
the  distillation-flasks.  After  deducting  the  average  amount  of  acid  neu- 
tralized by  the  ammonia  obtained  in  the  blank  determinations,  the  amount 
of  nitrogen  found  is  computed  by  the  factor  1  c.c.  of  N/5  acid  =.002802 
gram  of  nitrogen.  To  obtain  the  percentage  of  casein,  the  percentage  of 
nitrogen  is  multiplied  by  the  commonly  used  factor  6.25  or  by  the  more 
recently  obtained  and  probably  more  correct  factor  6.43. 


EXERCISE  54. 

• 

Determination  of  Nitrogen  in  Potassium  Nitrate  by  the  Forster  Modification  of 

the  Kjeldahl  Method. 

For  this  determination  it  is  advisable  to  use  a  sample  of  pure  potas- 
sium nitrate  as  a  check  on  the  process.  A  sample  of  commercial  sodium 
or  potassium  nitrate  may  then  be  analyzed. 

Twenty  grams  of  phenol  are  dissolved  in  340  grams  of  concentrated 
sulphuric  acid.  A  solution  of  potassium  sulphide  is  made  by  dissolving 
20  grams  of  the  commercial  article  in  500  c.c.  of  water.  Jf-gram  portions  of 
the  nitrate  are  weighed  out  and  transferred  to  digestion-flasks  and  dis- 


292  VOLUMETRIC  METHODS. 

solved  in  15  c.c.  of  the  phenol-sulphuric-acid  solution.  After  the  nitrate 
is  completely  dissolved,  1  to  2  grams  of  sodium  thiosulphate  are  added. 
When  the  reaction  due  to  the  latter  is  ended,  10  c.c.  of  concentrated  sul- 
phuric acid  is  added  and  about  1  gram  of  metallic  mercury.  The  mercury 
may  be  dropped  from  a  glass  tube  the  end  of  which  is  partially  closed  by 
fusion  or  drawing  out.  The  size  of  this  opening  may  readily  be  so  adjusted 
that  one  drop  shall  weigh  about  1  gram.  A  blank  determination  is  made 
by  treating  about  1  gram  of  sugar  with  the  reagents  as  already  given.  The 
digestion  and  the  subsequent  operations  are  carried  out  exactly  as  given  in 
the  preceding  exercise,  except  that  after  the  acid  solution  has  been  trans- 
ferred to  the  distillation-flask,  25  c.c.  of  the  potassium -sulphide  solution  are 
added.  1  c.c.  of  N/5  acid  is  equal  to  one  five-thousandth  of  the  molecular 
weight  of  potassium  nitrate  or  the  same  proportion  of  the  molecular  weight 
of  sodium  nitrate. 


CHAPTER  XXIII. 
TITRATION  OF  BORIC  AND  CARBONIC  ACIDS. 

ESTIMATION  OF  BORIC  ACID. 

THE  titration  of  boric  acid  cannot  be  carried  out  by  means  of 
any  indicator  in  the  ordinary  manner  of  titrating  an  acid,  on  account 
of  the  very  weak  acid  properties  of  this  substance.  Because  of 
this  peculiarity,  the  base  in  many  of  the  salts  of  boric  acid  may 
be  readily  titrated  by  means  of  a  strong  mineral  acid  and  methyl 
orange  as  the  indicator  without  the  removal  of  the  boric  acid, 
which  is  entirely  neutral  towards  methyl  orange.  Because  of 
the  difficulty  of  carrying  out  the  gravimetric  methods  which  have 
been  devised,  the  discovery  of  a  method  of  increasing  the  acid 
properties  of  this  acid  so  that  it  can  be  titrated  with  phenol- 
phthalein  as  the  indicator  has  been  very  welcome  to  chemists. 

400.  Thompson's  Glycerine  Method. — The  method  proposed 
by  R.  T.  Thompson  *  consists  in  the  addition  of  enough  glycerine 
to  constitute  30%  of  the  solution  when  the  titration  is  complete. 
In  such  a  solution  the  boric  acid  acts  as  a  monobasic  acid.  To 
secure  a  solution  which  shall  contain  no  free  alkali  nor  acid  except 
boric  acid,  the  borate  is  dissolved  in  water  and  the  solution  neu- 
tralized with  sulphuric  acid  after  the  addition  of  a  drop  of  methyl 
orange.  A  drop  or  two  of  acid  is  then  added  and  the  solution 
boiled  in  an  open  dish  for  a  few  minutes  to  expel  carbon  dioxide. 
A  dilute  solution  may  be  boiled  in  this  manner  for  fifteen  minutes 
without  the  loss  of  more  than  a  faint  trace  of  boric  acid.f  Dilute 
caustic-soda  solution  is  now  added  until  the  pink  color  of  the 
methyl  orange  just  disappears.  Glycerine  is  then  added  in  such 
amount  that  after  completion  of  the  titration  at  least  30%  shall  be 


*  Jour.  Soc.  Chem.  Ind.,  XII,  432. 

f  L.  de  Koningh,  Jour.  Am.  Chem.  Soc.,  1897,  385. 

293 


294  VOLUMETRIC  METHODS. 

present.  After  the  addition  of  a  drop  of  phenolphthalein  the  solu- 
tion is  titrated  with  fifth-normal  caustic  soda  which  is  free  from 
carbon  dioxide.  The  usual  pink  end-point  of  the  phenolphthalein 
is  obtained  when  a  slight  excess  of  the  alkali  has  been  added. 
More  glycerine  should  then  be  added,  and  if  the  pink  color  of  the 
phenolphthalein  fades,  more  alkali  must  be  added.  A  blank  deter- 
mination should  be  carried  out  to  determine  the  acidity  of  the 
glycerine.  For  this  purpose  the  same  amount  of  glycerine  should 
be  diluted  with  an  equal  volume  of  water  and  the  solution  tit- 
rated with  the  standard  alkali. 

If  ammonia  is  present,  it  must  first  be  removed  by  adding  a 
slight  excess  of  sodium  carbonate  and  evaporating  the  solution 
down  to  half  its  bulk.  By  this  treatment  any  iron  present  will  be 
precipitated  and  should  be  filtered  off.  The  neutralization  of 
the  boric  acid  with  caustic  soda  takes  place  according  to  the  fol- 
lowing equation: 

HB02  +NaOH  =NaB02  +H20. 

1  c.c.  of  normal  caustic  alkali  is  therefore  equal  to  .035  gram  of  B203. 

401.  Jones's  Mannitol  Method. — Another  method  of  titrating 
boric  acid  has  been  based  on  the  fact  that  in  the  presence  of  manni- 
tol  the  acidity  is  sufficiently  increased  to  allow  of  its  titration 
with  phenolphthalein  as  the  indicator.*  The  solution  may  be  neu- 
tralized in  the  same  manner  as  given  in  the  glycerine  method. 
Phenolphthalein  is  then  added  and  standard  alkali  introduced 
until  the  solution  is  red.  A  pinch  of  mannitol  is  then  thrown  in, 
which  bleaches  the  red  color.  Enough  alkali  is  added  to  produce 
a  slight  alkaline  reaction,  followed  by  mannitol  until  a  permanent 
red  color  is  produced  which  cannot  be  discharged  by  further  addi- 
tion of  mannitol.  1  or  2  grams  of  mannitol  is  usually  sufficient. 

The  author  of  this  method,  instead  of  using  methyl  orange  as 
the  indicator  to  secure  a  solution  free  from  acids  except  boric, 
utilizes  the  fact  that  a  mixture  of  potassium  iodide  and  iodate  is 
entirely  unaffected  by  free  boric  acid,  while  strong  mineral  acids 
liberate  iodine,  thus  destroying  the  free  mineral  acid  according  to 
the  following  equation : 

6HC1 +5KI +KI03  =6KC1  +  3I2  +3H20. 


*  L.  C.  Jones,  Am.  J.  Sci.,  1898,  pp.  147-153. 


TITRATION  OF  BORIC  ACID.  295 

The  free  iodine  is  removed  by  adding  sodium  thiosulphate  solution, 
which  combines  with  the  iodine,  producing  only  neutral  compounds 
as  follows: 

2Na2SA  +1,  =2NaI  +  Na2S406. 

The  amount  of  free  mineral  acid  present  may  be  estimated  by  de- 
termining the  amount  of  free  iodine,  as  will  be  shown  in  the  next 
chapter. 

The  solution  of  the  borate,  the  volume  of  which  should  be 
about  50  c.c.  and  which  should  contain  about  0.1  gram  of  boric 
acid,  is  made  slightly  acid  with  hydrochloric  acid.  To  precipitate 
carbonic  acid  5  c.c.  of  a  10%  solution  of  barium  chloride  is  added. 
A  mixture  of  a  solution  of  potassium  iodide  and  iodate  is  made 
in  a  separate  beaker.  About  2  grams  of  the  iodide  and  J  gram 
of  the  iodate  are  generally  sufficient  to  destroy  all  the  mineral 
acid  present,  unless  the  amount  is  excessive.  Starch  solution  is 
added  to  the  iodate  solution,  and  if  a  blue  color  develops,  it  is  re- 
moved by  the  cautious  addition, of  dilute  sodium  thiosulphate  solu- 
tion. A  drop  of  the  borate  solution  added  to  the  iodate  should 
produce  a  blue  color,  indicating  the  presence  of  free  hydrochloric 
acid,  and  the  certainty  of  the  presence  of  all  boric  acid  in  the  free 
state.  The  two  solutions  are  then  mixed,  and  the  iodine  liberated 
is  removed  by  the  addition  of  sodium  thiosulphate.  The  colorless 
solution  now  contains  only  starch,  neutral  chlorides,  potassium 
tetrathionate,  iodide,  and  iodate,  and  the  total  amount  of  the 
boric  acid  to  be  determined  in  the  free  condition.  Phenolphthalein 
is  now  added  and  the  boric  acid  titrated,  the  mannitol  being 
added  as  already  directed.  The  end-reaction  by  this  method  is 
sharper  than  that  in  Thompson's  method. 

402.  Decomposition  of  Organic  Matter  containing  Boric  Acid. — 
Boric  acid  must  frequently  be  determined  in  food  products,  and 
the  separation  of  the  organic  matter  is  first  necessary.  Thompson 
proceeds  as  follows:  100  grams  of  the  sample  is  made  strongly 
alkaline  with  lime-water,  evaporated  to  dryness  in  a  platinum 
dish,  and  cautiously  heated  until  the  organic  matter  is  thoroughly 
charred  so  as  to  give  a  colorless  solution.  20  c.c.  of  water  is 
added,  and  the  solution  acidified  with  hydrochloric  acid,  when  all 
but  carbon  will  be  in  solution.  The  whole  is  transferred  to  a 
100-c.c.  flask  and  0.5  gram  calcium  chloride  added.  A  few  drops  of 


296  VOLUMETRIC  METHODS. 

phenolphthalein  are  introduced,  then  caustic  soda  until  a  slight 
pink  color  is  produced,  and  finally  25  c.c.  of  lime-water.  By  this 
treatment  the  carbon  dioxide  as  well  as  other  weak  acids,  such  as 
phosphoric  acid,  which  may  be  present  are  precipitated  as  calcium 
salts.  This  solution  is  made  up  to  100  c.c.,  thoroughly  shaken, 
and  filtered  through  a  dry  paper.  To  50  c.c.  of  the  filtrate  normal 
sulphuric  acid  is  added  until  the  pink  color  disappears,  then 
a  drop  or  two  of  methyl  orange  is  introduced  and  acid  is  again 
added  until  the  yellow  color  is  just  changed  to  pink.  Fifth- 
normal  caustic  soda  is  then  added  drop  by  drop  until  the  liquid 
assumes  the  yellow  tinge,  excess  of  soda  being  carefully  avoided. 
The  solution  is  boiled  until  the  carbon  dioxide  is  entirely  expelled. 
It  is  then  cooled  and  the  boric  acid  titrated,  using  phenolphthalein 
and  glycerine  or  mannitol  as  the  indicator. 

DETERMINATION  OF  CARBONIC  ACID  IN  WATER. 

The  determination  of  the  percentage  of  carbonates  and  bicar- 
bonates  when  existing  as  alkali  salts,  and  in  the  case  of  the  former 
in  the  presence  of  caustic  alkali,  has  already  been  given.  The 
determination  of  carbonic  acid  when  present  as  an  insoluble  car- 
bonate is  best  carried  out  gravimetrically,  as  given  in  Chapter 
IX.  Carbon  dioxide  existing  in  solution,  as  hi  natural  or 
artificial  mineral  waters,  is  generally  determined  volumetrically. 
It  exists  in  three  conditions  in  such  waters:  as  FREE  CARBONIC 
ACID;  as  SEMICOMBINED  ACID,  or  BICARBONATES  of  the  alkali  or 
alkaline-earth  metals;  and  as  COMBINED  ACID,  or  CARBONATES  of 
the  alkali  metals. 

403.  The  Percentage  of  Free  Carbon  Dioxide  may  be  deter- 
mined by  titrating  100  c.c.  of  the  water  with  a  standard  solution 
of  caustic  soda  or  sodium  carbonate  after  the  addition  of  a  few 
drops  of  phenolphthalein.  The  alkali  should  be  dilute,  tenth  or 
twentieth  normal  being  convenient.  When  sufficient  alkali  has 
been  added  to  give  a  faint  pink  to  the  indicator,  all  the  carbon 
dioxide  has  been  converted  into  bicarbonate  and  a  slight  excess  of 
carbonate  is  present,  which  produces  the  color  with  the  indicator. 
The  titration  must  be  repeated,  adding  at  once  nearly  all  the 
alkali  used  in  the  first  titration  as  some  carbon  dioxide  may 
have  been  lost  during  the  stirring  of  the  acid  solution. 


TITRATION  OF  CARBONIC  ACID.  297 

If  free  carbon  dioxide  is  present,  normal  carbonates  must 
be  absent,  since  such  salts  would  react  with  the  free  acid  to 
form  bicarbonates  as  follows: 

Na2C03+ H2C03  =2NaHC03, 

Only  bicarbonates,  therefore,  can  exist  in  a  solution  containing 
free  carbonic  acid.  On  the  addition  of  caustic  soda  the  following 
reaction  takes  place: 

NaOH+ H2C03  =NaHCO,+ H20. 

40.058  parts  of  caustic  soda  are  therefore  equivalent  to  44  parts 
of  carbon  dioxide.  1  c.c.  of  N/10  caustic  soda  will  therefore  be 
equal  to  .004400  gram  of  C02. 

With  sodium  carbonate  solution  the  following  reaction  takes 
place : 

Na2C03  +H2C03  =2NaHC03. 

106.1  parts  of  sodium  carbonate  are  therefore  equal  to  44  parts 
of  carbon  dioxide.  1  c.c.  of  N/10  sodium  carbonate  solution  will 
therefore  be  equal  to  .0022  gram  of  C02.  The  use  of  an  alkaline 
solution  of  which  1  c.c.  is  equal  to  1  mg.  of  carbon  dioxide  is 
very  convenient.  2.412  g.  of  sodium  carbonate  are  equal  to  1 

g.  of  carbon  dioxide  f  ..  =  2.412  j.  Probably  the  most  con- 
venient method  of  procedure  is  to  dissolve  2.412  g.  of  pure 
anhydrous  sodium  carbonate  in  1  liter  of  distilled  water  from 
which  the  carbon  dioxide  has  been  expelled  by  boiling.  1  c.c.  of 
this  solution  will  be  equal  to  1  mg.  of  carbon  dioxide. 

If  the  water  is  neutral  to  phenolphthalein  only  bicarbonates 
can  be  present.  The  water  may  be  tested  for  the  presence  of 
FREE  MINERAL  ACIDS  by  titration  with  the  standard  solution  of 
alkali,  using  methyl  orange  as  the  indicator.  The  difference 
between  the  amounts  of  alkali  used  in  this  titration  and  the  one 
in  which  phenolphthalein  is  used  as  the  indicator  gives  the 
amount  of  alkali  necessary  to  neutralize  the  free  carbonic  acid. 
In  the  presence  of  free  mineral  acid  semi-combined  or  combined 
carbonic  acid  would  be  absent. 

404.  The  Amount  of  Semi-combined  Carbon  Dioxide  may  be 
determined  by  titrating  100  c.c.  of  the  water  with  dilute  standard 


298  VOLUMETRIC  METHODS. 

acid,  using  methyl  orange  as  the  indicator.  The  titration  with  acid 
and  methyl  orange  gives  the  amount  of  semi-combined  carbon  di- 
oxide only  when  the  water  is  acid  or  neutral  to  phenolphthalein,  as 
normal  carbonates  are  then  absent.  If  the  water  is  alkaline  to 
phenolphthalein  free  carbon  dioxide  is  absent  and  only  carbon- 
ates and  bicarbonates  can  be  present.  By  titrating  100  c.c.  of  such 
a  water  with  phenolphthalein  and  dilute  (N/20)  acid  until  the  pink 
color  is  removed  and  then  adding  methyl  orange  and  continuing 
the  addition  of  the  acid  until  the  pink  color  of  the  methyl  orange 
is  developed,  the  amount  of  combined  and  semi-combined  acid 
may  be  calculated.  The  combined  carbon  dioxide  is  calculated 
from  the  number  of  cubic  centimeters  of  acid  used  in  the  first 
stage  of  the  titration,  1  c.c.  of  N/20  acid  being  equal  to  .0022  gram 

/44  00  \ 
of  carbon  dioxide  f     '      ),  tho  following  reaction  taking  place: 

HC1  +Na2C03  =NaHC03  +NaCL 

The  number  of  cubic  centimeters  of  acid  used  in  the  second  stage 
of  the  titration  minus  the  amount  used  in  the  first  stage  gives  the 
amount  of  semi-combined  carbon  dioxide,  1  c.c.  of  N/20  acid 
being  equal,  as  before,  to  .0022  gram  of  carbon  dioxide. 

405.  The  Total  Amount  of  Carbon  Dioxide  may  be  obtained  by 
adding  together  the  amount  of  free  carbon  dioxide  to  that  semi- 
combined;  or  when  free  carbon  dioxide  is  absent,  by  adding  the 
amount  of  semi-combined  to  the  amount  combined.     The  amount 
of  free  and  semi-combined  carbon  dioxide  is  frequently  deter- 
mined by  PETTENKOFER'S  METHOD.     In  this  process  a  measured 
amount  of  a  standard  solution  of  calcium  or  barium  hydroxide 
is  added  to  a  measured  volume,  generally  100  c.c.,  of  the  water  to 
be  tested.     The  free  and  semi-combined  carbon  dioxide  is  precipi- 
tated as  barium  or  calcium  carbonate  with  consequent  decrease  of 
alkalinity  of  the  solution.    The  excess  of  alkali  is  titrated  back  with 
dilute  acid.     The  total  amount  of  carbon  dioxide  is  calculated 
from  the  excess  of  barium  hydroxide  plus  the  initial  alkalinity  of 
the  water : 

Ba(OH)2+ H2C03  -  BaC03+ 2H20; 
Ba(OH)2+ Ca(HC03)2  =  BaC03+ CaC03+ 2H20. 

406.  If  Sulphates  or  Carbonates  of  the  Alkalies  are  present,  the 
precipitated  barium  salts  carry  down  some  of  the  alkalies  unless 


TITRATION  OF  CARBONIC  ACID.  299 

the  barium  was  present  as  chloride,  so  as  to  leave  the  alkali  metals 
as  chlorides.  For  this  reason  a  little  neutral  barium  chloride 
solution  should  be  added. 

407.  Determination  of  Total  Amount  of  Carbon  Dioxide  in  the 
Presence  of  Magnesium.  —  If  magnesium  bicarbonate  is  present, 
an  error  is  introduced  into  the  determination  as  shown  by  the 
following  equations: 


+Ba(OH)2  =BaC03  +MgC03  +2H20; 
MgC03  +Ba(OH)2  =BaC03  +Mg(OH)2. 

As  the  magnesium  hydroxide  is  insoluble  the  alkalinity  of  the 
solution  has  been  decreased  by  2  molecules  of  barium  hydroxide, 
whereas  with  other  bicarbonates  but  1  molecule  is  neutralized. 
To  remedy  this  difficulty  Pettenkofer  added  2  c.c.  of  a  saturated 
solution  of  ammonium  chloride  so  that  the  magnesium  hydroxide 
should  remain  in  solution.  According  to  Trillion*  this  is  only 
a  partial  remedy  of  the  difficulty.  He  proposes  to  omit  the  addi- 
tion of  the  ammonium  chloride  and  apply  a  correction  to  the 
result  from  a  separate  determination  of  magnesium. 

The  DETERMINATION  WAS  CARRIED  OUT  according  to  Petten- 
kofer as  follows:  100  c.c.  of  the  water  are  placed  in  a  flask  and  to 
it  are  added  3  c.c.  of  a  10%  solution  of  calcium  or  barium  chloride 
and  2  c.c.  of  a  saturated  solution  of  ammonium  chloride,  as  well 
as  45  c.c.  of  a  solution  of  barium  hydroxide  of  such  a  strength 
that  1  c.c.  =1  mg.  of  carbon  dioxide.  The  flask  is  then  corked 
and  allowed  to  stand  for  twelve  hours.  50  c.c.  of  the  clear  liquid 
are  withdrawn  and  the  excess  of  alkali  titrated  with  standard 
acid.  Trillich  omits  the  addition  of  ammonium  chloride,  so  that 
the  magnesium  hydroxide  is  entirely  precipitated.  The  amount 
of  magnesium  hydroxide  is  calculated  from  a  gravimetric  deter- 
mination, and  its  equivalent  in  barium  hydroxide  subtracted 
from  the  amount  of  barium  hydroxide  neutralized  by  the  water. 

408.  Precipitation  of  Carbon  Dioxide  as  Calcium  Carbonate.  — 
When  water  contains  a  considerable  amount  of  carbon  dioxide 
so  that  there  is  danger  of  loss  during  the  manipulation,  a  sample 
should  be  collected  in  a  measuring-flask  which  contains  a  measured 
volume  of  an  alkaline  solution  of  calcium  chloride  made  as  follows: 
*  Zeit.  f.  angew.  Chem.,  1889,  pp.  117,  337. 


300  VOLUMETRIC  METHODS. 

To  one  part  of  crystallized  calcium  chloride  5  parts  of  water  are 
added  and  10  parts  of  ammonium  hydroxide  sp.  gr.  0.96.  This 
solution  is  placed  in  well-stoppered  bottles  and  allowed  to  stand 
several  days.  The  clear  supernatant  liquid  is  drawn  off  as  needed. 
On  the  addition  of  this  reagent  to  carbonated  water  all  of  the 
carbonic  acid  present  is  precipitated  as  calcium  carbonate.  The 
precipitate  is  at  first  amorphous,  but  on  standing  for  at  least 
twelve  hours  it  becomes  crystalline.  This  change  may  be  hastened 
by  warming  on  the  water-bath.  The  precipitate  must  not  be 
filtered  off  until  it  has  been  digested  in  this  manner  or  stood  at 
least  12  hours.  It  is  then  filtered  off  and  washed  free  from  alkali. 
The  carbon  dioxide  may  now  be  determined  in  the  calcium  car- 
bonate by  various  methods.  A  very  simple  method  consists 
in  dissolving  the  carbonate  in  excess  of  standard  acid  and  titrating 
back  the  excess  with  standard  alkali. 

409.  If  Bottled  Carbonated  Waters  are  to   be  Examined,  the 
bottles  may  be  opened  so  as  not  to  lose  carbon  dioxide  by  a  very 
simple  device.    An  ordinary  brass  cork-borer  of  suitable  size  is 
taken,  and  several  small  holes  are  drilled  a  little  farther  from  the 
cutting  end  than  the  length  of  the  cork  to  be  pierced.    A  rubber 
tube  is  securely  fastened  to  the  upper  end  of  the  borer,  and  con- 
nected to  a  convenient  absorption  apparatus  containing  some  of 
the  alkaline  calcium  chloride  solution.     The  cork  of  the  bottle 
is  pierced  with  the  cork-borer  by  holding  the  latter  firmly  in  the 
hand  and  turning  the  bottle  around.    When  one  of  the  small 
holes  in  the  borer  has  been  brought  below  the  cork  the  carbon 
dioxide  will  pass  out  through  the  cork-borer  and  into  the  absorp- 
tion apparatus.    When  no  more  carbon  dioxide  is  evolved  the 
bottle  is  warmed  by  placing  it  in  lukewarm  water,  which  is  then 
brought  nearly  to  a  boil.     The  absorption  apparatus  is  then  dis- 
connected, and  the  carbon  dioxide  swept  out  of  the  connecting 
tube  by  means  of  air  freed  from  carbon  dioxide.    The  contents  of 
the  bottle  are  added  to  the  liquid  in  the  absorption  apparatus, 
the  solution  digested  on  the  water-bath  for  some  time,  and  finally 
the  calcium  carbonate  is  filtered  off  and  washed.    The  calcium 
carbonate  may  then  be  titrated  with  standard  acid  and  alkali. 

410.  Determination  of   Carbon  Dioxide  in  Gases. — When  car- 
bon dioxide  occurs  in  considerable  amount  in  mixtures  of  gases 


TITRATION  OF  CARBONIC  ACID.  301 

it  is  determined  by  gasometric  methods.  When  it  occurs  in 
amounts  considerably  less  than  1%  it  is  customary  to  determine 
it  gravimetrically  by  passing  the  gas  through  an  appropriate 
series  of  absorption-tubes  so  that  the  carbon  dioxide  may  finally 
be  weighed.  More  frequently,  however,  the  carbon  dioxide  is 
determined  volumetrically. 

411.  Pettenkofer's  Method. — Many  methods  of  procedure  and 
forms  of  apparatus  have  been  proposed,  but  nearly  all  of  them 
follow  in  general  the  method  suggested  by  Pettenkofer,  which 
consists  in  absorbing  the  carbon  dioxide  in  a  measured  volume 
of  standard  alkali  and  titrating  back  the  excess.     Pettenkofer 
used  barium  hydroxide  and  oxalic  acid  as  the  standard  solutions. 
He  took  a  large-sized  bottle,  aspirated  the  air  to  be  examined 
through  it  until  the  air  already  present  was  completely  displaced, 
and  added  a  measured  amount  of  the  standard  alkali.    On  shaking 
the  bottle  vigorously  for  some  time  the  carbon  dioxide  was  com- 
pletely absorbed.    The  alkali  was  poured  into  a  cylinder  or  bottle, 
the   precipitate   allowed    to   settle,    a   measured   volume    with- 
drawn, and  the  excess  of  alkali  titrated  with  the  oxalic  acid,  using 
phenolphthalein  as  the  indicator. 

412.  Several   Difficulties    and    Errors   are   met   with   in   this 
method.    The  surface  of  the  glass  bottle  is  capable  of  holding 
barium  hydroxide  so  that  more  alkali  is  apparently  removed  from 
the  solution  than  corresponds  to  the  carbon  dioxide  present.    To 
overcome  this  difficulty  various  remedies  have  been  proposed. 
The  bottle  may  be  rinsed  with  some  of  the  barium  hydroxide 
solution,  washed  free  from  alkali  with  distilled  water,  and  then 
dried.    A  better  method  much  used  consists  in  coating  the  inside 
of  the  bottle  with  a  thin  layer  of  paraffine.    This  method  is  suc- 
cessful where  a  coat  of  paraffine  can  be  obtained  which  does  not 
peel  off.    Still  another  method  consists  in  shaking  the  bottle  with 
moist  freshly-precipitated  barium  carbonate.    A  thin  coating  of 
this  salt  adheres  quite  firmly  to  the  glass,  and  acts  quite  efficiently 
as  a  protective  agent. 

413.  Apparatus. — Another    source   of   error   is   found   in   the 
contamination   of   the   barium  hydroxide   solution  with   carbon 
dioxide  while  being  poured  from  the  large  bottle  into  a  suitable 
vessel  for  allowing  the  precipitate  to  settle.     In  this  case   the 


302 


VOLUMETRIC  METHODS. 


most  serious  contaminating  influence  is  the  breath  of  the  operator. 
A  form  of  apparatus  recently  described  by  Sewaschen  *  seems 
well  adapted  to  overcome  this  difficulty.  The  large  bottle  A  has 
a  capacity  of  about  6  liters.  The  exact  volume  is  carefully  deter- 
mined, most  conveniently  by  the  Morse-Blalock  bulbs.  The  vol- 
ume is  taken  to  a  mark  c  on  the  neck,  to  which  the  rubber  stopper 
B  must  always  be  inserted.  The  small  bottle  C  should  have  a 
capacity  of  very  nearly  100  c.c.,  the  exact  capacity  having  been 


6000 cc 


FIG.  51. 

carefully  determined  when  the  stopper  D  is  inserted  to  the  mark 
d  on  the  neck,  the  bottle  being  filled  so  that  no  air-bubbles  are 
left  under  the  stopper  and  the  glass  rod  b  inserted.  The  volume 
of  the  neck  from  the  point  d  must  also  be  determined  and  added 
to  that  of  the  large  bottle.  The  neck  of  the  small  bottle  must  be 
free  from  any  projecting  ridge,  and  must  fit  quite  snugly  into  the 
opening  in  the  stopper  B.  If  a  suitable  100-c.c.  bottle  is  not  at 
hand,  a  stout  100-c.c.  Florence  flask  may  be  used.  The  projecting 
rim  on  the  neck  may  be  quite  readily  cut  off  by  means  of  a  dia- 
mond, a  small  flame,  or  other  glass-cutting  device. 

414.  Manipulation. — To  use  this  apparatus,  the  bottle  A  is 
cleaned  and  dried  and  the  inner  surface  treated  by  one  of  the 
methods  already  given.  The  air  to  be  examined  is  aspirated 
through  the  bottle  after  the  stopper  B  is  inserted.  While  the 
large  bottle  is  being  filled  with  air  the  standard  solution  of  barium 
hydroxide  is  measured  out  into  the  small  bottle,  the  stopper  D 


*  Hygienische  Rundschau,  No.  9,  1897. 


TITRATION  OF  CARBONIC  ACID.  303 

inserted,  and  finally  the  rod  b.  The  large  bottle  is  turned  on  its 
side  and  the  neck  of  the  small  bottle  inserted  into  the  stopper  B 
after  removing  the  stopper  D.  On  turning  the  large  bottle  upright 
the  barium  hydroxide  solution  flows  out  of  the  small  bottle  and 
comes  in  contact  with  the  air  in  the  large  bottle.  The  latter  is 
shaken  occasionally  for  about  two  hours,  when  the  absorption  of 
the  carbon  dioxide  will  be  complete.  By  inverting  the  large 
bottle  the  barium  hydroxide  solution  will  flow  into  the  small 
bottle,  which  is  then  disconnected  from  the  large  bottle,  the 
stopper  D  inserted,  and  the  barium  carbonate  allowed  to  settle, 
which  requires  from  twelve  to  twenty-four  hours.  Measured 
volumes  are  then  withdrawn  and  titrated  with  the  standard 
acid. 

415.  Barium    Hydroxide  has  been  used  most  largely  as    the 
alkaline  solution  in  which  to  absorb  the  carbon  dioxide.    The 
barium  hydroxide  is  seldom  free  from  traces  of  the  alkalies,  which 
exert  a  disturbing  influence  on  the  titration  when  oxalic  acid  is 
used  and  barium  carbonate  is  present.     The  potassium  or  sodium 
oxalate  formed  in  the  solution  reacts  with  the  barium  carbonate 
as  follows: 

K2C204+ BaC03  =BaC204+ K2C03. 

The  potassium  carbonate  is  converted  into  oxalate  by  the  further 
addition  of  oxalic  acid.  Carbon  dioxide  is  liberated,  and  the 
potassium  oxalate  reacts  with  more  of  the  barium  carbonate, 
according  to  the  equation  given.  More  oxalic  acid  is  therefore  used 
than  corresponds  to  the  amount  of  free  alkali  originally  present. 
This  action  is  entirely  prevented  by  the  addition  of  a  little  barium 
chloride  to  the  solution,  which  converts  any  potassium  or  sodium 
salts  present  into  chlorides.  The  solution  must  be  carefully  pro- 
tected from  carbon  dioxide  by  one  of  the  methods  already  given. 
It  is  standardized  by  titration  against  standard  acid. 

416.  Oxalic  Acid  has  been  most  largely  used  to  titrate  the 
excess  of  the  alkali.    2.8636  *  grams  of  the  pure  recrystallized  acid 
dissolved  in  one  liter  of  distilleJ  water  gives  a  solution  1  c.c.  of 
which  is  equal  to  1  mg.  of  carbon  dioxide.    As  dilute  solutions 
of  oxalic  acid  are  not  stable,  a  fresh  solution  must  be  made  as 
needed  by  weighing  out  the  crystallized  acid. 

*  Note  to  student. — Show  how  this  number  is  calculated. 


304  VOLUMETRIC  METHODS. 

417.  Sulphuric  Acid  of  equivalent  strength  may  also  be  used 
with  advantage.    The  presence  of  barium  carbonate  does  not  inter- 
fere so  much  with  the  titration  when  sulphuric  acid  is  used.     Some 
workers  who  use  this  acid  titrate  the  excess  of  the  barium  hy- 
droxide without  removing  any  of  the  barium  carbonate.     Alkalies, 
if  present,  do  not  interfere  with  the  titration  if  sulphuric  acid  is 
used.     It  may  be  made  by  diluting  a  stronger  solution.     45.45 
c.c.  of    a  normal  solution  or  227.27*  c.c.  of  a  fifth-normal  acid 
diluted  to  a  liter  will  give  an  acid  1  c.c.  of  which  is  equal  to 
1  mg.  of  carbon  dioxide.    The  acid  may  also  be  made  by  diluting 
concentrated  acid  so  as  to  make  an  approximate  solution,  which  is 
then  standardized  by  titration  with  the  barium  hydroxide  solu- 
tion, which  in  turn  is  standardized  by  titration  against  weighed 
amounts  of  pure  crystallized  oxrJic  acid.     Phenolphthalein  is  used 
as  the  indicator  in  all  titrations. 

418.  The  Results  of  the  Analysis  must  be  expressed  in  per 
cent  by  volume,  the  amount  of  carbon  dioxide  in  the  air  being 
given  in  parts  by  volume  in  10,000.     The  temperature  of  the  air 
and  the  barometric  pressure  must  be  taken  at  the  time  the  large 
bottle  is  filled.     In  very  careful  work  the  tension  of  water  vapor  in 
the  air  must  also  be  taken.  The  volume  of  the  air  at  0°  and  760  mm. 
pressure    must    then    be    calculated    according    to    the    formula 

V(P—  v) 

7o°-760  =  (l  +  0.00366670760'  where  V  is  the  V°lume  °f  the  krge 
bottle  used,  P  is  the  barometric  pressure  in  millimeters,  and  t  the 
temperature  when  the  sample  of  air  is  taken,  p  is  the  tension  of 
water  vapor  in  millimeters  of  mercury.  The  amount  of  carbon 
dioxide,  being  obtained  in  milligrams,  must  be  reduced  to  cubic 
centimeters  by  multiplying  the  number  of  milligrams  found  by  .508. 

EXERCISE  55. 
Determination  of  Carbon  Dioxide  in  the  Air. 

419.  Standard   Solutions.  —  Prepare  a  saturated  solution  of  barium  hy- 
droxide by  treating  15  to  20  grams  of  barium  hydroxide  with  warm  dis- 
tilled water.      Place  in  a  well-stoppered  bottle,  and   when  the  insoluble 
matter  has  settled,  draw  off  by  a  siphon  or  large  pipette  about  250  c.c. 
and  dilute  to  1  liter.     Protect  the  solution  from  the  carbon  dioxide  of  the 
air  by  placing  the  solution  in  a  bottle  arranged  as  shown  in  Fig.  50,  p.  290. 

*  Note  to  student. — Show  how  this  number  is  calculated. 


TITRATION  OF  CARBONIC  ACID.  305 

Measure  out  227.27  c.c.  of  N/5  sulphuric  acid  by  means  of  a  burette 
and  dilute  to  1  liter.  If  standard  sulphuric  acid  is  not  at  hand,  weigh  out 
2.8647  grams  of  oxalic  acid  and  dilute  to  a  liter.  If  the  oxalic  acid  is  used, 
about  0.2  gram  of  barium  chloride  must  be  added  to  the  barium  hydroxide 
solution. 

The  bottles  described  on  p.  302  are  prepared : 

420.  Collecting  the  Sample. — The  rubber  stopper  is  inserted  in  the  large 
bottle  A,  and  into  the  hole  of  the  stopper  is  inserted  a  smaller  two-holed 
rubber  stopper  through  which  two  glass  tubes  pass,  one  of  which  extends 
to  the  bottom  of  the  bottle,  while  the  other  passes  through  the  stopper 
only.     The  tube  extending  to  the  bottom  of  the  bottle  is  connected  to  a 
long  tube  passing  out  of  a  window.     The  smaller  glass  tube  is  connected 
to  a  Bunsen  suction-pump,  which  is  allowed  to  draw  air  through  the  bottle 
for  half  an  hour.      The  small  bottle  is  filled  with  the  barium  hydroxide 
solution,  the  rubber  stopper  inserted  so  as  not  to  enclose  any  air-bubbles, 
and  after  pressing  it  down  to  the  mark,  the  excess  of  barium  solution  is 
removed  and  the  glass  plug  inserted. 

When  the  large  bottle  has  been  filled  with  the  outside  air,  the  stopper 
containing  the  glass  tubes  is  removed.  The  stopper  is  also  removed  from  the 
small  bottle,  which  is  inserted  into  the  hole  in  the  stopper  of  the  large  bottle, 
which  for  this  purpose  is  placed  on  its  side.  The  temperature  is  taken  from 
a  thermometer  which  has  been  hung  by  the  large  bottle.  The  barometer  is 
also  read.  The  large  bottle  is  shaken  at  intervals  for  about  two  hours. 
It  is  then  inverted,  the  small  bottle  containing  the  barium  hydroxide 
solution  is  removed,  and  the  stopper  inserted. 

421.  Titration. — In  the  meantime  the  standard  solutions  are  compared, 
the  titrations  being  carried  out  rapidly  to  prevent  absorption  of  carbonic 
acid.     Phenolphthalein  is  used  as  the  indicator.     If  oxalic  acid  is  used,  the 
solution  in  the  small  bottle  must  stand  overnight  before  being  titrated. 
If  sulphuric  acid  is  used,  it  may  be  titrated  when  convenient,  the  glass  plug 
being  withdrawn  from  the  stopper,  and  25  c.c.  of  the  solution  withdrawn 
by  means  of  a  pipette  and  titrated  immediately.     Two  or  three  titrations 
should  be  made  with  25-c.c.  portions.     The  difference  between  the  amount 
of  acid  used  for  these  25-c.c.  portions  and  the  amount  used  in  the  stand- 
ardization for  25  c.c.  is  found  and  multiplied  by  the  ratio  of  the  volume  of 
the  small  bottle  to  25.     This  gives  the  number  of  cubic  centimeters  of 
acid  equivalent  to  the  carbon  dioxide  in  the  sample  of  air  taken.     Calculate 
the  number  of  cubic  centimeters  of  carbon  dioxide  by  multiplying  the  num- 
ber of  milligrams  by  .508.     Calculate  the  volume  of  air  taken  at  760  mm. 
and  0°.     Divide  the  former  by  the  latter  and  multiply  by  10,000  to  find 
the  number  of  parts  of  carbon  dioxide  per  10,000. 


OXIDATION  AND  REDUCTION  METHODS. 

CHAPTER  XXIV. 

POTASSIUM  PERMANGANATE  AND  BICHROMATE 
SOLUTIONS. 

422.  Oxidation.  —  A  number  of  very  excellent  volumetric 
methods  are  based  on  the  fact  that  many  substances  exist  in  two 
states  of  oxidation.  In  the  case  of  arsenic,  we  have  arsenious 
oxide,  As203,  and  arsenic  oxide,  As205.  Sulphur  forms  several 
compounds  of  varying  oxygen  content,  as  sulphurous  acid,  H2S03, 
and  sulphuric  acid,  H2S04.  Substances  capable  of  giving  up  oxy- 
gen, such  as  POTASSIUM  PERMANGANATE,  KMn04,  convert  the  former 
into  the  latter.  The  oxidizing  substance  itself  may  not  contain 
oxygen,  but  may  be  able  to  oxidize  by  acting  on  water  or  some  inter- 
mediary substance.  IODINE  is  such  a  substance  which  is  capable 
of  converting  sodium  arsenite  into  sodium  arsenate,  as  follows: 

Na3As03+ 1,+ H20  =  NasAs04+ 2HI. 

An  alkali  must  be  present  to  unite  with  the  hydriodic  acid  so 
that  the  action  might  be  represented  as  follows: 

Na3As03 + 12 + 2NaOH  =  Na3As04 + 2NaI  +  H20. 

The  mechanism  of  the  reaction  is  probably  quite  complicated, 
as  many  reactions  are  possible  under  the  circumstances.  It  may, 
however,  be  carried  out  so  that  2  atoms  of  iodine  are  equal  to 
1  atom  of  oxygen. 

The  result  of  the  oxidation  may  not  even  be  the  addition  of 
oxygen,  as  when  ferrous  chloride,  FeCl2,  is  converted  into  ferric 
chloride,  FeCl3.  Perhaps  the  simplest  DEFINITION  of  OXIDATION 
is  the  change  in  valence  of  an  element  by  the  addition  of  negative 
atoms  or  radicles  or  the  removal  of  positive  atoms  or  radicles.  The 
change  in  valence  may  not  always  be  an  increase,  though  it 
generally  is.  The  oxidation  of  ammonia  may  result  in  a  decrease 

306 


AVAILABLE  OXYGEN.  307 

of  valence.  It  is  apparent  that  the  reactions  taking  place  during 
oxidation  and  reduction  are  more  complicated  than  those  involved 
in  acidimetry  and  alkalimetry. 

423.  Normal  Oxidizing  and  Reducing  Solutions. — The  strength 
of  all  oxidizing   and    reducing   solutions   may    nevertheless    be 
expressed  in  as  simple  terms  as  the  acid  and  alkaline  solutions. 
The  basis  selected  is  oxygen,  the  strength  of  all  standard  oxidizing 
solutions  being  expressed  in  terms  of  AVAILABLE  OXYGEN  or  its 
equivalent  in  chlorine,  iodine,  etc.     A  normal  oxidizing  solution  is 
defined  as  one  which  contains  8  grams  of  available  oxygen  or  its 
equivalent  per  liter.     HYDROGEN  has  been  selected  as  the  basis  of 
reducing  solutions,  1  liter  of  a  normal  reducing  solution  containing 
1  gram  of  available  hydrogen  or  its  equivalent. 

424.  Available    Oxygen. — The    distinction   between    available 
and  non-available  oxygen  must  be  carefully  made,  just  as  the 
acid  hydrogens  of  a  molecule  must  be  distinguished  from  the 
non-acid  atoms  in  acidimetry.     Of  the  4  atoms  of  oxygen  in  the 
molecule   of   POTASSIUM   PERMANGANATE   but   2J   are   available. 
The    decomposition   of  2  molecules  of   this  compound   may  be 
represented  as  follows: 

2KMn04  =  K20  +  2MnO  +  50. 

The  oxidation  is  generally  carried  out  in  acid  solution,  the  potas- 
sium oxide  and  manganous  oxide  combining  with  the  acid  as 
follows : 

K20+ 2MnO +3H2S04  =K2S04+  2MnS04 +3H20. 

Five  atoms  of  oxygen,  or  80  parts  by  weight,  being  free  for  oxida- 
tion purposes,  one-tenth  of  the  gram  molecular  weight  of  2  mole- 
cules of  potassium  permanganate,  or  31.63  grams  must  be  dis- 
solved in  1  liter  to  make  a  normal  solution. 

The  decomposition  of  POTASSIUM  BICHROMATE  is  similar: 

K2Cr207  =K20  +Cr203  +30. 

This  substance  is  also  used  in  acid  solution,  the  potassium  and 
chromium  oxides  reacting  with  sulphuric  acid  as  follows: 

K20 +Cr203+ 4H2S04  =K2S04+ Cr2(S04)3+4H20. 

As  3  atoms  of  oxygen,  or  48  grams,  are  available,  one-sixth  of 
the  gram  molecular  weight  of  potassium  dichromate  must  be  dis- 


308  VOLUMETRIC  METHODS. 

solved  in  a  liter  to  obtain  a  solution  containing  8  grams  of  avail- 
able oxygen  per  liter.  IODINE  reacts  as  follows  in  the  presence 
of  a  substance  capable  of  absorbing  oxygen: 

I2+H20=2HI-K). 

Two  atoms  of  iodine  liberate  1  atom  of  oxygen.  The  weight  of 
1  atom  of  iodine  in  grams  must  therefore  be  dissolved  in  a  liter 
to  give  a  normal  solution. 

425.  Indicators. — No  indicator  is  known  which  directly  shows 
the  presence  or  absence  of  available  oxygen  or  hydrogen  in  a 
solution.  POTASSIUM  PERMANGANATE  serves  as  its  own  indicator. 
As  it  has  an  intense  reddish-purple  color  in  water  solution,  and 
when  reduced  in  acid  solution  forms  the  colorless  potassium 
and  manganous  sulphates,  the  least  excess  of  the  oxidizing  solu- 
tion is  indicated  by  the  characteristic  permanganate  color.  As 
long  as  even  a  trace  of  a  reducing  substance  is  present  the  color 
fades  out  by  reduction  of  the  permanganate. 

As  POTASSIUM  DICHROMATE  is  yellow  and  on  being  reduced 
gives  the  green  chromic  salts  it  is  impossible  to  detect  the  presence 
of  a  small  amount  of  the  yellow  dichromate  in  the  presence  of 
the  large  amount  of  green  chromic  salts.  The  dichromate  solu- 
tion is  generally  used  with  a  ferrous  iron  solution  as  the  reducing 
substance.  In  acid  solution  potassium  dichromate  instantly 
converts  any  ferrous  iron  present  into  ferric  iron.  A  very  delicate 
test  for  ferrous  iron  is  found  in  the  blue  color  which  even  very 
small  amounts  produce  in  a  solution  of  POTASSIUM  FERRIC YANIDE. 
A  solution  of  ferrous  iron  to  which  dichromate  solution  has 
been  added  may  be  tested  for  the  presence  of  the  former  by 
taking  out  a  drop  of  the  solution  and  adding  it  to  a  drop  of  the 
indicator.  When,  after  the  successive  addition  of  small  amounts 
of  dichromate  solution,  a  test  for  ferrous  iron  is  no  longer  obtained, 
the  tit  ration  of  the  ferrous  iron  is  complete. 

Solutions  of  IODINE  are  deep  red  to  yellow  according  to  the 
concentration.  After  reduction  the  iodine  forms  colorless  salts. 
If  the  substance  oxidized  also  forms  colorless  salts  the  titration 
may  be  conducted  by  using  the  IODINE  COLOR  as  the  indicator. 
This  gives  fairly  accurate  results,  but  a  much  more  delicate  indi- 
cator for  iodometric  work  is  found  in  STARCH  SOLUTION,  which 


POTASSIUM  PERMANGANATE.  309 

gives  an  intense  blue  color  when  the  least  excess  of  free  iodine 
is  present.  Because  of  the  delicacy  of  this  method  of  obtaining 
the  end-point,  iodometric  titrations  are  among  the  most  accurate 
known  to  the  chemist. 

POTASSIUM   PERMANGANATE. 

426.  Decomposition  of  the  Permanganate  by  Manganese  Diox- 
ide.— This  salt  may  now  be  purchased  in  a  high  state  of  purity. 
As  it  is  not  hygroscopic,  solutions  made  by  weighing  out  the  C.P. 
article  and  dissolving  in  water  are  accurate  within  less  than  J%. 
Sulphates,  nitrates,  and  chlorides  are  the  common  impurities 
which  must  be  absent  from  the  C.P.  article.  It  is  almost  impossi- 
ble, however,  to  prepare  a  sample  of  this  salt  which  is  free  from 
small  amounts  of  manganese  dioxide.  The  injurious  effect  of 
this  impurity  on  solutions  of  potassium  permanganate  has  been 
clearly  shown  by  Morse  and  his  pupils.*  He  has  proved  that 
the  permanganate  is  slowly  decomposed  by  even  traces  of  the 
dioxide  with  liberation  of  oxygen  and  formation  of  more  of  the 
dioxide.  As  the  rate  of  decomposition  is  proportional  to  the 
amount  of  the  dioxide,  the  decomposition  is  accelerated  with  the 
age  of  the  solution.  The  remedy  consists  in  the  complete  removal 
of  the  dioxide.  This  is  easily  accomplished  by  filtering  the  per- 
manganate solution  through  asbestos  which  has  been  digested  with 
strong  aqua  regia  and  well  washed.  Organic  matter,  dust,  etc., 
which  may  reduce  the  permanganate  with  formation  of  the  diox- 
ide must  be  rigorously  excluded  from  the  solution  after  nitration. 
All  bottles  and  glassware  with  which  the  solution  may  come  in 
contact  must  be  freed  from  organic  matter  by  washing  with 
dichr ornate  cleaning  mixture.  As  distilled  water  usually  contains 
ammonia  and  organic  matter,  it  is  well  to  allow  the  permanganate 
solution  to  stand  some  time  before  filtration  so  as  to  entirely 
decompose  this  organic  matter.  The  distilled  water  may  also  be 
purified  by  adding  a  very  little  potassium  permanganate,  boiling 
until  the  permanganate  is  decomposed,  and  then  filtering  off  the 
manganese  dioxide  through  asbestos.  If  the  permanganate  solution 
is  made  up  in  this  manner  and  kept  out  of  strong  sunlight  it  is  very 

*Am.  Chem.  Jour.,  18,  411;   23,  313. 


VOLUMETRIC  METHODS. 

permanent,  readily  keeping  its  strength  for  months.     Portions  of  the 
solution  withdrawn  from  the  bottle  should  not  be  returned  to  it. 

STANDARDIZATION  OF  POTASSIUM  PERMANGANATE 

SOLUTIONS. 

427.  By  Pure  Iron  Wire. — As  has  been  said,  permanganate 
solutions  made  by  weighing  out  the  pure  salt  are  within  at  least 
J%  of  the  calculated  strength.  A  higher  degree  of  accuracy  is 
obtained  by  carefully  standardizing  the  solution.  The  most  largely 
used  and  perhaps  the  most  reliable  method  consists  in  the  use  of 
known  amounts  of  iron.  Soft-iron  wire  containirg  from  99.6  to 
99.8%  of  iron  may  readily  be  obtained.  The  exact  percentage  of 
iron  may  be  obtained  by  a  gravimetric  determination  of  the  iron  as 
given  in  Exercise  11,  or  of  the  total  carbon,  as  given  on  page  374,  the 
iron  being  found  by  difference.  Formerly  the  practice  was  com- 
mon of  dissolving  the  iron  in  dilute  sulphuric  acid,  the  air  having 
been  excluded  by  means  of  carbon  dioxide.  Under  these  circum- 
stances the  iron  is  converted  into  ferrous  sulphate,  which  was 
then  titrated  with  the  permanganate.  As  one  of  the  impurities 
in  the  iron  is  carbon  which  is  combined  to  a  greater  or  less  extent 
as  a  carbide  of  iron,  the  solution  of  the  wire  in  acid  leads  to  the 
formation  of  hydrocarbons,  which  are  oxidized  by  the  permangan- 
ate. As  one  part  of  carbon  may  reduce  ten  and  one-half  parts  of 
potassium  permanganate,  and  if  the  carbon  is  present  as  a  hydro- 
carbon it  may  reduce  twice  this  amount  of  the  permanganate, 
the  error  from  this  source,  even  if  the  amount  of  carbon  present 
is  very  small,  may  be  very  considerable.  Titrations  of  solutions 
of  iron  wire  have  been  made  which  indicate  as  much  as  104%  of 
metal  in  the  wire  if  calculated  on  the  assumption  that  the  only 
reducing  substance  present  is  ferrous  iron.  Blair  recommends 
the  oxidation  of  the  hydrocarbons  by  means  of  potassium  chlo- 
rate *  or  potassium  permanganate. f  If  this  method  is  pursued 
the  iron  need  not  be  dissolved  in  an  atmosphere  free  from  oxygen, 
as  it  must  be  reduced  after  oxidation  of  the  organic  material.  If 
the  chlorate  is  used  the  iron  is  dissolved  in  hydrochloric  acid,  a 
few  crystals  of  potassium  chlorate  added,  and  the  excess  of 

*  Blair,  The  Chemical  Analysis  of  Iron,  4th  Ed.,  p.  220. 
t  Ibid.,  p.  95. 


POTASSIUM  PERMANGANTE. 


311 


chlorine  expelled  by  boiling.      If  permanganate  is  used  the  iron 
may  be  dissolved  in  sulphuric  acid. 

428.  By  Standard  Iron  Solution.  —  A  very  excellent  reagent 
for    standardizing    permanganate'  solutions   is   recommended   by 
Blair.*     100  grams  of  pure  wrought  iron  free  from  manganese  and 
arsenic,  and  in  which  the  phosphorus   has  been  determined,  is 
weighed  out  and  dissolved  in  nitric  acid.      The  solution  is  evap- 
orated  to    dryness   and    the  residue 

heated  in  a  platinum  crucible  or  dish 
as  hot  as  possible  with  the  blast-lamp. 
The  ignited  material  is  ground  in  an 
agate  mortar,  dissolved  in  hydro- 
chloric acid,  evaporated  to  dryness 
to  dehydrate  the  silica,  taken  up 
with  hydrochloric  acid,  and  the  silica 
filtered  off.  The  filtrate  is  diluted  to 
about  4  liters.  Portions  of  this  solu- 
tion of  15  to  25  c.c.  are  weighed  in 
small  flasks  and  the  iron  precipitated 
and  weighed  as  ferric  oxide.  The 
weight  of  the  phosphoric  acid  must 
be  subtracted  from  the  weight  of  the 
oxide.  The  percentage  of  phosphorus 
in  the  iron  multiplied  by  1.6f  will  give 
the  percentage  by  which  the  weight  of 
the  ferric  oxide  should  be  reduced.  After  determining  in  this  man- 
ner the  exact  amount  of  iron  in  a  weighed  portion  of  the  solution, 
the  permanganate  may  be  standardized  by  taking  weighed  por- 
tions of  the  ferric  chloride  solution,  reducing  the  iron,  and  titrating. 

429.  Reduction  of  Iron  Solutions. — Various  methods  of  reduc- 
ing the  solutions  of  iron  are  in  use.    The  most  common  substance 
used  is  GRANULATED  MOSSY  ZINC.    The  solution  of  the  iron  is  placed 
in  a  flask  of  about  250  c.c.  capacity.     It  may  be  closed  with  a 
Bunsen  valve,  made  by  cutting  a  slit  a  quarter  of  an  inch  long  with 
a  sharp  knife  in  a  small  rubber  tube  which  is  then  closed  at  one 
end  with  a  glass  plug,  while  the  other  end  is  passed  over  a  short 
piece  of  glass  tubing  which  passes  through  the  rubber  stopper 

*  Blair,  The  Chemical  Analysis  of  Iron,  4th  Ed.,  p.  218. 
f  Note  to  student. — How  is  this  number  obtained  ? 


FIG.  52. 


312 


VOLUMETRIC  METHODS. 


into  the  flask,  as  shown  in  Fig.  52.  The  gases  within  the  flask  can 
readily  pass  out  through  this  valve,  while  the  air  cannot  enter. 
The  air  in  the  flask  is  expelled  by  adding  a  little  sodium  carbonate. 
A  few  grams  of  zinc  are  then  added,  and  the  solution  warmed 
nearly  to  boiling.  The  sides  of  the  flask  must  not  be  heated  with 
the  flame  of  the  burner  above  the  point  filled  by  the  solution. 
If  hydrochloric  acid  is  present,  the  complete  reduction  of  the  iron 
is  indicated  by  the  change  in  color  from  the  yellow  of  the  ferric 
chloride  to  the  light  green  of  ferrous  chloride.  Sulphuric  acid  is  then 
added  and  the  solution  heated  until  all  of  the  zinc  is  dissolved. 
Cold,  recently  boiled  water  is  then  added  and  the  solution  allowed 
to  cool.  When  the  stopper  is  removed  from  the  flask  it  should 
be  replaced  as  soon  as  possible,  a  little  sodium  carbonate  having 
been  added  to  expel  the  air. 

430.  Exclusion  of  the  Air. — Another  very  convenient  method 
of  excluding  the  air  consists  of  inserting  into  the  flask  a  one-holed 

rubber  stopper  through  which  passes 
a  glass  tube  bent  to  a  slightly  acute 
angle,  so  that  when  the  flask  is  in- 
clined the  long  end  of  the  glass  tube 
may  be  vertical,  as  shown  in  Fig.  53. 
This  end  dips  into  a  saturated  solu- 
tion of  sodium  bicarbonate.  On  heat- 
ing the  flask  the  air  is  expelled,  and 
on  cooling  a  partial  vacuum  is  formed 
so  that  the  bicarbonate  solution  is 
drawn  up  into  the  flask.  On  meeting 
the  acid  solution,  carbon  dioxide  is 
evolved,  which  prevents  the  remain- 
der of  the  solution  from  enterirg  the 
flask.  After  the  addition  of  zinc  and 
FIG.  53.  reduction  of  the  iron,  sulphuric  acid 

is  introduced  to  dissolve  the  remainder  of  the  zinc.  The  carbon- 
ate solution  is  removed,  boiled  and  cooled  distilled  water  is  added, 
and  the  iron  solution  titrated. 

431.  Titration  of  Iron  in  Zinc. — As  it  is  difficult  to  obtain  zinc 
which  is  entirely  free  from  iron,  the  amount  of  zinc  used  must  be 
roughly  weighed.     The  same  amount  of  zinc  is  weighed  out  and 


POTASSIUM  PERMANGANATE.  313 

treated  with  sulphuric  acid  in  a  flask  exactly  as  in  reducing  the 
iron.  After  all  of  the  zinc  has  been  dissolved  the  solution  is 
titrated  with  the  permanganate.  The  amount  of  solution  used 
in  this  blank  determination  is  subtracted  from  the  volume  of 
permanganate  used  in  titrating  the  iron. 

The  REDUCTION  of  the  iron  solution  by  means  of  STANNOUS 

CHLORIDE,     AMMONIUM     SULPHITE,     and    HYDROGEN    SULPHIDE     are 

more  especially  adapted  to  the  reduction  of  iron  in  iron  ores,  and 
will  be  described  under  that  heading  on  page  318.  These 
methods  are  equally  well  adapted  to  the  reduction  of  iron  solutions 
for  the  purpose  of  standardization. 

432.  Ferrous    Ammonium    Sulphate    may    also    be    used    for 
standardizing  permanganate  solutions.     This  salt  is  remarkably 
permanent.     When  dry  it  may  be  kept  for  a  long  time  without 
any  appreciable  oxidation  of  the  iron.     It  also  retains  its  water  of 
crystallization  with  considerable  persistence.     If  a  sample  of  this 
salt  is  at  hand  which  is  free  from  ferric  iron  and  has  been  com- 
pared with  a  standard  of  known  purity,  it  offers  a  very  convenient 
method  of  standardizing  permanganate  solutions,  as  the  salt  may 
be  weighed  out,  dissolved  in  water,  and,  after  the  addition  of 
dilute  sulphuric  acid,  may  be  titrated  with   the  permanganate 
immediately.    The  iron  begins  to  oxidize  very  soon  after  being 
dissolved,  so  that  the  solution  must  be  titrated  immediately.    As 
this  salt  may  be  titrated  just  as  readily  with  bichromate  solutions, 
its  purity  may  be  tested  by  titrating  a  weighed  portion  with  a 
standard  solution  of  potassium  dichr ornate. 

433.  Oxalic  Acid  and  the  Oxalates,  such  as   potassium  tetrox- 
alate  prepared  and  tested  exactly  as  given  in  Chapter  XXI  for 
standardizing  alkalies  and  acids,  may  be  used  for  standardizing 
permanganate  solutions.     It  is  evident  that  this  class  of  sub- 
stances affords  a  means  of  comparing  standard  acids  with  standard 
oxidizing  solutions.     Testing  the  purity  of  the  oxalic  acid  by 
titrating  it  with  a  standard  acid  is  of  value  only  when  the  absence 
of  alkalies  from  the  oxalic  acid  is  assured.    This  is  readily  accom- 
plished by  igniting  a  portion  of  the  acid.     It  should  volatilize 
completely.    The    titration    of    oxalic    acid   with    permanganate 
solution  is  generally  carried  out  in  a  warm  solution  acidified  with 
sulphuric  acid.     In  the  cold  the  oxidation  of  the  oxalic  acid  is 
very  slow.     It  may  also  be  accelerated  by  the  addition  of  man- 


314  VOLUMETRIC  METHODS. 

ganous  sulphate  solution.  If  the  solution  is  warmed,  the  temper- 
ature must  not  be  allowed  to  exceed  60°,  as  at  a  higher  temperature 
small  amounts  of  oxalic  acid  may  be  volatilized.  At  this  temper- 
ature the  beaker  can  be  taken  in  the  hand  without  serious  discom- 
fort. The  permanganate  solution  must  be  added  slowly  and  with 
vigorous  stirring  to  the  oxalic  acid  solution.  Much  manganese 
dioxide  must  not  be  allowed  to  form  in  the  solution  or  an  excess 
of  permanganate  will  be  required,  as  the  manganese  dioxide  decom- 
poses some  of  the  permanganate  with  evolution  of  gaseous  oxygen. 

434.  Direct  Measurement  of  Oxygen. — An  excellent  method  of 
standardization  which  does,  not  require  the  preparation  of  a  pure 
substance  has  been  given  by  G.  Lunge.*    In  this  method  a  measured 
volume  of  the  permanganate  solution  is  decomposed  by  means 
of  hydrogen  peroxide  and  sulphuric  acid  according  to  the  follow- 
ing reaction: 

2KMnO4+ 3H2S04-h  5H202  =K2S04  +  2MnS04+  8H20  +  502. 
Twice  the  amount  of  available  oxygen  present  in  the  permanga- 
nate is  evolved  in  the  gaseous  form.  This  gas  is  measured 
and  the  weight  of  KMn04  calculated.  LUNGE'S  NITROMETER  is 
used  for  carrying  out  the  determination.  The  accuracy  of  the 
determination  depends  largely  on  the  accuracy  with  which  the 
nitrometer  has  been  graduated  or  calibrated.  As  the  solutions 
used  are  already  saturated  with  air  the  evolution  of  the  oxygen 
is  very  quickly  complete.  The  gas  is  measured  in  the  nitrometer 
over  mercury.  The  details  of  the  process  and  a  diagram  of  the 
nitrometer  are  given  in  Exercise  56,  page  315. 

435.  Standard  Sulphuric  Acid  may  be  used  for  standardizing  a 
permanganate  solution.    A  measured  amount  of  the  standard  acid 
is  placed  in  a  beaker  and  a  sufficient  amount  of  hydrogen  per- 
oxide to  reduce  about  25  c.c.  of  the  permanganate  solution  is 
added.    The  peroxide  is  titrated  with  the  permanganate  solution. 
The  faint  red  color  giving  the  end-point  is  removed  with  a  drop 
of  the  peroxide.     Methyl  orange  is  added  and  the  excess  of  sul- 
phuric acid  titrated  with  standard  alkali. 

The  difference  between  the  amount  of  acid  originally  added  and 
that  found  by  titration  gives  the  amount  of  acid  required  for  the 
reaction  given  m  the  preceding  paragraph.      It  is  evident  from 
*Chem.  Ind.,  1885,  168;   Ber.,  18,  1072;   Zeit.  angew.  Ch.,  1890,  10. 


POTASSIUM  PERMANGANATE. 


315 


this  equation  that  25  c.c.  of  N/5  potassium  permanganate  will 
require  15  c.c.  of  N/5  sulphuric  acid.  From  this  relation  the 
strength  of  the  potassium  permanganate  solution  can  be  calculated. 
The  methods  of  standardization  in  which  a  known  amount  of 
iron  is  used  are  preferred  for  obvious  reasons  wheb^he  perman- 
ganate solution  is  to  be  employed  for  the  determinatfon  of  iron. 

EXERCISE  56. 
Preparation  and  Standardization  of  N/5  Potassium  Permanganate  Solution. 

436.  Preparation  and  Preservation  of  the  Solution. — Weigh  out  6.35  grams 
of  KMn04.  Warm  about  half  a  liter  of  distilled  water  to  about  50°.  Add 
the  permanganate  with  stirring  to  the  warm  water. 
Prepare  two  carbon  filters,  using  asbestos  which  has 
been  digested  with  warm  concentrated  aqua  regia  for 
about  an  hour  and  then  well  washed  with  water  and 
finally  transferred  to  a  stoppered  bottle  with  distilled 
water.  These  filters,  as  well  as  all  vessels  with  which 
the  permanganate  solution  is  to  come  in  contact,  should 
be  freed  from  organic  matter  by  digestion  with  cleaning 
mixture,  made  by  dissolving  a  few  crystals  of  potassium 
dichromate  in  concentrated  sulphuric  acid.  Suck  the 
asbestos  down  with  the  filter-pump,  being  careful  to 
leave  a  film  projecting  upwards  around  the  edge.  By 
means  of  rubber  stoppers  arrange  the  filters  as  shown 
in  Fig.  54  Filter  the  permanganate  solution,  being 
careful  not  to  apply  too  strong  suction  for  fear  of  break- 
ing the  liter  flask.  Rinse  the  beaker  carefully  and 
wash  the  asbestos  until  the  water  comes  through  color- 
less. After  being  washed,  the  asbestos  in  the  second 
filter  should  be  very  nearly  pure  white.  If  much  brown 
manganese  dioxide  is  seen,  the  solution  must  be  filtered 
again,  the  filters  being  interchanged,  and  clean  asbestos 
put  in  what  is  now  the  second  filter.  After  cooling,  the 
solution  is  diluted  to  the  mark  with  distilled  water.  This 
solution  should  be  kept  in  the  dark  and  out  of  con- 
tact with  organic  matter.  Portions  taken  out  of  the 
bottle  should  not  be  returned  to  it. 


T 


STANDARDIZATION. 


437.  First  Method.  By  Means  of  Pure  Iron  Wire. — Clean  thoroughly  by 
rubbing  with  sandpaper  and  filter-paper  some  soft  iron  wire  in  which  the 
percentage  of  iron  has  been  determined.  When  cleaned,  wind  in  a  spiral 
on  a  lead-pencil  with  as  little  contact  with  the  fingers  as  possible.  Weigh 
carefully  about  0.250  gram,  place  it  in  a  250-c.c.  Florence  flask,  and  dissolve 


316  VOLUMETRIC  METHODS. 

with  gentle  heat  in  10  c.c.  concentrated  hydrochloric  acid  and  20  c.c.  (A 
water.  When  the  iron  is  dissolved,  throw  in  a  few  crystals  of  potassium 
chlorate  and  boil  gently  a  few  minutes  with  a  small  Bunsen-burner  flame. 
Carefully  avoid  heating  with  the  flame  the  sides  of  the  flask  above  the 
liquid.  Add  about  2  grams  of  granulated  zinc  which  is  free  from  iron. 
Close  the  mouth  of  the  flask  with  a  rubber  stopper  through  which  passes  a 
bent  glass  tube  dipping  into  a  solution  of  sodium  bicarbonate,  as  shown  in 
Fig.  53.  Heat  nearly  to  boiling  till  the  .solution  is  of  a  pure  green  color,  free 
from  the  slightest  tinge  of  yellow.  If  the  zinc  is  all  dissolved  before  this 
point  is  reached,  add  another  gram  and  continue  heating  until  all  of  the  iron 
is  reduced.  Allow  the  flask  to  cool  somewhat.  The  bicarbonate  solution 
will  rise  in  the  glass  tube  and  enter  the  flask  only  to  be  expelled  by  the 
carbon  dioxide  evolved  when  it  reaches  the  acid  solution.  Add  a  mixture 
of  10  c.c.  of  concentrated  sulphuric  acid  and  20  c.c.  of  water  and  again  heat 
until  no  particles  of  undissolved  zinc  remain  in  the  solution  or  on  the  sides 
of  the  flask.  Allow  the  solution  to  cool  and  then  add  cold  but  recently 
boiled  distilled  water  until  the  flask  is  about  two-thirds  full.  Titrate 
immediately  with  the  permanganate  solution  until  a  rose-tint  is  produced 
which  is  permanent  for  two  minutes.  Repeat  the  determination  until 
duplicates  are  obtained  agreeing  within  less  than  0.1  c.c. 

438.  Second  Method.     By  Means  of  Mohr's  Salt. — Weigh  out  19.62  grams 
of  pure  recrystallized  ferrous  ammonium  sulphate  (FeSO4.(NH4)2S04.6H2O). 
Transfer  to  a  beaker,  dissolve  in  about  100  c.c.  of  water,  and  transfer  to  a 
250-c.c.    flask.       Add    40  c.c.    dilute    sulphuric    acid,    shake,    cool,    and 
dilute  to  the  mark.     This  solution  will  be  exactly  N/5  normal.    Titrate  imme- 
diately 25-c.c.  portions  with  the  permanganate  solution  until  duplicates 
are  obtained  and  calculate  the  strength  of  the  permanganate  solution  in 
terms  of  normal. 

439.  Third  Method.     By  Means  of  Potassium  Tetroxalate. — Weigh  out  be- 
tween 0.3  and  0.35  gram  of  recrystallized  potassium  tetroxalate  and  transfer 
to  a  beaker.      Add  about  50  c.c.  of  water  and  a  few  cubic  centimeters  of 
dilute  sulphuric  acid  and  dissolve  by  gently  heating  the  solution  to  50°  or 
60°.     Be  very  careful  not  to  heat  any  higher.     Add  the  permanganate 
solution  slowly  with  constant   stirring.     If  the  solution  does   not   clear 
readily,  warm  gently  again.     When  nearly  all  of  the  oxalic  acid  has  been 
oxidized,  the  solution  will  decolorize  the  permanganate  almost  instantly. 
At  this  point  add  the  permanganate  cautiously  until  a  faint  permanent 
pink  is  obtained.      If   a  permanent   brown   coloration    or    precipitate  is 
obtained,    too    much    permanganate    has    been    added.       Repeat    until 
duplicates  have  been  obtained.     One  part  of  the  oxalate  equals  0.4978  part 
KMnO4. 

440.  Fourth  Method.     Determination  of  Oxygen  Evolved   with   Hydrogen 
Peroxide. — Set  up  the  Lunge  nitrometer  as  shown  in  Fig.  55.      The  box 
is   designed   to    prevent   loss   of  mercury  if  any  is   accidentally   spilled. 
Suspend    a    thermometer    near  the    nitrometer.      Fill   with    mercury   and 


POTASSIUM  PERMANGANATE. 


317 


test  the  joints  for  leaks  by  lowering  the  level-tube  A  so  as  to  produce 
a  vacuum  in  B  and  noticing  if  the  height  of  the  mercury  column  in  B 
remains  constant  for  ten  minutes.  Now  raise  the  level-tube  and  pro- 
duce pressure  in  the  apparatus,  and  again  wait  ten  minutes  to  see  if  the 
mercury  column  moves.  Measure  out  into  a  beaker  15  c.c.  of  the  perman- 
ganate solution,  and,  after  acidifying  with  10  c.c.  of  dilute  sulphuric  acid, 
add  from  a  burette  hydrogen  peroxide  quite  rapidly  with  stirring  until  the 


FIG.  55. 

solution  is  clear.     Repeat  the  determination,  measuring  out  the  hydrogen 
peroxide  solution  first. 

Measure  out  into  the  bottle  C  the  amount  of  hydrogen  peroxide  solution 
found  to  be  equivalent  to  15  c.c.  of  the  permanganate  solution,  adding  a 
slight  excess.  Add  about  10  c.c.  dilute  sulphuric  acid.  Measure  into  the 
inner  tube  15  c.c.  of  the  permanganate  solution.  Insert  the  stopper  firmly 
and  bring  the  mercury  in  the  two  tubes  to  the  same  level.  Turn  the  stop- 
cock of  the  nitrometer  through  180°  and  bring  the  mercury  in  B  just  up  to 
the  key  of  the  stop-cock,  thus  expelling  the  air  through  the  funnel-tube. 


318  VOLUMETRIC   METHODS. 

After  clamping  the  level-tube,  reverse  the  key  of  the  stop-cock  and  observe 
if  the  mercury  falls  or  rises  on  connecting  the  tube  B  with  the  bottle.  If 
it  falls  a  little,  air  should  be  drawn  out  of  C  by  lowering  the  level-tube. 
The  stop-cock  is  then  reversed,  and  the  air  expelled  as  before.  This 
operation  must  be  repeated  until  the  mercury  column  in  B  neither  falls  nor 
rises  when  the  key  of  the  stop-cock  is  reversed.  If  the  mercury  in  B  rises, 
air  must  of  course  be  admitted  through  the  funnel-tube. 

The  barometer  and  the  thermometer  suspended  near  the  gas  burette 
should  now  be  read.  The  apparatus  should  then  be  allowed  to  stand  undis- 
turbed for  five  minutes  to  see  if  the  air  in  the  bottle  is  at  the  room  tem- 
perature. Any  change  in  temperature  will  be  indicated  by  movement  of 
the  mercury  in  B  up  or  down,  in  which  case  the  amount  of  air  in  C  must  be 
again  adjusted.  The  bottle  is  now  tilted  slightly  and  gently  shaken,  so 
that  the  two  liquids  come  in  contact.  The  level-tube  is  lowered  so  that  no 
considerable  pressure  is  developed  in  B.  The  shaking  is  continued  until  all 
of  the  permanganate  is  decomposed.  The  mercury  columns  are  brought  to 
the  same  level  and  the  apparatus  allowed  to  stand  for  five  minutes.  If  a 
change  in  the  volume  of  the  gas  has  taken  place,  the  bottle  is  again  shaken 
and  the  test  repeated.  When  no  more  gas  is  evolved,  the  final  adjustment  of 
pressure  is  made.  The  mercury  surfaces  are  brought  as  nearly  on  a  level  as 
possible  with  the  eye.  A  drop  of  water  is  introduced  into  the  funnel-tube 
and  the  stop-cock  slowly  turned.  If  the  pressure  within  the  tube  B  is  not 
identical  with  the  atmospheric  pressure,  the  drop  of  water  will  start  to 
move,  the  direction  indicating  in  which  tube  the  pressure  is  greater.  By 
raising  or  lowering  the  level-tube  slightly,  the  inequality  can  generally  be 
overcome  in  a  few  minutes. 

When  atmospheric  pressure  has  been  secured  within  the  tube  B,  the 
volume  of  the  gas  is  carefully  read,  and  after  five  minutes  read  again.  If 
no  change  has  occurred,  the  thermometer  and  barometer  are  again  read. 
As  the  entire  determination  need  not  consume  more  than  one  hour,  neither 
the  temperature  nor  the  pressure  usually  change  an  appreciable  amount. 
Compute  the  volume,  at  0°  and  760  mm.  pressure,  of  the  oxygen  evolved. 
In  this  computation,  the  pressure  of  water  vapor  must  be  taken  into 
account  by  deducting  from  the  observed  barometric  pressure  the  vapor 
tension  of  water  at  the  observed  temperature.  1  c.c.  of  dry  oxygen  at  0° 
and  760  mm.  pressure  weighs  .0014298  gram.  1  gram  of  oxygen  equals  1.977 
grams  KMnO4.  Repeat  the  determination  until  duplicates  are  obtained. 

DETERMINATION  OF  IRON  IN  IRON  ORES. 

441.  Dissolving  the  Ore. — By  digesting  the  powdered  ore  on 
the  hot-plate  with  a  mixture  of  equal  parts  of  water  and  con- 
centrated hydrochloric  acid  the  iron  in  some  ores  is  completely 
dissolved;  but  most  iron  ores  contain  a  small  amount  of  ferrous 


DETERMINATION  OF  IRON  319 

silicate  or  titanate  which  is  not  decomposed  by  hydrochloric  acid. 
After  digesting  the  ore  until  the  residue  is  white  or  unaffected 
by  further  digestion  with  hydrochloric  acid,  it  is  filtered  off  and 
washed  free  from  iron.  After  burning  the  paper  the  silica  is 
volatilized  by  treatment  with  hydrofluoric  and  sulphuric  acids. 
The  iron  remains  in  the  crucible  and  may  be  dissolved  by  digestion 
with  hydrochloric  acid,  or  it  may  be  fused  with  acid  potassium 
sulphate.  The  insoluble  material  may  also  be  decomposed  by 
fusion  with  sodium  carbonate.  If  the  ore  contains  organic  mater- 
ial a  little  potassium  chlorate  must  be  added  to  the  hydrochloric 
acid. 

442.  Reduction  of   the  Iron  by  Zinc. — The  solution  of  the  ore 
may  be  deoxidized  by  means  of  zinc  as  given  for  the  standardiza- 
tion of  the  permanganate  solution.     This  method  introduces  an 
error  in  the  determination  if  titanium  is  present,  as  it  is  partially 
reduced  by  the  zinc.     The  reduction  by  ammonium  bisulphite 
or  hydrogen  sulphide  does  not  affect  the  titanium.     It  is  carried 
out  as  directed  in  Exercise  57. 

443.  The  Reduction  of  Iron  by  Means  of  Stannous  Chloride  is 
very  rapid  and  with  the  addition  of  phosphoric  acid  to  the  solu- 
tion to  remove  the  color  of  the  ferric  chloride  the  titration  is  very 
exact.     The   hydrochloric   acid  solution    of    the    iron    is    heated 
nearly  to  boiling  and  stannous  chloride  solution  added  cautiously 
until  the  yellow  color  of  the  ferric  chloride  is  replaced  by  the  light 
green  of  ferrous  chloride.     A  slight  excess  of  slannous  chloride 
is  added  and  then  all  at  once  with  vigorous  shaking  of  the  flask  60 
c.c.  of  mercuric  chloride  are  added.     60  c.c.  of  an  acid  manganous 
sulphate  solution  containing  phosphoric  acid  is  added  and  after 
diluting   the  solution  with  cold  water  it  is  titrated  immediately 
with   the  permanganate   solution.     The   manganous   sulphate   is 
introduced   to  prevent  reduction  of   the  permanganate    by    the 
hydrochloric  acid. 

The  stannous  chloride  solution  is  made  by  dissolving  30  grams 
of  tin  in  200  to  300  c.c.  of  strong  hydrochloric  acid  and  diluting 
to  1  liter  after  filtering  through  asbestos. 

The  mercuric  chloride  solution  is  made  by  dissolving  50  grams 
of  mercuric  chloride  in  1  liter  of  water. 

One  liter  of  the  manganous  sulphate  solution  should  contain  66$- 


320  VOLUMETRIC  METHODS. 

grams  of  crystallized  manganous  sulphate,  333J  c.c.  of  phosphoric 
acid  (sp.  gr.  1.3),  and  133  c.c.  of  concentrated  sulphuric  acid. 

444.  Reduction  of  Iron  by  Hydrogen  Sulphide. — The  iron  may 
also  be  reduced  with  hydrogen  sulphide.  For  this  purpose  the 
solution  is  placed  in  a  flask  closed  with  a  two-holed  rubber  stopper, 
through  which  passes  a  glass  tube  extending  nearly  to  the  bottom 
of  the  flask.  The  solution  is  heated  nearly  to  boiling  and  hydrogen 
sulphide  passed  through  the  long  glass  tube  until  the  iron  is  com- 
pletely reduced.  The  stream  of  hydrogen  sulphide  is  replaced 
by  a  stream  of  carbon  dioxide  and  the  hydrogen  sulphide  expelled 
by  boiling  the  solution  for  a  few  minutes.  The  flask  is  cooled 
while  the  stream  of  carbon  dioxide  is  passing  and  the  iron  tit- 
rated immediately  with  the  permanganate  solution.  The  sulphur 
is  not  oxidized  by  permanganate  in  the  cold.* 

EXERCISE  57. 
Determination  of  Ferrous  and  Total  Iron  in  an  Iron  Ore. 

445.  Solution  of  the  Ore. — Weigh  out  0.5  gram  of  the  finely  powdered  ore, 
transfer  to  a  100-c.c.  beaker,  and  add  10  c.c.  of  concentrated  hydrochloric 
acid  and  10  c.c.  of   water.     Cover  the  beaker  with  a  watch-crystal  and 
digest  on  the  hot-plate  until  the  residue  is  colorless  or  unaffected  by  further 
digestion  with  the  acid.     The  solution  must  not  be  allowed  to  go  to  dry- 
ness.      If  necessary  add  a  little  dilute  hydrochloric  acid  as  the  solution 
evaporates. 

Decant  the  solution  into  a  250-c.c.  flask,  transfer  the  insoluble  material 
to  a  small  filter-paper,  and  wash  with  small  portions  of  hot  water.  Throw 
the  moist  paper  into  a  platinum  crucible  and  burn  the  paper.  Add  20  to 
30  drops  of  concentrated  sulphuric  acid  and  as  much  hydrofluoric  acid 
and  warm  gently.  If  the  material  dissolves,  continue  heating  the  crucible 
until  the  hydrofluoric  acid  is  entirely  expelled.  If  solution  is  not  com- 
plete add,  after  expulsion  of  the  hydrofluoric  acid,  about  ^  gram  of  acid 
potassium  sulphate  and  heat  the  covered  crucible  sufficiently  to  fuse  the 
sulphate.  When  the  material  is  entirely  dissolved,  let  the  crucible  cool 
and  dissolve  the  contents  in  hot  water,  if  necessary  adding  a  little  hydro- 
chloric acid. 

446.  Reduction  of  the  Iron  by  Means  of  Sulphurous  Acid. — In  the  mean- 
time the  main  portion  of  the  ore  is  reduced  by  means  of  ammonium  bisul- 
phite.    Ammonia  is  added  until  a  small  permanent  precipitate  of  ferric 
hydrate  is  produced.     5  c.c.  of  a  solution  of  ammonium  bisulphite,  made  by 

*  Jour.  Am.  Chem.  Soc.,  Vol.  XVII,  p.  78,  1895;  Chem.  News,  Vol.  LXXIII, 
p.  123,  1896. 


ANALYSIS  OF  IRON  ORES. 


321 


saturating  strong  ammonia  with  sulphur  dioxide,  is  added  and,  after  shaking 
vigorously,  the  solution  is  gently  heated  until  it  is  colorless.  The  solution 
of  the  material  insoluble  in  hydrochloric  acid  is  now  added,  as  well  as  a  mix- 
ture of  10  c.c.  of  concentrated  sulphuric  acid  and  20  c.c.  of  water.  The 
solution  is  then  boiled  until  the  steam  no  longer  smells  of  sulphur  dioxide. 
The  flask  is  filled  with  boiled  distilled  water  without  mixing  with  the  solu- 
tion of  iron.  The  flask  may  be  covered  with  a  small  watch-crystal  and 
cooled  by  placing  it  in  cold  water.  The  solution  may  then  be  poured  into  a 
large  beaker  or  porcelain  dish  and  titrated  with  the  standard  permanganate 
solution.  Calculate  the  percentage  of  metallic  iron  present  in  the  ore. 


DETERMINATION    OF    FERROUS    IRON. 

447.  Solution  of  the  Ore. — Weigh  out  ?  gram  of  the  ore  and  transfer 
to  the  flask  A,  which  should  have  a  capacity  of  about  250  c.c.  The  simplest 
method  of  weighing  the  ore  is  to  use  a  glass  tube  about  10  cm.  long,  having 
a  diameter  of  about  ^  cm.  and  sealed  at 
one  end.  Weigh  the  tube  roughly,  intro- 
duce about  £  gram  of  the  ore  and  weigh 
carefully.  Insert  the  tube  in  the  neck  of 
the  flask,  and  by  gently  tapping  the  tube, 
deposit  the  ore  on  the  bottom  of  the  flask. 
Withdraw  the  tube  and  weigh  again. 
The  difference  between  the  last  two 
weights  is  the  weight  of  the  ore  in  the 
flask. 

Insert  the  stopper  with  the  tubes,  as 
shown  in  Fig.  56.  Place  a  solution  of 
sodium  bicarbonate  in  the  beaker  and 
pass  a  stream  of  carbon  dioxide  from  a 
Kipp  generator  through  the  tube  B  for 
about  ten  minutes.  Disconnect  the  tube  B 
at  the  joint  C  and  attach  a  funnel.  Pour 
in  30  c.c.  of  dilute  hydrochloric  acid,  raising 
the  tube  D  out  of  the  bicarbonate  solution. 


FIG.  56. 


Remove  the  funnel  and  again  connect  the  tube  B  at  C  and  continue  passing 
the  current  of  carbon  dioxide.  Dissolve  the  ore  in  the  acid  by  means  of 
gentle  heat  from  the  Bunsen  burner. 

448.  Titration. — When  no  black  particles  remain  in  the  insoluble  por- 
tion remove  the  burner,  replace  the  beaker  of  bicarbonate  solution  with  a 
beaker  of  boiled  and  cooled  distilled  water,  disconnect  the  tube  B  and  close  C 
with  a  clamp.  Dissolve  3  grams  of  zinc  in  10  c.c.  of  concentrated  sulphuric 
acid  and  20  c.c.  of  water.  When  the  solution  of  the  iron  is  cold,  add  the  acid 
solution  of  zinc  and  titrate  at  once. 


322  VOLUMETRIC  METHODS. 

449.  Calculation. — Compute  the  percentage  of  iron  present  as  ferrous 
oxide.  1  c.c.  N/5  potassium  permanganate  is  equal  to  .01438  gram  of  fer- 
rous oxide.  Compute  the  percentage  of  ferric  oxide  present.  For  this 
purpose  subtract  the  number  of  cubic  centimeters  of  permanganate  solu- 
tion reduced  by  the  ferrous  iron  in  £  gram  of  the  ore  from  the  number  of 
cubic  centimeters  required  to  titrate  the  total  amount  of  iron  present.  The 
difference  gives  the  number  of  cubic  centimeters  of  permanganate  solution 
required  to  titrate  the  iron  present  as  ferric  oxide  in  ^  gram  of  ore.  1  c.c.  of 
N/5  permanganate  solution  is  equal  to  .01598  gram  of  ferric  oxide. 


ANALYSIS  OF  MANGANESE  ORES. 

The  only  ores  of  manganese  which  are  of  commercial  importance 
are  PYROLUSITE  arid  several  related  ores  which  consist  essentially 
of  manganese  dioxide  combined  with  small  amounts  cf  the  lower 
oxides  of  manganese  together  with  more  or  less  iron,  silica,  and 
other  impurities.  Formerly  the  manganese  ore  was  of  value 
chiefly  on  account  of  the  oxygen  present  by  which  free  chlorine, 
bleaching  powder,  etc.,  could  be  produced.  Recently  an  ex- 
tended use  for  manganese  has  been  found  in  connection  with 
the  production  of  iron  and  steel,  so  that  nine-tenths  of  all  man- 
ganese ores  are  said  to.  be  used  for  this  purpose.  The  valuation  of 
the  manganese  ore  may  therefore  require  the  determination  of 
the  available  oxygen  or  the  percentage  of  manganese  present. 

450.  Determination  of  Available  Oxygen.  —  As  the  oxygen 
present  is  used  to  evolve  chlorine,  the  most  rational  method  of 
determination  would  seem  to  be  to  treat  the  ore  with  hydrochloric 
acid  and  determine  the  amount  of  chlorine  evolved.  This  may 
be  done  with  great  accuracy,  and  the  process  will  be  described 
in  the  chapter  on  iodometric  methods.  This  method  is  not  so 
largely  used  at  present  as  those  in  which  the  ore  is  dissolved 
in  acid  ;n  the  presence  of  a  known  amount  of  a  reducing  agent, 
and  the  excess  of  the  latter  determined  by  means  of  a  standard 
solution  of  an  oxidizing  agent.  Oxalic  acid  and  ferrous  iron  have 
been  most  largely  used  as  reducing  agents  with  potassium  per- 
manganate and  potassium  dichromate  respectively  as  the  standard 
oxidizing  solutions. 

The  finely  powdered  material  is  dried  by  spreading  it  out  on 
a  watch-crystal  and  heating  for  from  four  to  six  hours  on  a  water- 


DETERMINATION  OF  MANGANESE.  323 

bath,  or  in  an  air-bath  maintained  at  100°.  The  dried  material 
must  be  weighed  quickly,  as  it  is  hygroscopic.  If  oxalic  acid  is 
used  the  ore  may  be  dissolved  in  an  open  beaker,  as  this  reducing 
agent  is  not  affected  by  the  oxygen  of  the  air.  If  ferrous  iron 
is  used  the  air  must  be  excluded  by  one  of  the  methods  used  during 
the  reduction  of  iron  with  zinc.  It  is  advisable,  both  when  using 
oxalic  acid  and  ferrous  iron,  which  is  most  conveniently  introduced 
as  ferrous  sulphate  or  ferrous  ammonium  sulphate,  to  weigh  out 
the  dry  salts  instead  of  making  a  standard  solution  of  these  re- 
ducing agents.  The  reducing  solutions,  especially  of  the  iron, 
would  rquire  standardization  every  day,  while  one  titration  of 
the  dry  salt  if  preserved  in  a  well-stoppered  bottle  would  be  suffi- 
cient. The  errors  of  measuring  out  portions  of  a  solution  are  also 
considerably  greater  than  the  errors  of  weighing  a  salt. 

DETERMINATION  OF  MANGANESE  BY  VOLHARD'S 

METHOD. 

Manganese  may  be  determined  very  quickly  and  accurately 
by  the  method  of  Volhard.  The  manganese  must  be  in  neutral 
solution  as  a  manganous  salt.  On  adding  potassium  perman- 
ganate to  such  a  solution  all  of  the  manganese  present,  including 
that  which  was  added  as  permanganate,  will  be  precipitated  as 
the  dioxide.  The  precipitate  has  a  fairly  strong  acid  character, 
so  that  varying  amounts  of  the  bases  present  in  the  solution  will 
be  carried  down  with  the  precipitate.  The  manganous  manga- 
nese present  is  apt  to  be  carried  down  in  this  manner,  forming 
salts  of  the  general  character  (MnO^MnO.  Less  permanganate 
solution  will  be  used  in  this  case  than  theory  requires.  This 
difficulty  is  almost  entirely  overcome  by  having  a  considerable 
amount  of  zinc  sulphate  present  in  the  solution.  The  zinc  will 
then  be  carried  down  with  the  manganese  dioxide  in  preference 
to  the  manganous  manganese. 

451.  Determination  of  Amount  of  Error. — The  extent  of  the 
error  from  this  source  may  be  ascertained  by  a  blank  determination 
as  follows:  A  given  volume  of  the  permanganate  solution,  30  c.c. 
for  instance,  is  carefully  measured  out  into  a  beaker  and  reduced 
by  acidifying  with  sulphuric  acid  and  adding  hydrogen  peroxide 


324  VOLUMETRIC  METHODS. 

in  small  portions,  carefully  avoiding  excess.  The  solution  is  then 
transferred  to  a  liter  flask  and  neutralized  by  the  addition  of 
zinc  oxide.  If  several  grams  of  zinc  have ,  not  been  used  some 
neutral  zinc  sulphate  should  be  added.  The  solution  is  then 
diluted  to  400  c.c.  and  warmed  to  80°.  19  c.c.  of  the  perman- 
ganate solution  are  then  added  rapidly  and,  after  vigorous  shak- 
ing of  the  flask,  the  permanganate  is  added  in  0.1-c.c.  portions, 
until  a  slight  excess  is  present  as  shown  by  the  pink  color  of  the 
solution.  The  solution  is  vigorously  shaken  after  each  addition 
of  permanganate.  On  standing  a  few  minutes  the  precipitate 
settles  so  that  the  color  of  the  solution  can  be  seen  on  holding 
the  flask  up  to  the  light. 

452.  Calculation. — If  the  reaction  proceeded  wholly  according 
to  the  equation 

2KMn04+ 2H20  +3MnS04  =2H2S04  +K2S04  +5Mn02, 

exactly  20  c.c.  of  the  permanganate  solution  would  be  required 
to  titrate  the  manganese  obtained  from  the  reduction  of  30 
c.c.  of  the  same  solution.  Generally,  a  little  less  than  20  c.c.  will 
be  used,  the  deficiency  averaging  about  1%  of  the  total  volume. 
The  volume  of  the  permanganate  solution  used  in  a  given  titration 
or  the  value  of  1  c.c.  in  manganese  must  be  increased  in  the  pro- 
portion found  by  the  blank  determination. 

453.  Method  of  Adding  the  Permanganate  Solution. — If  the  per- 
manganate solution  is  added  in  small  portions  to  the  solution  of 
the  manganous  salt  the  error  will  be  greater  than  when  the  total 
amount  of  permanganate  is  added  all  at  once.     If  the  amount  of 
manganese  present  is  entirely  unknown  one  or  more  preliminary 
titrations  must  be  made.     In  the  first  the  permanganate  solution 
is  added  several  cubic  centimeters  at  a  time  until  the  end-point  has 
been  reached  or  passed.     In  the  second  nearly  the  total  amount  of 
permanganate  is  added  all  at  once  and  the  end-point  reached  by  the 
addition  of  amounts  less  than  a  cubic  centimeter.     In  the  third 
titration  the  total  amount  of  the  permanganate  solution  less  a  few 
tenths  of  a  cubic  centimeter  is  added  at  once  and  the  end-point 
reached  by  adding  portions  of  a  tenth  of  a  cubic  centimeter  or  less. 

454.  Interfering  Metals. — Of  the  other  metals  which  occur  in 
manganese  ores  IRON  is  most  common,  but  it  does  not  interfere 


DETERMINATION  OF  MANGANESE.  325 

with  the  titration  of  the  manganese  since  it  is  precipitated  on 
neutralizing  the  solution  with  zinc  oxide.  COPPER  is  also  precipi- 
tated by  means  of  the  zinc  oxide.  NICKEL  and  LEAD,  if  present, 
produce  high  results.  The  lead  may  be  removed  by  treating  the 
solution  with  sulphuric  acid  or  may  be  removed  with  cobalt  and 
nickel  as  sulphides.  The  solution  is  neutralized  with  ammonia 
and  ammonium  sulphide  added.  It  is  then  acidified  with  hydro- 
chloric acid  and  filtered  from  the  sulphides  of  cobalt,  nickel,  and 
lead.  The  hydrogen  sulphide  is  expelled  by  boiling  and  the 
manganese  titrated  after  neutralization  of  the  solution  with  zinc 
oxide.  CHROMIUM  as  well  as  COBALT  interferes  if  present  even  in 
small  amounts.  Manganese  may  be  separated  from  chromium  by 
precipitation  as  dioxide  from  a  strong  nitric  acid  solution  by 
means  of  potassium  chlorate.  It  is  filtered  off,  washed,  and,  after 
solution  in  hydrochloric  acid  and  neutralization  with  zinc  oxide, 
titrated  with  the  permanganate  solution. 

455.  Strength  of  Solution. — As  the  Volhard  reaction  may  be 
represented  as  follows,  MnO  +  0  =  Mn02,  one  liter  of  a  fifth-normal 
permanganate  solution  would  be  er-ual  to  1/10  of  the  molecular 
weight  of  Mn02  in  grams  if  the  oxygen  were  evolved  from  the 
potassium  permanganate  in  the  usual  manner.  As  in  the  Volhard 
reaction  it  decomposes  as  follows, 

2KMn04  =  K20  +  2Mn02 + 30, 

a  fifth-normal  solution  has  only  three-fifths  of  its  usual  strength. 
One  liter  of  the  fifth-normal  solution  is  therefore  equal  to  3/50  of 
the  molecular  weight  of  Mn02,  i.e.,  1  c.c.  is  equal  to  .00522  gram 
of  manganese  dioxide. 

EXERCISE  58. 
Determination  of  Available  Oxygen  in  Pyrolusite. 

(a)  By  Oxalic  Acid. 

The  ore  is  finely  powdered,  spread  out  on  a  watch-crystal,  and  dried  four 
to  six  hours  on  the  water-bath  or  in  an  air-bath  kept  at  100°.  Portions 
weighing  0.870  gram  are  quickly  weighed  out  and  transferred  to  beakers. 
Portions  of  pure  recrystallized  oxalic  acid  weighing  1.2605  grams  are  added 
to  the  beakers  containing  the  ore.  About  50  c.c.  of  water  and  a  few  cubic 
centimeters  of  dilute  sulphuric  acid  are  added  to  each  beaker  and  the  solu- 
tion heated  gently  until  the  ore  is  dissolved.  The  temperature  must  not 


326  VOLUMETRIC  METHODS. 

be  allowed  to  rise  above  60°.  Without  filtering  off  the  insoluble  residue, 
which  is  generally  white,  the  warm  solution  is  titrated  with  standard  per- 
manganate solution.  More  dilute  sulphuric  acid  is  added  if  the  permanganate 
color  fades  slowly  or  manganese  dioxide  is  precipitated.  The  percentage 
of  Mn02  present  is  found  by  subtracting  the  number  of  cubic  centimeters 
of  N/5  potassium  permanganate  used  from  100. 

If  pure  recrystallized  oxalic  acid  is  not  at  hand,  the  C.P.  article  may  be 
used.  It  is  tested  as  follows:  One-gram  portions  of  the  oxalic  acid  are 
weighed  out  and  titrated  against  the  permanganate  solution.  The  amount 
of  oxalic  acid  which  will  reduce  exactly  100  c.c.  of  the  permanganate  solution 
is  then  calculated.  This  amount  is  then  weighed  out  instead  of  1.2605  grams 
of  the  pure  acid.  It  is  advisable  to  test  even  the  most  carefully  prepared 
article  by  titration  against  the  permanganate  solution  used. 

(6)  By  a  Ferrous  Salt. 

Determine  the  percentage  of  MnO2  in  the  same  or  another  sample  of 
pyrolusite  as  follows:  0.870  gram  of  the  finely  powdered  and  dried  material 
is  weighed  out  and  transferred  to  a  250-c.c.  flask.  Add  a  little  distilled 
water  and  some  dilute  sulphuric  acid  and  then  a  gram  or  two  of  sodium 
bicarbonate.  Insert  a  rubber  stopper  fitted  with  a  bent  glass  tube  dipping 
into  a  solution  of  sodium  bicarbonate  exactly  as  used  in  the  standardization 
of  the  permanganate  solution  with  iron  wire.  An  amount  of  ferrous  sul- 
phate or  of  ferrous  ammonium  sulphate  which  has  been  found  by  titration 
to  be  exactly  equal  to  100  c.c.  of  the  permanganate  solution  is  now  added. 
The  solution  is  warmed  gently  until  no  more  black  particles  of  the  pyrolusite 
remain  undissolved.  The  excess  of  ferrous  sulphate  is  titrated  with  the 
permanganate  solution,  a  red  color  permanent  for  one-half  minute  being 
taken  as  the  end-point,  both  in  this  titration  and  in  testing  the  purity  of 
the  ferrous  salt.  The  number  of  cubic  centimeters  of  N/5  permanganate 
used,  subtracted  from  100  gives  the  percentage  of  MnO2  present. 


EXERCISE  59. 
Determination  of  Manganese  in  Pyrolusite. 

456.  Solution  of  the  Ore. — 1.5  grams  of  the  dried  ore  are  weighed  out 
and  digested  with  30  c.c.  of  concentrated  hydrochloric  acid  until  no  black 
particles  remain  or  until  no  further  solvent  action  is  noticed.  The  insoluble 
residue  is  filtered  off  on  a  small  paper,  the  solution  being  allowed  to  flow 
into  a  250-c.c.  flask.  After  washing  the  paper  free  from  manganese,  the 
residue  is  fused  with  sodium  and  potassium  carbonates  unless  the  absence 
of  manganese  has  been  shown  by  a  previous  test.  The  moist  paper  is 
thrown  into  a  platinum  crucible  and  burned.  Four  to  six  times  as  much 
sodium  and  potassium  carbonates  are  added  and  the  contents  of  the  crucible 


DETERMINATION  OF  TIN.  327 

fused.  The  presence  of  manganese  is  generally  indicated  by  the  charac- 
teristic green  color.  Whether  the  fusion  is  green  or  not  the  fused  material 
is  dissolved  in  water,  the  solution  acidified  with  hydrochloric  acid  and 
added  to  the  main  solution  of  the  ore. 

457.  Neutralization  of  the  Solution. — The  solution  is  diluted  to  the  mark 
and  50-c.c.  portions  withdrawn  with  a  calibrated  pipette.     These  portions 
are  placed  in  Florence  flasks  of  750-  to  1000-c.c.  capacity.     Zinc  oxide  is 
added  in  small  portions  with  vigorous  shaking  of  the  flask  until  the  solution 
is  neutral.     At  this  point  the  iron  which  is  almost  invariably  present  begins 
to  be  precipitated  as  the  hydroxide.     The  precipitation  of  the  iron  is  hastened 
by  warming  the  solution.     The  zinc  oxide  should  be  add?d  cautiously  until 
{he  solution  is  colorless,  indicating  the  complete  precipitation  of  the  iron. 
A  large  excess  of  the  zinc  oxide  should  be  avoided.     If  too  much  has  been 
added,  so  that  the  solution  is  turbid  or  the  precipitate  whitish  instead  of 
red,  it  may  be  removed  by  the  cautious  addition  of  hydrochloric  acid.     A 
slight  excess  of  zinc  oxide  is  not  disadvantageous. 

458.  Titration. — The  solution  is  diluted    to    about  400  c.c.,  warmed  to 
80°,  and  titrated  with  the  permanganate  solution,  adding  4-  or  5-c.c.  por- 
tions at  once  and  shaking  vigorously.      After  allowing  the  precipitate  to 
settle  a  little,  the  color  of  the  solution  can  easily  be  observed.     When  the 
characteristic  permanganate  color  has  been  given  to  the  solution,  the  titra- 
tion  is  interrupted,  and  to  the  second  flask,  after  neutralizing,  diluting  to 
400  c.c.,  and  warming,  is  added  the  total  amount  of  permanganate  solution 
which  was  added  to  the  first  flask  less  the  final  portion  which  gave  the  end- 
point.      After  thorough  shaking,  1-c.c.  portions  of  the  permanganate  are 
added  until  the  end-point  is  reached.     To  the  third  flask  the  total  amount  of 
permanganate  solution  within  1  c.c.  is  added,  and  after  shaking,  the  end- 
point  is  reached  by  adding  0.1 -c.c.  portions  of  the  permanganate  solution. 
The  fourth  and  fifth  portions  of  the  solutions  are  also  titrated. 

Calculate  the  per  cent  of  manganese  in  the  ore. 

459.  Determination  of  Tin. — Tin  may  readily  be  determined 
volumetrically  by  titrating  the  ferrous  iron  produced  by  the 
reduction  of  ferric  chloride  by  stannous  chloride  or  metallic 
tin.  If  the  tin  is  obtained  as  the  stannous  salt,  it  is  treated 
with  an  excess  of  ferric  chloride,  when  the  following  reaction 
takes  place: 

2FeCl3 + SnCl2  =  2FeCl2 + SnCl4. 

By  determining  the  amount  of  ferrous  iron  produced,  the  amount 
of  tin  present  in  the  stannous  condition  is  found.  This  determina- 
tion is  of  value  because  the  tin  salt  is  frequently  used  as  a  reducing 
agent.  Cupric  chloride  may  be  used  in  place  of  the  ferric  chloride. 
If  the  tin  is  in  the  metallic  condition  it  may  be  dissolved 


328  VOLUMETRIC  METHODS. 

in  hydrochloric  acid  with  exclusion  of  air  and  the  solution  of 
stannous  chloride  treated  with  ferric  chloride.  The  metallic  tin 
may  also  be  treated  directly  with  ferric  chloride  solution,  in  which 
it  dissolves  readily,  especially  if  it  is  finely  divided.  No  hydrogen 
is  evolved,  but  the  tin  dissolves  according  to  the  following  equa- 
tion: 

Sn+4FeCl3  =4FeCl2+SnCl4. 

If  iron  is  present  in  the  tin  the  solution  in  hydrochloric  acid 
must  be  allowed  to  stand  twelve  hours  after  the  addition  of  metal- 
lic zinc.  The  tin  is  completely  precipitated  and  may  be  removed 
from  the  zinc  with  a  brush,  washed,  dissolved  in  ferric  chloride 
and  the  ferrous  iron  determined.  The  tin  in  stannic  compounds 
may  be  determined  in  the  same  manner,  the  solution  of  the  stannic 
compound  in  hydrochloric  acid  being  treated  with  zinc  as  directed 
above. 

EXERCISE  60. 
Analysis  of  Stannous  Chloride. 

Make  a  solution  of  ferric  chloride  by  dissolving  60  grams  of  the  com- 
mercial salt  in  a  liter  of  water.  One  gram  of  the  stannous  chloride  is  weighed 
out,  placed  in  a  beaker,  and  50  c.c.  of  the  ferric  chloride  solution  added  together 
with  a  few  cubic  centimeters  of  dilute  hydrochloric  acid.  50  c.c.  of  the  ferric 
chloride  solution  is  placed  in  a  similar  beaker  and  the  same  amount  of 
hydrochloric  acid  added.  When  the  stannous  chloride  is  dissolved,  add  a  few 
grams  of  sodium  sulphate  and  at  least  200  c.c.  of  boiled  and  cooled  distilled 
water  to  each  beaker  and  titrate  the  tin  solution  with  standard  permanganate 
solution  until  a  faint  rose  color  permanent  for  one-half  minute  is  obtained. 
Add  permanganate  to  the  ferric  chloride  solution  drop  by  drop  until  the 
same  color  is  obtained.  The  amount  of  permanganate  added  to  this 
solution  must  be  subtracted  from  the  amount  added  to  the  solution  of  the 
tin.  Calculate  the  amount  of  crystallized  stannous  chloride,  SnCl2.2H2O, 
present.  1  c.c.  N/5  permanganate  equals  .02259  gram  of  this  salt. 

DETERMINATION  OF  CALCIUM. 

460.  Titration  of  Calcium  Oxalate. — Calcium  may  be  deter- 
mined very  rapidly  and  accurately  by  titrating  the  oxalic  acid  in 
the  calcium  oxalate  precipitate  with  standard  permanganate 
solution.  The  calcium  may  be  precipitated  in  the  usual  manner 
by  means  of  ammonium  oxalate  and  digesting  the  precipitate 


DETERMINATION  OF  CALCIUM.  329 

until  the  solution  is  clear.  The  precipitate  is  thoroughly  washed 
with  hot  water.  If  it  is  large  it  is  advisable  to  wash  entirely  by 
decantation.  The  filter-paper  through  which  the  wash-water  is 
passed  is  treated  with  a  little  warm  dilute  sulphuric  acid  and 
then  washed  with  water,  allowing  the  liquid  to  flow  into  the  beaker 
containing  the  bulk  of  the  precipitate.  When  the  precipitate 
on  the  filter-paper  has  been  entirely  dissolved  and  washed  out, 
the  portion  in  the  beaker  is  dissolved  by  gently  warming  the 
solution  and  adding  more  dilute  sulphuric  acid  if  necessary.  The 
oxalic  acid  is  then  titrated  in  the  usual  manner. 

461.  Titration  of  the  Excess  of  Oxalic  Acid. — The  washing 
and  dissolving  of  the  precipitate  may  be  obviated,  if  the  calcium 
is  precipitated  hi  a  measuring-flask  of  convenient  size,  by  neutral- 
izing with  ammonia  and  adding  a  measured  excess  of  oxalic  acid. 
For  this  purpose  a  standard  solution  may  be  used,  or  the  pure 
crystals  may  be  weighed  out.  After  digesting  the  precipitate 
until  the  solution  is  clear  it  is  diluted  to  the  mark  on  the  flask 
with  water  and  after  thorough  shaking  is  allowed  to  settle  a  few 
minutes.  As  the  precipitate  occupies  a  small  volume  a  few  drops 
of  water  should  be  added  after  the  solution  is  up  to  the  mark. 
The  exact  amount  to  be  added  may  be  found  by  dividing  the 
weight  of  the  precipitate  by  its  specific  gravity. 

The  solution  is  filtered  through  a  dry  paper  and  the  oxalic 
acid  in  a  measured  volume  of  the  filtrate  titrated  with  standard 
permanganate  solution  after  acidifying  with  sulphuric  acid.  The 
solution  need  not  be  cooled  to  the  room  temperature  if  the  portion 
of  the  filtrate  to  be  titrated  is  measured  out  as  soon  as  filtered. 
A  very  convenient  method  of  procedure  is  to  precipitate  the 
calcium  in  a  500-c.c.  flask  and  after  digestion  allow  to  cool 
somewhat,  dilute,  and  shake.  After  standing  a  few  minutes  the 
solution  is  filtered  through  a  dry  paper  into  a  250-c.c.  measuring- 
flask.  The  solution  is  allowed  to  flow  into  this  flask  until  it  is 
filled  to  a  little  above  the  mark.  The  excess  is  taken  out 'with  a 
glass  tube  used  like  a  pipette.  The  solution  is  then  poured  into  a 
beaker,  the  flask  rinsed  out,  and  the  oxalic  acid  titrated. 

When  a  large  number  of  calcium  determinations  must  be 
made  the  volumetric  method  is  convenient  and  rapid.  If  only  a 
few  need  be  made  it  is  doubtful  if  time  would  be  saved. 


330  VOLUMETRIC  METHODS. 


POTASSIUM  BICHROMATE  SOLUTION. 

462.  Conditions  of  Use. — The  use  of  this  substance  as  an  oxi- 
dizing solution  is  limited  by  the  fact  that  it  does  not  give  up  its 
oxygen  to  many  substances  which  can  be  oxidized  by  potassium 
permanganate  or  iodine.     The  titration  must  always  be  so  con- 
ducted that  the  end-point  is  obtained  by  titrating  ferrous  iron, 
using  potassium  ferricyanide  as  the  outside  indicator.     On  the 
other  hand  the  solution  has  the  advantage  that  it  is  very  stable, 
being  unaffected  by  dust  or  the  organic  matter  which  might  come 
in  contact  with  it.     It  may  be  used  in  the  Mohr  burette,  which  is 
closed  with  a  rubber  tube  and  a  pinch-cock.     Dilute  hydrochloric 
acid  is  not  oxidized  as  it  is  by  potassium  permanganate.      The 
presence  of  this  acid  in  the  solution  does  not  interfere  with  the 
titration   when    carried    out    cold.     Potassium   dichromate    may 
also  be  easily  obtained  very  pure,  so  that  the  solution  may  be  made 
by  weighing  out  the  pure  salt.     Its  oxidizing  power  is  consider- 
ably increased  when  used  in  hot  concentrated  solution.     It  differs 
from  potassium  permanganate  in  that  under  these  conditions  it 
does  not  lose  an  appreciable  amount  of  oxygen  in  the  free  con- 
dition. 

463.  Pure  Potassium  Dichromate  may  generally  be    obtained 
commercially.    Sulphates  and  chlorides  are  the  common  impuri- 
ties.    If  the  pure  salt  is  not  at  hand,  it  may  be  made  by  recrys- 
tallizing  the  impure  substance.     The  salt  contains  no  water  of 
crystallization  and  is  not  hygroscopic.     It  may  be  fused  without 
loss  of  oxygen,  and  this  method  of  drying  is  frequently  used.     If 
dust  or  organic  matter  is  present,  the  fused  salt  is  reduced.     If 
heated  higher  than  necessary  to  fuse  it,  it  may  lose  oxygen.     As 
the  salt  dried  at  100°  loses  only  an  inappreciable  amount  when 
fused,  .this  method  of  drying  seems  to  be  unnecessary.     9.817 
grams  of  the  pure  salt  are  weighed  out,  dissolved  in  water  and 
diluted  to  a  liter  to  make  a  fifth-normal  solution. 

464.  Standardization  by  Means  of  Iron  Wire. — The  strength  of 
the  solution  may  be  verified  by  standardization  with  a  known 
amount  of  iron  as  directed  for  the  standardization  of  potassium 


POTASSIUM  BICHROMATE.  331 

permanganate  solution.  The  oxidation  of  the  organic  matter  in 
the  hydrochloric  acid  solution  of  the  iron  wire  by  means  of  potas- 
sium chlorate  is  unnecessary,  as  the  dichromate  is  not  reduced  by 
the  hydrocarbons  present.  The  iron  wire  may  therefore  be  dis- 
solved in  dilute  sulphuric  acid,  the  air  being  excluded  by  means 
of  carbon  dioxide  by  the  methods  already  given.  If  the  standard- 
ized solution  of  ferric  chloride  is  used,  it  is  not  advisable  to  reduce 
it  by  means  of  zinc,  as  the  zinc  salts  react  with  the  ferricyanide 
used  as  the  indicator,  forming  the  white  zinc  salt  which  obscures 
the  end-point.  The  reduction  by  means  of  stannous  chloride 
solution  is  well  adapted  for  use  when  the  iron  is  to  be  titrated  with 
dichromate  solution.  The  stannous  chloride  is  added  cautiously 
to  the  hot  ferric  chloride  solution  until  the  characteristic  color 
of  the  ferric  chloride  has  disappeared.  The  slight  excess  of  stan- 
nous chloride  is  removed  by  adding  a  considerable  amount  of 
mercuric  chloride  solution.  After  diluting  the  solution  somewhat 
and  adding  dilute  sulphuric  acid  the  ferrous  iron  is  immediately 
titrated  with  the  dichromate  solution.  The  reduction  with  ammo- 
nium bisulphite  and  hydrogen  sulphide  is  carried  out  exactly  as 
directed  for  titration  with  the  permanganate  solution. 

465.  Standardization  by  Means   of  Pure    Ferrous  Ammonium 
Sulphate  is  carried  out  in  the  same  manner  as  when  a  permanga- 
nate solution  is  standardized  with  this  salt,  the  end-point  of  the 
titration  being  ascertained  by  means  of  potassium  ferricyanide. 
The  standardization  by  means  of  oxalic  acid  or  oxalates  is  not 
available  for  dichromate  solutions,  nor  is  the  decomposition  by 
means  of  hydrogen  peroxide  and  measuring  the  gas  evolved.     A 
potassium  permanganate  solution  which  has  been  carefully  stand- 
ardized by  any  of  these  methods  may  be  used  as  a  standard  for 
a  dichromate  solution,  using  a  ferrous  salt  to  make  the  comparison. 
In  this  case  the  errors  of  three  titrations  would  be  included  in  the 
final  result. 

466.  Analysis  of   Iron  Ores. — When  a  dichromate  solution  is 
used  for  the  determination  of  the  iron  in  an  iron  ore,  it  is  decom- 
posed in  exactly  the  same  manner  as  when  the  iron  is  titrated 
with  a  permanganate  solution.     Zinc  is  not  used  for  the  reduction, 
.and  when   stannous   chloride   is  employed,  the  addition   of  the 
phosphoric  acid  and  the  manganous  sulphate  is  unnecessary.     As 


332  VOLUMETRIC  METHODS 

hydrochloric  acid  is  not  readily  oxidized  by  potassium  dichromate, 
the  solution  of  iron  need  not  be  so  thoroughly  cooled  as  when 
permanganate  is  used. 

EXERCISE  61. 
Preparation  of  Standard  N/5  Potassium  Bichromate  Solution. 

Weigh  out  9.817  grams  of  pure  dry  potassium  dichromate.     Dissolve  in 
warm  water,  transfer  the  solution  to  a  liter  flask,  cool,  and  dilute  to  the  mark. 

467.  Standardization  by  Ferrous  Ammonium  Sulphate. — 1.962  grams  of  the 
pure  salt  are  weighed  out  and   dissolved  in  water,  dilute  sulphuric  acid 
is   added,  and    the    solution    titrated  immediately  with    the    dichromate 
solution. 

468.  Indicator. — The  potassium  ferricyanide  solution   to  be  used  as  the 
indicator  must  be  made  immediately  before  it  is  required  for  use.     A  crystal 
about  the  size  of  a  pea  is  placed  in  a  small  beaker  and  75  to  100  c.c.  of  dis- 
tilled water  added.     The  solution  is  most  readily  withdrawn  by  means  of  a 
short  glass  tube.     A  row  of  single  drops  are  placed  by  means  of  this  glass  tube 
on  a  white  porcelain  plate.     On  touching  one  of  these  drops  with  a  rod  dipped 
in  a  solution  of  ferric  chloride  only  a  light-brown  color  should  be  produced. 
If  any  blue  color  develops,  the  ferric  chloride  should  be  boiled  after  the  addi- 
tion of  a  drop  or  two  of  nitric  acid  and  the  test  repeated.      If  a  blue  color  is 
still   developed,  the   ferricyanide  is  contaminated  with  ferrocyanide  and 
cannot  be  used.     When  an  indicator  has  been  obtained  which  is  satisfactory, 
the  progress  of  the  titration  is  ascertained  by  touching  drops  of  the  indicator 
with  drops  of  the  solution  taken  out  on  the  stirring-rod. 

469.  Titration. — To  avoid  loss  of  ferrous  iron,  nearly  all  of  the  calcu- 
lated amount  of  the  dichromate  solution  should  be  added  to  the  iron  solu- 
tion before  a  test  is  made.     Two  or  three  drops  of  the  dichromate  solution 
are  then  added  at  a  time  and  the  te3t  made  until  only  a  faint  blue  color  is 
developed.     A  drop  at  a  tune  is  then  added  until  the  spot  test  shows  no  blue 
color.     No  change  in  the  color  of  the  spot  test  can  then  be  noticed  on  adding 
more  dichromate  to  the  iron  solution. 

470.  The  Standardization  by  Means  of  Soft-iron  Wire  in  which  the  per- 
centage of  iron  has  been  determined  is  carried  out  as  follows:    Prepare  a 
Bunsen  valve  as  directed  on  p.  311  and  a  250-c.c.  Florence  flask.     Clean 
some  soft-iron  wire,  in  which  the  percentage  of  iron  has  been  determined,  by 
rubbing  with  sandpaper  and  filter-paper.      Make  it  into  a  spiral  by  wind- 
ing on  a  lead-pencil.     After  cleaning  do  not  touch  the  wire  with  the  fingers. 
Weigh  from  0.25  to  0.3  gram  and  transfer  to  the  flask.     Add  about  15  c.c.  of 
dilute  sulphuric  acid  and  a  gram  or  two  of  sodium  bicarbonate.     Close  the 
flask  with  the  rubber  stopper  and  the  Bunsen  valve  and  warm  gently  until 
the  iron  is  dissolved.     Add  cold  boiled  water  until  the  flask  is  about  half  full, 


DETERMINATION  OF  CHROMIUM.  333 

then  throw  in  a  gram  or  two  of  sodium  bicarbonate,  and  if  the  solution  is  not 
strongly  acid,  add  a  few  cubic  centimeters  of  dilute  sulphuric  acid  and  titrate 
immediately  with  the  dichromate  solution. 


EXERCISE  62. 
Determination  of  Iron  in  an  Iron  Ore. 

To  determine  the  iron  in  an  iron  ore,  ^  gram  of  the  powdered  ore  is 
weighed  and  dissolved  exactly  as  directed  in  Exercise  57,  page  320.  The 
solution  of  the  iron  is  reduced  by  means  of  stannous  chloride.  It  is  heated 
nearly  to  boiling  and  the  stannous  chloride  solution  added  drop  by  drop  until 
the  color  of  the  solution  is  a  pure  green  and  a  slight  excess  of  the  stannous 
chloride  is  present.  Cool  the  solution  by  allowing  cold  water  to  flow  over 
the  flask.  £0  c.e.  of  mercuric-chloride  solution  is  ndded  all  at  once  and 
the  solution  vigorously  agitated  by  rotating  the  flask  with  the  hand.  The 
iron  is  immediately  titrated  with  the  dichromate  solution.  1  c.c.  of  N/5 
dichromate  solution  is  equal  to  .01118  gram  of  iron,  or  .01598  gram  of  ferric 
oxide.  Compute  the  percentage  of  metallic  iron  and  also  of  ferric  oxide  in 
the  ore. 

471.  Determination  of  Chromium  in  Chrome  Iron  Ores. — 
These  ores  are  most  readily  decomposed  by  fusion  with  an  alkaline 
oxidizing  mixture  by  which  the  chromium  is  converted  into  chro- 
mate,  which  on  treatment  with  water  dissolves,  leaving  the  iron 
as  oxide.  After  destroying  the  excess  of  the  oxidizing  agent,  the 
chromate  may  be  determined  by  means  of  a  known  amount  of 
ferrous  iron.  The  best  fusion  mixture  for  this  purpose  seems  to 
be  4  to  5  parts  of  sodium  hydroxide  and  3  to  4  parts  of  sodium 
peroxide,  or  8  to  10  parts  of  sodium  peroxide  alone.  The  fusion 
must  be  conducted  in  a  silver,  nickel,  or  copper  crucible,  as  plati- 
num is  strongly  attacked  by  this  mixture. 


EXERCISE  63. 
Determination  of  Chromium  in  Chrome  Iron  Ore. 

472.  Decomposition  of  the  Ore. — Grind  the  ore  in  an  agate  mortar  until 
it  is  reduced  to  the  finest  powder.  Weigh  out  £  gram  and  transfer  to  a 
nickel  or  copper  crucible.  Add  4  to  5  grams  of  sodium  peroxide,  being  care- 
ful to  select  the  yellow  material.  The  white  crust  on  top  is  sodium  carbonate 
and  oxide  resulting  from  decomposition  of  the  peroxide.  Mix  the  material 


334  VOLUMETRIC  METHODS. 

well  with  a  platium  wire  or  glass  rod.     This  can  be  more  efficiently  done  if 
the  peroxide  is  placed  in  the  crucible  first  and  the  ore  on  top. 

Heat  at  first  with  a  very  small  flame.  After  about  ten  minutes  the 
material  should  be  entirely  liquid.  It  should  be  kept  in  this  condition  for 
ten  or  fifteen  minutes.  Allow  to  cool  until  a  crust  forms  on  top,  add  1  gram 
of  sodium  peroxide,  and  fuse  again  for  about  five  minutes.  After  cooling, 
transfer  the  crucible  to  a  porcelain  dish  and  dissolve  the  fused  mass  in  hot 
water.  Take  out  and  rinse  the  crucible.  If  the  solution  is  purple,  add  a 
little  more  sodium  peroxide.  Boil  the  solution  for  ten  minutes  to  decom- 
pose the  sodium  peroxide.  Filter  off  and  wash  thoroughly  the  insoluble 
material.  Any  portion  of  this  material  which  is  not  soluble  in  hydrochloric 
acid  must  be  fused  with  a  little  more  sodium  peroxide,  as  it  may  be  unde- 
composed  portions  of  the  ore. 

473.  Titration  of  the  Chromic  Acid. — Acidify  the  filtrate  with  dilute  sul- 
phuric acid  and  add  an  excess  of  about  15  c.c.  Add  a  weighed  amount  of 
ferrous  ammonium  sulphate  which  is  more  than  sufficient  to  reduce  the 
chromium  present.  The  presence  of  an  excess  of  the  ferrous  salt  may  be 
ascertained  by  touching  a  drop  of  the  ferricyanide  indicator  with  a  glass 
rod  which  has  been  dipped  into  the  solution.  A  sufficient  amount  of  the 
ferrous  salt  should  be  placed  in  a  weighing-bottle  and  carefully  weighed. 
After  adding  an  excess  to  the  chromate  solution  the  residue  is  weighed. 
The  amount  added  is  found  by  difference.  5  grams  of  the  ferrous  ammo- 
nium sulphate  will  generally  be  found  to  be  a  sufficient  amount.  The  excess 
of  ferrous  iron  is  titrated  with  standard  dichromate  solution. 

The  value  of  the  ferrous  salt  is  ascertained  by  titrating  2-gram  portions 
with  the  dichromate  solution.  Calculate  the  volume  of  the  dichromate 
solution  equivalent  to  5  grams  of  the  ferrous  salt  or  the  amount  added  to  the 
solution  of  the  chrome  ore.  The  difference  between  this  number  and  the 
number  of  cubic  centimeters  used  in  titrating  the  excess  of  ferrous  iron 
gives  the  amount  of  chromate  present  in  the  solution  of  the  ore  in  cubic 
centimeters  of  N/5  dichromate  solution.  Calculate  the  percentage  of 
O203  in  the  ore,  1  c.c.  of  the  N/5  dichromate  solution  being  equal  to  .005073 
gram  of  Cr203. 

PROBLEMS. 

Problem  n.  The  calcium  in  \  grain  of  a  sample  of  dolomite  was  pre- 
cipitated with  1  gram  of  crystallized  oxalic  acid  and  the  excess  titrated 
with  34  c.c.  of  N/5  potassium  permanganate  solution.  Calculate  the  per 
cent  of  calcium  oxide  in  the  dolomite.  How  much  dolomite  and  oxalic 
acid  must  be  taken  so  that  the  per  cent  of  calcium  oxide  will  be  given  by 
the  number  of  cubic  centimeters  of  N/5  potassium  permanganate? 

Problem  12.  In  the  analysis  of  a  sample  of  stannous  chloride,  a  half- 
gram  portion  was  treated  with  ferric  chloride  solution  and  titrated  with 
N/5  potassium  permanganate  solution  requiring  20  c.c.  The  tin  in  another 
half-gram  portion  was  weighed  as  stannic  oxide,  .330  grams  being  obtaL,.  1. 
Calculate  the  per  cent  of  SnCl2.2H2O  and  SnCl4  present. 


CHAPTER  XXV. 
IODOMETHIC    METHODS. 

474.  Conditions  of  Use  of  Iodine  Solutions. — lodometric  meth- 
ods are  largely  used,  and  are  very  accurate  on  account  of   the 
great  delicacy  of  the  indicator  available,  starch  solution  giving  a 
distinct  blue  color  with  a  very  small  amount  of  free  iodine.    Iodine 
acts  as  an  oxidizing  agent  in  acid  and  neutral  solutions.     The  caustic 
alkalies,  as  well  as  solutions  of  the  alkali  carbonates,  combine 
with  the  iodine,  rendering  titration  in  such  solutions  impossible. 
Sodium  or  potassium  bicarbonate  is  riot  acted  on  by  iodine,  and 
as  solutions  of  these  salts  are  alkaline  toward  all  acids  stronger 
than  carbonic  acid,  by  working  with  solutions  of  these  salts,  iodine 
may  be  used  to  oxidize  many  substances  which  require  an  alkaline 
solution.     It  is  therefore  possible  to  use  iodine  to  oxidize  a  great 
variety  of  chemical  substances.     The  solubility  of  iodine  in  water 
solutions  of  potassium  iodide,  as  well  as  in  alcohol,  chloroform, 
glacial  acetic  acid,  and  other  organic  solvents,  still  further  enlarges 
its  use,  since  both  the  iodine  and  the  substance  acted  on  must  be 
in  solution. 

The  use  of  iodine  is  somewhat  restricted  because  its  solutions 
are  not  stable,  while  many  of  the  methods  of  standardization  are 
indirect.  The  •  volatility  of  the  iodine  also  introduces  difficulties 
in  its  use.  Iodine  solutions  must  not  come  in  contact  with  rubber, 
cork,  or  other  organic  material.  Paraffine  is  unacted  on. 

475.  Solvents  of  Iodine. — The  most    commonly  used  solution 
of  iodine  is  made  by  dissolving  the  element  in  a  water  solution  of 
potassium  iodide.     An  unstable  compound  of  the  formula,  KI3  is 
formed    under    these    circumstances.     A    considerable    excess    of 
potassium  iodide  must  be  present  in  order  to  dissolve  the  iodine. 
Alcoholic  solutions  of  iodine  have  been  largely  used  for  the  deter- 
mination of  the  so-called  iodine  number  of  oils.     For -this. purpose 

335 


336  VOLUMETRIC  METHODS. 

a  solution  in  glacial  acetic  acid  has  recently  superseded  the 
alcoholic  solutions  formerly  used. 

476.  Reducing  Solutions. — As  many  of  the  methods  of  stand- 
ardizing iodine  solutions  consist  in  first  standardizing  a  reducing 
solution,  which  is  then  compared  with  the  iodine  solution,  the 
reducing  solution  must  always  be  at  hand  in  iodometric  work. 
The  solution  most  commonly  used  for  this  purpose  is  SODIUM 
THIOSULPHATE.     The  iodine  is  oxidized  according  to  the  following 
equation : 

2Na2S203 +I2  =Na2S406  +2NaI. 

A  solution  of  SODIUM  ARSENITE  is  sometimes  used,  especially  for 
the  titration  of  bleaching-powder.  It  reacts  with  the  iodine  as 
follows : 

Na2HAs03  + 13  +2NaHC03  =  Na2HAs04  +  2NaI  +  H20  +  2C02. 

Neither  of  these  solutions  is  absolutely  stable,  though  more  so  than 
the  iodine  solutions.  The  most  active  decomposing  influence  on 
the  sodium  thiosulphate  solution  seems  to  be  the  action  of  carbon 
dioxide  in  the  presence  of  oxygen  and  sunlight.*  The  solution 
should  therefore  be  made  by  dissolving  the  thiosulphate  in 
water  from  which  the  carbon  dioxide  has  been  expelled  by  boil- 
ing. The  carbon  dioxide  may  then  be  excluded  from  the  bottle 
by  means  of  a  soda-lime  tube  or  the  solution  may  be  protected 
from  the  air  by  pouring  a  layer  of  petroleum-oil  over  it  and 
siphoning  out  the  solution.  Made  and  protected  in  one  of  these 
ways,  the  solution  will  remain  unchanged  for  months. 

477.  Preparation  of  Solutions. — Although  the  commercial  re- 
sublimed  iodine  may  frequently  be  obtained  free  from  all  impuri- 
ties except  moisture,  which  may  be  removed  by  drying  in  a  desicca- 
tor over  sulphuric  acid,  a  given  sample  cannot  be  relied  on  until 
tested.      For  this  purpose   a   portion   is   sublimed   according  to 
the  direction  given  in  Chapter  II,  p.  34.     A  solution  made  from 
this  resublimed  iodine  may  be  titrated  against  a  thiosulphate  solu- 
tion, which  is  then  titrated  against  a  solution  of  a  weighed  portion 
of  the  unpurified  iodine  dissolved  in  potassium  iodide  solution. 

The  sodium  thiosulphate  cannot  be  weighed  out  exactly  because 
of  uncertain  hydration,  unless  it  has  been  recrystallized  and  care- 

*  Topf.  Zeit.  anal.  Chem.,  26,  150. 


STANDARDIZA  TION.  337 

fully  dried.  A  salt  may  sometimes  be  obtained  in  this  manner 
which  gives  solutions  of  exact  strength. 

STANDARDIZATION. 

Neither  the  iodine  nor  the  sodium  thiosulphate  solution  should 
be  relied  on  without  standardization,  although  if  both  solutions 
are  made  from  carefully  purified  material,  and  on  comparison  by 
titration  are  found  to  be  of  the  strength  calculated  from  the  weight 
of  material  used,  they  may  be  employed  without  further  verifica- 
tion, except  in  the  most  careful  work. 

478.  Resublimed    Iodine. — The  thiosulphate  solution  may  be 
standardized  by  means  of  small  weighed  portions  of  resublimed 
iodine.     As  the  iodine  is  hygroscopic,   it  should  be  resublimed 
immediately  before  use.     The  loss  by  volatilization  may  also  be 
considerable  unless  the   weighed   portions  are   immediately  dis- 
solved in  potassium  iodide  and  titrated  without  delay. 

479.  Potassium  Bichromate,  as  well  as  various  other  oxidizing 
substances,  have  been  recommended  for  use  in  standardizing  the 
thiosulphate    solutions.     Potassium   dichromate    can   readily   be 
obtained  pure.     It  will  liberate  a  definite  amount  of  iodine  from  a 
potassium  iodide  solution  acidified  with  hydrochloric  acid.     The 
reaction  takes  place  according  to  the  following  equation: 

K2Cr207  +  6KI  +  14HC1  =  8KC1  +  2CrCl3 + 3I2 + 7H20. 

A  sufficient  amount  of  potassium  iodide  must  be  present  to  dis- 
solve the  iodine  liberated,  which  is  then  titrated  with  the  thio- 
sulphate solution.  The  decomposition  of  the  potassium  dichro- 
mate is  not  complete  unless  the  potassium  iodide,  as  well  as  the 
hydrochloric  acid,  is  present  in  considerable  excess.  If  a  suffi- 
cient amount  of  hydrochloric  acid  is  not  present,  the  blue  starch 
iodide  color  will  reappear  on  adding  more  hydrochloric  acid 
after  titrating  the  iodine  with  the  thiosulphate  solution.  The  fol- 
lowing proportions  will  generally  be  found  satisfactory:  To  10  c.c. 
of  fifth-normal  potassium-dichromate  solution,  10  c.c.  of  a  10  per 
cent  potassium-iodide  solution  and  5  c.c.  of  strong  hydrochloric 
acid  is  added.  On  account  of  the  volatility  of  the  iodine,  it  is 
advisable  to  carry  out  the  operation  in  a  glass-stoppered  bottle. 

480.  Potassium  Permanganate  reacts  more  readily  with    the 
hydriodic  acid.    If  a  standard  solution  is  at  hand,  a  measured 


338  VOLUMETRIC  METHODS. 

volume  may  be  added  to  a  solution  of  potassium  iodide  acidified 
with  hydrochloric  acid  and  the  iodine  liberated  titrated  at  once. 
The  decomposition  takes  place  according  to  the  following  equation: 

KMn04+5KI  f  8HC1  =  6KC1  +MnCl2+4H20  +51. 

481.  The   lodates  of  Potassium   and   Sodium  have  also  been 
recommended  for  use  in  a  similar  manner.     They  are  decomposed 
by  hydriodic  acid  instantly  and  completely.     The  acid  potassium 
iodate  is  very  readily  obtained  in  a  high  state  of  purity.     This 
salt  has  the  disadvantage  of  being  sparingly  soluble  in  water,  and 
on  account  of  the  large  percentage  of  available  oxygen  the  quanti- 
ties to  be  weighed  out  are  small,  necessitating  very  careful  weigh- 
ing.    If  a  large  amount  is  weighed  out,  the  solution  must  be  made 
up  to  a  definite  volume  and  portions  measured  out.     The  decom- 
position takes  place  according  to  the  following  equation: 

KH(T03)2+10KI+11HC1  =  11KCH-6H20+6I2. 

482.  Barium  Thiosulphate. — The  iodine  solution  is  standardized 
by  titration  with  a  sodium  thiosulphate  solution  which  has  been 
standardized  by  one  of  the  methods  already  given.     It  may  be 
directly  standardized  by  the  titration  of  a  weighed  amount  of  pure 
barium  thiosulphate.     This  salt  may  be  made  by  mixing  together 
a  warm  solution  of  50  grams  of  sodium  thiosulphate  in  300  c.c.  of 
water  and  40  grams  of   barium  chloride  dissolved  in  the  same 
amount  of  water.     The  solution  should  be  stirred  vigorously  while 
the  salt  crystallizes  out.     It  is  filtered  off,  washed  with  water  until 
free  from  chlorides,  and  dried  in  the  air.     It  contains  one  molecule 
of  water  of  crystallization,  its  formula  being  BaS203.H20.     It  is 
very  sparingly  soluble  in  water,  a  solution  made  by  shaking  an 
excess  of  the  salt  with  water  at  17.5°  for  fifteen  minutes  being 
exactly  N/100  in  strength.     Because  of  its  high  molecular  weight 
careful   weighing    is   unnecessary.      Quantities  weighed    out  for 
the  standardization  of  iodine  solutions  are  treated  with  water  in 
a  beaker.     Because  of  the  slight  solubility  of  the  salt,  the  iodine 
solution  is  added  to  the  beaker  while  undissolved  thiosulphate  still 
remains.    When  all  or  nearly  all  of  the  salt  is  dissolved,  starch 
solution  is  added  and  the  addition  of  the  iodine  solution  continued 
until  a  permanent  blue  color  is  produced. 


IODINE  SOLUTION.  339 

483.  Pure  Arsenious  Oxide  may  also  be  used  for  the  standard- 
ization of  iodine  solutions.     Weighed  portions  are  dissolved  in 
sodium  bicarbonate  solution,   and  after  the  addition  of  starch 
solution  the  iodine  is  added  until  the  end-point  is  reached. 

484.  The  Starch  Solution  is  made  by  placing  a  gram  or  two 
of  starch  in  a  porcelain  mortar,  adding  a  little  water,  and  grind- 
ing to  a  smooth  paste.      150  or  200  c.c.  of  boiling  water  are 
then  poured  into  the  mortar  while  stirring  continually  with  the 
pestle.     The  solution  is  then  poured  into  a  beaker  containing  an 
equal  volume   of  cold  distilled  water.     After  the  solution  has 
been  allowed  to  stand  and  settle  for  some  time,  the  clear  liquid  is 
decanted  for  use.    This  starch  solution  is  sensitive  only  when 
fresh  and  must  be  prepared  each  day.    If  saturated  with  sodium 
chloride,  or  a  few  drops  of  chloroform  added,  it  keeps  much  longer. 
Various  other  methods  of  making  a  permanent  starch  solution 
have  been  suggested,  but  the  solution  made  fresh  each  day  seems 
to  be  the  most  satisfactory. 

If  the  starch  is  added  to  a  solution  containing  a  considerable 
amount  of  free  iodine,  a  compound  of  starch  and  iodine  is  pro- 
duced which  is  decomposed  very  slowly  by  the  thiosulphate. 
The  indicator  is  therefore  added  only  when  nearly  all  of  the  free 
iodine  has  been  removed  by  the  sodium  thiosulphate.  Potassium 
iodide  as  well  as  free  iodine  must  be  present  to  produce  the  blue 
color  with  starch.  As  solutions  of  potassium  iodide  are  not  stable, 
this  salt  must  be  kept  in  the  solid  form  and  only  freshly  made 
solutions  used. 

EXERCISE  64. 

Preparation  and  Standardization  of  N/io  Iodine  and  Sodium 
Thiosulphate  Solutions. 

485-  Sodium  Thiosulphate  Solution. — If  recrystallized  and  carefully  dried 
sodium  thiosulphate  is  at  hand,  weigh  out  carefully  24.83  grams,  dissolve 
in  water,  and  dilute  the  solution  to  a  liter.  If  the  ordinary  C.P.  salt  is  used, 
weigh  out  about  25  grams  for  a  liter  of  solution.  In  either  case  use  distilled 
water  which  has  been  thoroughly  boiled  to  expel  the  carbon  dioxide  and 
place  the  solution  in  a  bottle,  through  the  stopper  of  which  a  siphon  passes 
for  withdrawing  the  solution.  Pour  enough  naphtha  over  the  solution  to 
form  a  layer  about  1  cm.  deep,  or  put  a  straight  calcium  chloride  tube  in  the 
second  hole  of  the  stopper.  Fill  the  tube  with  fresh  soda-lime. 


340  VOLUMETRIC  METHODS. 

486.  Iodine  Solution.— Weigh  12.697  grams  of  recently  sublimed  iodine  in 
a  weighing-bottle.     Keep  the  stopper  tightly  closed,  except  when  putting 
in  or  taking  out  iodine.     Only  glass,  porcelain,  or  platinum  should  come  in 
contact  with  the  iodine.     Be  careful  not  to  spill  crystals  on  the  balance-pan 
or  in  the  case.     About  18  grams  of  pure  potassium  iodide  are  weighed  out 
and  dissolved  in  about  150  c.c.  of  water.     The  potassium  iodide  should  be 
tested  by  dissolving  about  1  gram  in  10  c.c.  of  water  and  acidifying  with 
dilute  hydrochloric  acid.     The  solution  must  be  colorless  and  remain  so  on 
the  addition  of  a  few  cubic  centimeters  of  starch  solution. 

The  iodine  is  transferred  to  a  liter  flask  by  removing  the  stopper  of  the 
weighing-bottle  and  inserting  the  bottle  into  the  neck  of  the  flask,  which 
for  this  purpose  is  held  in  a  horizontal  position.  On  raising  the  flask  to  an 
upright  position  and  tapping  the  weighing-bottle,  the  bulk  of  the  iodine  will 
be  transferred  to  the  flask.  The  few  remaining  crystals  are  removed  by 
adding  a  little  of  the  potassium  iodide  solution  and  shaking  the  stoppered 
weighing-bottle  until  dissolved.  The  solution  is  poured  into  the  flask  and 
the  weighing-bottle  and  stopper  are  thoroughly  rinsed  with  the  potassium 
iodide  solution.  The  remainder  of  this  solution  is  added  to  the  liter  flask, 
which  is  stoppered,  and  shaken  at  intervals  until  all  of  the  iodine  is  dissolved. 
If  the  last  of  the  iodine  crystals  dissolve  slowly,  a  few  crystals  of  potassium 
iodide  may  be  dropped  into  the  flask  and  the  concentrated  solution  formed 
left  undisturbed  in  contact  with  the  iodine  crystals.  The  solution  may  also 
be  very  gently  warmed  on  the  water-bath. 

487.  Comparison  of  Iodine  and  Sodium  Thiosulphate  Solutions. — When  all 
of  the  iodine  has  been  dissolved,  the  solution  is  diluted  to  the  mark  and 
transferred  to  a  glass-stoppered  bottle,  which  is  best  kept  in  a  cool,  dark  place. 
The  solution  is  immediately  compared  with  the  sodium  thiosulphate  solu- 
tion.    25  c.c.  of  iodine  solution  is  measured  out  into  a  beaker  and  the  thiosul- 
phate solution  added  until  the  color  has  been  almost  entirely  removed, 
leaving  the  solution  a  light  yellow.     A  few  cubic  centimeters  of  the  starch 
solution    are   then  added  and  the  addition  of  the  thiosulphate  solution 
continued  until  the  intense-blue  color  is  just  removed.     Until  experience  is 
gained,  it  is  advisable  to  add  iodine  solution  drop  by  drop  until  the  blue 
color  is  restored  before  reading  the  burette.     There  is  some  danger  of  over- 
stepping the  mark  when  the  end-point  is  obtained  by  removing  the  blue 
color  with  the  thiosulphate  solution. 


STANDARDIZATION  OF  THE  SOLUTIONS. 

488.  First  Method.  Barium  Thiosulphate. — Weigh  out  several  portions 
of  pure  barium  thiosulphate  of  about  £  gram.  If  exactly  0.6688-gram  por- 
tions are  weighed,  each  will  be  equal  to  25  c.c.  of  tenth-normal  iodine  solu- 
tion. Transfer  the  portions  to  beakers  and  add  from  50  to  100  c.c.  of  water. 
Without  waiting  for  the  salt  to  dissolve  completely,  add  the  iodine  solution 


IODINE  SOLUTION.  341 

slowly  with  constant  stirring  until  nearly  all  of  the  thiosulphate  is  dissolved. 
Starch  solution  is  then  added  and  then  iodine  solution  until  a  slight  perma- 
nent blue  color  is  produced. 

489.  Second  Method.     Arsenious  Oxide. — Weigh  out  4.95  grams  of  pure 
arsenious  oxide.     Place  in  a  beaker  and  add  about  13  grams  of  caustic 
soda.     Dissolve  in  50  c.c.  of  distilled  water,  transfer  to  a  liter  flask,  and 
saturate  the  solution  with  carbon  dioxide.     At  first  the  delivery-tube  must 
not  be  immersed  in  the  liquid,  as  the  latter  might  be  sucked  up  the  tube  by 
the  complete  absorption  of  the  gas.     When  the  gas-bubbles  pass  through 
the  liquid  undiminished  in  size,  the  latter  is  saturated.     Rinse  the  delivery- 
tube  and  dilute  the  solution  to  the  mark.     This  solution  will  be  exactly 
decinormal.     Titrate  25-c.c.  portions  with  the  iodine  solution. 

490.  Third  Method.     Acid  Potassium  lodate. — Weigh  out  0.3251  gram  of 
acid  potassium  iodate.      Dissolve  in  50  c.c.  of  water,  heating  gently  if 
necessary.     Transfer  the  solution  to  a  100-c.c.  flask,  rinsing  the  beaker  care- 
fully, and  dilute  to  the  mark.     This  solution  will  be  exactly  decinormal. 
Measure  out  25  c.c.  into  a  beaker,  add  about  1  gram  of  potassium  iodide 
dissolved  in  a  little  water  and  a  few  cubic  centimeters  of  dilute  hydro- 
chloric acid.     Titrate  immediately  with  the  thiosulphate  solution. 

491.  Fourth  Method.     Resublimed  Iodine. — Weigh  out  about  0.3  gram  of 
recently  sublimed  iodine,  using  a  tightly  stoppered  weighing-bottle.     Add 
10  c.c.  of  a  10%  solution  of  potassium  iodide  and  shake  until  the  iodine  is 
completely  dissolved.     Transfer  the  solution  to  a  beaker  and  rinse  out  the 
weighing-bottle  with  water  and,  if  necessary,  a  little  potassium  iodide  solu- 
tion.    Titrate  immediately  with  the  thiosulphate  solution. 

492.  Calculation  of  the  Results. — The  iodine  solution,  having  been  made 
from  pure  iodine,  must  be  considered  exactly  tenth-normal  when  made, 
unless  some  iodine  was  known  to  be  lost.     The  titration  of  the  thiosulphate 
solution  immediately  after  the  iodine  solution  was  made  is  considered  as  a 
standardization  of  the  former  and  is  so  calculated.     The  average  of  the  titra- 
tions  which  agreed  within  less  than  0.2  or  at  most  0.3%  is  taken.   If  duplicate 
titrations  agreeing  within  this  limit  are  not  readily  obtained,  some  error  is 
being  made  in  the  work,  which  must  be  discovered  by  varying  the  method 
of  procedure.     If,  for  instance,  24.05  c.c.  of  the  thiosulphate  solution  is 
found  to  be  equal  to  25  c.c.  of  the  iodine  solution,  the  strength  of  the  solu- 
tions being  inversely  as  the  number  of  cubic  centimeters  of  each  used,  the 
strength  of  the  thiosulphate  solution  will  be  given  by  the  following  pro- 
portion:   24.05  :  25.00  :  :  0.1N  :  x.     z  =  .10395N. 

By  the  first  method  of  standardization  the  iodine  solution  is  compared 
with  a  weighed  amount  of  barium  thiosulphate,  which  is  exactly  equal  to 
25  c.c.  of  tenth-normal  solution.  By  the  same  proportion  as  given  above, 
the  strength  of  the  iodine  solution  may  be  computed.  If,  for  instance,  an 
average  of  24.95  c.c.  of  iodine  solution  were  used,  the  iodine  solution  is 
.1002N  by  the  proportion  24.95  :  25.00  :  :  0.1N  :  x.  As  this  value  differs 


342  VOLUMETRIC  METHODS. 

from  tenth-normal  by  only  0.2%,  the  iodine  solution  is  considered  tenth- 
normal,  since  the  errors  of  titration  may  amount  to  0.2%. 

The  results  of  the  second  method  of  standardization  are  calculated  in  the 
same  manner.  The  third  as  well  as  the  fourth  methods  being  direct 
standardizations  of  the  thiosulphate  solution,  its  strength  is  calculated  from 
the  results. 

If  several  days  have  elapsed  between  the  standardization  of  the  thiosul- 
phate solution  and  its  comparison  with  the  iodine  solution  this  comparison 
must  be  repeated.  The  average  of  the  values  found  for  the  thiosulphate 
solution  is  taken  and  used  in  the  proportion  when  calculating  the  strength 
of  the  iodine  solution.  If,  for  instance,  25.18  c.c.  of  the  iodine  solution 
was  found  to  be  equal  to  24.12  c.c.  of  the  thiosulphate  solution,  and  the 
average  strength  of  this  solution  to  be  .1039N,  the  proportion  will  be 
25.18  :  24.12  :  :  ;1039  :  x,  from  which  x,  or  the  strength  of  the  iodine 
solution,  will  be  .09952N. 

493.  Analysis  of  Reducing  Agents. — All  substances  which  may 
be  analyzed  by  iodometric  methods  may  be  divided  into  two 
classes,  according  to  whether  they  act  as  reducing  agents  towards 
iodine  or  as  oxidizing  agents  towards  hydriodic  acid.  The  reducing 
agents  are  analyzed  either  by  titrating  directly  with  standard 
iodine  solution,  or  by  adding  an  excess  of  the  iodine  solution  and 
titrating  back  with  standard  thiosulphate  solution.  The  method 
of  titrating  a  THIOSULPHATE  or  ARSENIOUS  OXIDE  has  already 
been  given  in  the  methods  of  standardization.  Antimonons  oxide 
may  be  titrated  in  exactly  the  same  manner  as  arsenious  oxide, 
tartaric  acid  or  cream  of  tartar  being  used  to  dissolve  the  anti- 
monous  oxide.  The  titration  of  a  SULPHITE  will  be  given  in 
Exercise  66,  page  346.  The  SULPHIDES,  which  may  be  completely 
decomposed  with  liberation  of  hydrogen  sulphide,  may  be 
analyzed  in  a  similar  manner,  the  hydrogen  sulphide  being 
passed  through  excess  of  standard  iodine  solution.  The  follow- 
ing reaction  takes  place:  H2S  +  I2-2HI  +  S.  The  amount  of 
unreduced  iodine  is  then  determined.  Easily  decomposed  sul- 
phides, such  as  those  of  cadmium  and  zinc,  the  alkali  .sulphides, 
and  hydrogen  sulphide  water,  are  completely  decomposed  by 
adding  dilute  acid  and  iodine  solution.  Other  reducing  sub- 
stances which  can  be  determined  iodometrically  are  ANTIMONY 

OXIDE,  ALKALI  CYANIDES,  STANNOUS  CHLORIDE,  and  OILS  and  FATS, 

as  will  be  given  in  the  chapter  on  this  subject. 


IODOMETRIC  METHODS.  343 

494.  Oxidizing  Substances,  on  the  contrary,  are  treated  with 
an  excess  of  potassium  iodide,  and  the  solution  acidified.    The 
hydriodic  acid  formed  is  oxidized  to  iodine,  which  combines  with 
the  excess  of  potassium  iodide  and  is  then  titrated  with  standard 
thiosulphate  solution.     The  standardization  of  the  thiosulphate 
solution  by  means  of  ACID  POTASSIUM  IODATE  is  an  illustration  of 
this  method.     In  this  manner  FREE  CHLORINE,  BROMINE,  HYPO- 

CHLORITES,  BROMATES,  FERRIC  CHLORIDE,  NITROUS  ACID,  HYDROGEN 

PEROXIDE,  and  PERSULPHURIC  ACID  may  be  analyzed.  Oxidizing 
substances  decomposed  with  difficulty,  such  as  CHLORATES,  CHRO- 
MATES,  and  minerals  containing  MANGANESE  DIOXIDE,  may  be 
analyzed  by  treatment  with  concentrated  hydrochloric  acid  and 
distillation  of  the  chlorine  which  is  absorbed  in  potassium  iodide 
solution.  The  iodine  liberated  is  then  titrated  with  standard 
thiosulphate  solution. 

495.  Antimony  may  be  determined  by  first  oxidizing  with  potas- 
sium chlorate  (1  gram)  in  strong  hydrochloric-acid  solution.    The 
excess  of  chlorine  is  boiled  out,  the  solution  cooled,  and  excess 
of  potassium  iodide  added  (1  gram).    The  antimony  is  reduced 
from  Sb2Cl5  to  Sb2Cl3  with  liberation  of  iodine  which  is  titrated 
with   thiosulphate   solution.    Copper  and   iron   must  be   absent 
from  the  solution. 

496.  Copper  may  be   determined   in  a  similar  manner.     The 
nitric-acid  solution  of  the  copper  is  neutralized  with  ammonia  or 
sodium  carbonate  and  then  acidified  with  acetic  acid.    Ten  c.c. 
of  a  10  per  cent  solution  of   potassium   iodide  is  added  to  the 
copper  solution,  which  should    not    exceed    15  c.c.   in  volume. 
Iodine  is  liberated  according  to  the  following  reaction: 

Cu  (C2H302)2 + 2KI  =  Cul  + 1 + 2KC2H302. 

The  iodine  liberated  is  titrated  with  sodium-thiosulphate  solu- 
tion. Lead,  bismuth,  arsenic,  antimony,  and  iron  should  be 
absent  from  the  solution. 

497.  Acids  may  also  be  determined  iodometrically  by  using  a 
solution  of  potassium  iodate  and  potassium  iodide  in  the  propor- 
tion of  1  atom  of  the  former  to  5  atoms  of  the  latter.    On  acidi- 
fying such  a  solution,  the  following  reaction  takes  place: 


344 


VOLUMETRIC  METHODS. 


By  titrating  the  iodine  liberated,  the  amount  of  acid  added  may 
be  computed,  since  1  atom  of  iodine  is  liberated  by  1  molecule  of 
a  monobasic  acid.  Bases  may  also  be  determined  by  adding  an 
excess  of  acid  and  titrating  the  iodine  liberated  after  addition  of 
the  iodate  and  iodide  solution.  In  this  manner  the  extremely 
sensitive  indicator  of  iodometric  titrations  may  be  used  in  acid 
titrations.  Although  in  this  manner  a  much  sharper  end-point 
may  be  obtained  than  by  the  use  of  the  ordinary  indicators  for 
acids  and  bases,  the  accuracy  of  a  given  titration  will  be  no  greater 
unless  the  measurement  of  volumes  and  the  purification  of  reagents 
is  carried  out  with  the  same  high  degree  of  accuracy. 

498.  Bunsen's  Distillation  Apparatus.  —  For  carrying  on  the 
distillation  of  the  chlorine  from  oxidizing  agents,  such  as  manga- 
nese dioxide,  various  forms  of  apparatus  have  been  devised.  One 
of  the  best  is  that  of  Bunsen,  shown  in  Fig.  57.  The  oxidizing 


FIG.  57. 

substance  to  be  analyzed  is  placed  in  the  small  bulb  a,  which  is 
connected  with  the  tube  b  by  means  of  a  ground-glass  joint  over 
which  a  rubber  tube  of  suitable  size  is  stretched.  This  joint  must 
be  tight,  so  as  to  prevent  loss  of  chlorine,  which  must  not,  however, 
be  allowed  to  come  in  contact  with  the  rubber.  The  receiver  c 
is  half  filled  with  10%  potassium  iodide  solution,  which  is  decom- 
posed by  the  chlorine  with  liberation  of  iodine.  The  chlorine  is 
driven  out  of  the  flask  a  and  the  tube  b  with  the  excess  of  hydro- 
chloric acid  and  the  steam  produced  by  boiling  the  solution.  A 


ANALYSIS  OF  MANGANESE  DIOXIDE. 


345 


slow  stream  of  carbon  dioxide  may  be  produced  by  placing  a  few 
pieces  of  magnesite  in  the  flask  a.  This  is  not  necessary,  however. 
499.  Mohr's  Distillation  Apparatus. — Another  form  of  apparatus 
which  was  devised  by  Mohr  is  shown  in  Fig.  58.  In  this  apparatus 
the  joints  are  made  by  cork  stoppers  coated  with  paraffine,  which 


FIG.  58. 

is  not  acted  on  by  chlorine.  The  receiver  is  a  tube  about  30  cm. 
long  and  3  cm.  wide,  which  is  immersed  in  a  cylinder  filled  with 
cold  water.  The  distillation-flask  6  should  have  a  capacity  of 
50  to  75  c.c. 


EXERCISE  65. 
Determination  of  Available  Oxygen  in  Pyrolusite. 

Weigh  out  between  0.2  and  0.25  gram  of  the  finely  powdered  pyrolusite 
and  introduce  into  the  distillation-flask  of  either  of  the  forms  of  apparatus 
already  described.  Use  a  small  glass  tube  for  introducing  the  ore  into  the 
flask,  obtaining  the  weight  of  the  material  by  weighing  the  tube  before  and 
after  introducing  the  ore.  About  1.5  grams  of  potassium  iodide  are  dissolved 
in  50  c.c.  of  distilled  water  and  placed  in  the  receiver,  which  must  be  kept 
cool  by  immersion  in  ice-water.  20  to  30  e.c.  of  concentrated  hydrochloric 
acid  are  introduced  into  the  distillation-flask,  which  is  immediately  con- 
nected with  the  delivery-tube  and  the  latter  immersed  in  the  potassium 


346  VOLUMETRIC  METHODS. 

iodide  solution.  The  paraffine-coated  cork  at  a  is  loosely  inserted  into  the 
receiver. 

The  distillation-flask  6  is  now  gently  heated  until  the  manganese  dioxide 
is  completely  dissolved,  leaving  a  white  residue  which  is  not  affected  by 
further  digestion.  The  solution  is  finally  brought  to  a  boil,  and  while  the 
Eunsen  burner  is  still  under  the  distillation-flask,  the  delivery-tube  is  with- 
drawn from  the  receiver  and  rinsed  with  distilled  water.  The  iodine  solu- 
tion is  transferred  to  an  Erlenmeyer  flask  and  the  receiver  washed  with 
water.  A  few  cubic  centimeters  of  potassium  iodide  solution  are  placed  in 
the  receiver,  into  which  the  delivery-tube  is  again  passed,  and  the  contents 
of  the  distillation-flask  again  boiled  for  a  few  minutes.  If  no  more  iodine 
is  liberated  in  the  potassium  iodide  solution,  the  decomposition  of  the 
manganese  dioxide  is  complete,  otherwise  the  distillation  must  be  continued 
for  some  time,  the  contents  of  the  receiver  being  finally  added  to  the  main 
solution. 

The  iodine  liberated  is  titrated  with  standard  thiosulphate  solution. 
1  c.c.  of  N/10  sodium  thiosulphate  is  equal  to  .004350  gram  of  Mn02  or 
.003545  gram  of  chlorine.  Compute  the  percentage  of  MnO2  as  well  as 
the  percentage  of  chlorine  liberated. 

EXERCISE  66. 
Determination  of  Sulphur  Dioxide  in  Sodium  Sulphite. 

Weigh  out  1  gram  of  the  sulphite  and  placg  in^an  Erlenmeyer  flask. 
Add  100  c.c.  of  N/10  iodine  solution  all  at  once.  It  is  advisable  to  measure 
out  the  iodine  solution  with  a  pipette  or  a  burette  into  a  small  beaker.  The 
solution  may  then  be  poured  from  the  beaker  into  the  flask.  Shake  the 
flask  until  the  salt  is  dissolved.  Rinse  the  remainder  of  the  iodine  solu- 
tion in  the  beaker  into  the  flask  and  titrate  the  excess  with  standard  thio- 
sulphate solution.  1  c.c.  of  N/10  iodine  solution  is  equal  to  .003203  gram 
of  SO2  or  .01261  gram  of  Na2SO3.7H2O.  Compute  the  percentage  of  sulphur 
dioxide  and  also  of  the  crystallized  salt. 

500.  The  Active  Substance  in  Bleaching-powder  is  calcium 
oxychloride  Cl-Ca-0-Cl.  On  adding  water,  this  compound  is 
decomposed  into  calcium  chloride  and  calcium  hypochlorite, 
Ca(C10)2.  The  value  of  the  bleaching-powder  depends  on  the 
amount  of  hypochlorite  present.  The  result  of  an  analysis  is 
usually  expressed  in  percentage  by  weight  of  available  chlorine, 
although  in  France  it  is  customary  to  report  the  analysis  in  terms 
of  the  number  of  liters  of  chlorine  gas  measured  at  0°  and  760 
mm.,  which  may  be  obtained  from  1  kilogram  of  the  bleaching- 
powder. 


ANALYSIS  OF  BLEACHING-POWDER.  347 

501.  Sample.— The   analysis   may  be   carried  out  by  a  large 
number  of  methods,  of  which  two  of  the  most  largely  used  will  be 
given.    In  order  to  secure  a  fair  sample,  a  considerable  amount 
is  weighed  out  and  dissolved  in  a  definite  volume  of  water.    Meas- 
ured portions  of  this  solution  are  taken  for  analysis.    While  calcium 
hypochlorite  is  quite  soluble,  bleach  ing-powder  contains   a  con- 
siderable amount  of  insoluble  material.     Even  when  the  bleaching- 
powTder  is  ground  to  a  smooth  paste  with  water,  the  insoluble 
material  always  contains  some  bleaching  material.     The  insoluble 
material  is,  therefore,  not  filtered  off,  but  is  shaken  up  when  por- 
tions are  measured  out.     A  pipette  should  be  used  for  this  pur- 
pose, so  as  to  obtain  a  fair  proportion  of  the  insoluble  material. 

502.  lodometric  Titration. — The  amount  of  available  chlorine 
may  be  estimated  by  adding  potassium  iodide  to  a  portion  of 
the  solution  of  the  bleaching-powder  and  acidifying.     The  follow- 
ing reaction  takes  place: 

Ca(C10)2+  4KI + 4HC1  =  CaCl2 + 4KC1 + 2I2+  2H20. 

As  some  calcium  chlorate  is  usually  present,  the  solution  is  not 
made  strongly  acid,  as  chlorine  would  in  that  case  be  produced  by 
the  decomposition  of  this  salt.  For  this  reason,  if  hydrochloric 
acid  is  employed,  it  is  added  in  very  slight  excess.  The  difficulty 
is  entirely  obviated  by  the  use  of  acetic  acid.  The  iodine  liber- 
ated is  then  titrated  by  means  of  sodium  thiosulphate  solution. 

503.  Titration   with  Sodium  Arsenite. — The  hypochlorite  may 
also  be  titrated  directly  with  sodium  arsenite  solution.     To  ascer- 
tain the  end-point  drops  of  the  solution  are  touched  to  starch- 
iodide  paper.     When  the  hypochlorite   is  entirely  decomposed, 
the  starch-iodide  paper  is  no  longer  made  blue. 


EXERCISE  67. 
Determination  of  Available  Chlorine  in  Bleaching-powder. 

504.  Solution. — Weigh  out  7.091  grams  of  bleaching-powder.  As  the 
material  is  hygroscopic,  and  also  loses  chlorine,  a  weighing-bottle  must  be 
used.  Transfer  the  material  to  a  clean  porcelain  mortar  and  grind  to  a 
smooth  paste  with  the  addition  of  a  little  water.  Add  about  75  c.c.  of 
water,  mix  thoroughly  with  the  pestle,  allow  to  settle  a  few  minutes,  and 


348  VOLUMETRIC  METHODS. 

decant  into  a  liter  flask,  leaving  the  bulk  of  the  bleaching-powder  in  the 
mortar.  Repeat  this  operation  until  all  of  the  material  has  been  trans- 
ferred to  the  liter  flask,  which  is  then  filled  to  the  mark  with  distilled  water. 

505.  lo  do  me  trie    Titration.  —  The   solution   is    thoroughly   shaken     and 
50-c.c.  portions  of  the  milky  liquid  are  measured  out  with  a  pipette  and 
transferred  to  beakers.     About  1  gram  of  potassium  iodide  dissolved  in  a 
little  water  is  added  to  one  of  the  beakers.     The  solution  is  acidified  with 
acetic  acid  and  the  iodine  liberated  titrated  with  N/10  sodium  thiosulphate 
solution.     Duplicate  determinations  with  the  other  portions  are  made  in 
the  same  manner.     1  c.c.  of  the  tenth-normal  thiosulphate  solution  is  equal 
to  .003545  gram  of  chlorine.     If  exactly  7.091  *  grams  of  the  bleaching- 
powder  was  weighed  out,  the  number  of  cubic  centimeters  of  N/10  thiosul- 
phate solution  used  will  be  equal  to  the  percentage  of  available  chlorine 
in  the  bleaching-powder. 

506.  Titration  with  Sodium  Arsenite. — 50-c.c.  portions  of  the  solution  of 
bleaching-powder  may  also  be  titrated  with  the  sodium  arsenite  solution 
made  for  standardizing  the    iodine  solution    (page  341).     STARCH-IODIDE 
PAPER  may  be  made  as  follows :  One  gram  of  starch  is  ground  to  a  paste  witn 
a  little  water  and  100  c.c.  of  boiling  water  added  with  stirring.     The  solution 
is  filtered  and  to  the  filtrate  0.1  gram  of  potassium  iodide  is  added.     Strips 
of  filter-paper  are  dipped  in  this  solution  and  dried  at  40°  to  50°.      Strips 
of  this  paper  are  moistened  with  distilled  water  and  laid  on  a  porcelain  or 
glass  plate.     Nearly  all  the  sodium  arsenite  solution  thought  necessary  is 
added  to  the  solution  of  the  bleaching-powder.     After  stirring  thoroughly, 
a  drop  is  taken  out  and  touched  to  the  test  paper.     The  depth  of  the  color 
indicates  the  amount  of  arsenite  solution  still  to  be  added.     When  consid- 
erable  hypochlorite  is  still  present,  the  spot   becomes   almost  black  and 
then  colorless  again  as  the  iodine  is  converted  into  the  trichloride  IC13. 
Several  cubic  centimeters  of  the  arsenite  solution  should  then  be  added. 
When  the  spot  test  is  only  light  blue,  the  arsenite  solution  should  be  added 
by  drops  until  the  blue  color  has  entirely  disappeared.      The  calculation 
is  made  in  the  same  manner  as  with  the  iodine  solution. 


PROBLEM. 

Problem  13.  A  mixture  of  crystallized  sodium  thiosulphate,  sodium  sul- 
phite, and  sodium  sulphate  was  analyzed  in  the  following  manner  :  A  gram 
portion  was  dissolved  in  water  and  titrated  with  N/10  iodine  solution,  49  c.c. 
being  required.  The  hydriodic  acid  in  this  solution  was  then  titrated 
with  N/5  sodium-hydroxide  solution,  9.5  c.c.  being  required.  The  total 
amount  of  sulphur  in  a  half-gram  portion  was  then  determined  as  barium 
sulphate,  of  which  .821  grams  was  obtained.  Calculate  the  amount  of 
HA  and  Na2S04.10H:2O  present  in  the  mixture. 


*  Note  for  student:   Show  how  this  figure  is  obtained. 


PRECIPITATION  METHODS. 

CHAPTER  XXVI. 

DETERMINATION    OF    CHLORIDES,    CYANIDES, 
AND  SILVER. 

507.  Theory  of  Indicators. — In  the  volumetric  methods  already 
discussed  the  products  of  the  reactions  have  been  soluble,  and  the 
end-point  has  been  obtained  by  the  action  of  a  slight  excess  of  one 
of  the  reagents  on  the  indicator. 

In  ACIDIMETRY  and  ALKALIMETRY  the  indicator  is  either  an  add 
or  a  base,  but  possessing  such  a  weak  chemical  affinity  that  it  cannot 
remain  combined  until  an  excess  of  the  appropriate  reagent  (base 
or  acid)  is  present.  METHYL  ORANGE,  for  instance,  is  a  weak  acid 
having  a  bright-red  color,  while  its  salts  are  light  yellow.  If  methyl 
orange  is  added  to  a  solution  containing  a  free  base,  the  salt  of 
methyl  orange  and  the  base  will  be  formed  and  the  yellow  color 
of  the  salt  will  be  given  to  the  solution.  If  carbon  dioxide  in 
excess  is  added  to  this  solution,  the  color  remains  yellow  because 
the  carbonic  acid  formed  is  weaker  than  the  methyl-orange  acidr 
and  is  therefore  unable  to  displace  the  latter  from  its  combination 
with  the  base.  If  a  strong  acid  like  hydrochloric  is  added  to  the 
solution,  it  remains  yellow  as  long  as  there  is  more  base  present 
than  can  combine  with  the  hydrochloric  acid  added.  When  a 
slight  excess  of  the  strong  acid  has  been  added,  the  base  combined 
with  the  weakly  acidic  methyl  orange  is  taken  by  the  strong  acid 
The  free  methyl  orange  then  gives  the  reddish  color  to  the  solution. 

In  PRECIPITATION  METHODS,  on  the  other  hand,  advantage  is 
most  frequently  taken  of  the  relative  solubilities  of  the  salts  of  a 
metal  to  obtain  the  end-point.  The  colored  salt  which  indicates 
the  end  of  a  reaction  must  be  a  soluble  compound,  while  the  ele- 
ment to  be  determined  must  form  an  insoluble  compound. 

349 


350  VOLUMETRIC  METHODS. 

508.  Silver  Chromate   as   Indicator. — In  the  determination  of 
chlorides  by  means  of  silver  nitrate  solution,  a  little  neutral  potas- 
sium chromate  is  added  to  the  solution.      On  adding  the  silver 
nitrate  solution  the  silver  is  precipitated  as  silver  chloride.     When 
enough  silver  nitrate  has  been  added  to  precipitate  all  of  the 
chlorine,  the  red  silver  chromate  is  formed,  which  indicates  the 
end  of  the  reaction.     As  long  as  chlorides  exist  in  the  solution, 
any  silver  chromate  formed  will  be  decomposed,  since  if  any  of 
this  salt  is  present,  it  will  tend  to  keep  the  solution  saturated, 
and  any  silver  which  goes  into  solution  as  chromate  will  be  imme- 
diately precipitated  as  chloride  because  of  the  insolubility  of  the 
latter  salt.     Only  when  chlorides  are  absent  from  the  solution 
can  silver  chromate  be  permanently  formed.     As  the  silver  chro- 
mate is  decomposed  by  even  a   small   amount  of  free   acid,  the 
solution  must  be  neutral  or  slightly  alkaline  with  sodium  car- 
bonate. 

509.  Ferric  Sulphocyanate  as  Indicator. — Silver  also  forms  an 
insoluble  salt  with  sulphocyanic  acid.     This  acid  produces  an  in- 
tensely red  color  with  solutions  of  ferric  iron.    The  titration  of  silver 
is  conducted  by  adding  a  standard  solution  of  the  sulphocyanate 
to  the  silver  solution  containing  some  ferric  sulphate  or  nitrate. 
As  long  as  silver  is  present  in  the  solution  the  sulphocyanic  acid 
is  precipitated,  so  that  the  iron  compound  which  is  soluble  cannot 
exist.     When  all  of  the  silver  has  been  precipitated,  the  next  drop 
of  the  sulphocyanate  solution  forms  a  little  of  the  red  ferric  com- 
pound which  indicates  the  end  of  the  reaction.     As  the  silver 
sulphocyanate  is  insoluble  in  dilute  acids,  and  the  ferric  sulpho- 
cyanate is  readily  formed  under  these  conditions,  the  titration 
may  be  carried  on  in  acid  solutions.     The  titration  of  chlorides 
in  acid  solution  may  also  be  carried  out  by  this  method  which  is 
known  as  VOLHARD'S.  A  measured  amount  of  silver  nitrate  solution 
which  is  more  than  sufficient  to  precipitate  the  chloride  present  is 
added  to  the  solution.     After  the  addition  of  some  ferric  iron  as 
indicator,  the  excess  of  silver  is  titrated  back  with  standard  sul- 
phocyanate solution.     Because  of  the  solubility  of  silver  chloride 
in  solutions  of  the  thiocyanates,  this  method  does  not  give  abso- 
lutely correct   results  unless  the  silver  chloride  is  filtered  off  and 
the  excess  of  silver  determined  in  the  filtrate. 


TITRATION  OF   CYANIDES.  351 

510.  Silver   Chloride    as   Indicator.  —  Chlorides   may   also   be 
titrated  with  silver  nitrate  solution  without  the  addition  of  an 
indicator.     After  each  addition  of  silver  nitrate  the  solution  is 
thoroughly  shaken.     The  silver  chloride  readily  collects  so  as  to 
leave  a  clear  solution  in  which  the  formation  of  a  white  precipitate 
on  the  addition  of  another  drop  of  silver  nitrate  may  readily  be 
observed.     The  silver  chloride  does  not  collect  readily  unless  a 
considerable  amount  of  chloride  is  present  and  the  volume  of  the 
solution  is  not  too  large. 

511.  Titration    of    Cyanides. — The    alkali    cyanides    may    be 
titrated  with  a  silver  nitrate  solution  in  the  same  manner  as  chlo- 
rides, the  end-point  being  obtained  when  no  more  precipitate  is 
formed.     The  reaction  takes  place  in  two  stages  as  represented 
by  the  following  equations: 

2KCN  +  AgN03  =  KCN.AgCN  +  KN03; 
KCN.AgCN  +  AgN03=  2AgCN  +  KN03. 

Until  one-half  of  the  cyanide  has  been  converted  into  the  silver 
salt,  any  precipitate  of  silver  cyanide  which  forms  is  dissolved, 
by  the  undecomposed  cyanide  on  stirring  the  solution.  Whem 
the  reaction  represented  by  the  first  equation  is  completed,  further 
addition  of  silver  nitrate  forms  a  permanent  precipitate  of  silver 
cyanide  according  to  the  second  equation.  The  precipitate  readily 
collects  in  flocks  on  vigorously  stirring  the  solution,  leaving  a 
clear  liquid  in  which  the  absence  of  a  precipitate  on  the  addition 
of  a  drop  of  silver  nitrate  may  be  readily  observed  and  serves  to 
indicate  the  end-point. 

This  titration  may  also  be  conducted  so  that  the  reaction 
represented  by  the  first  equation  is  carried  out.  The  silver  solu- 
tion is  added  until  a  slight  permanent  precipitate  is  formed.  If 
CHLORIDES  are  present  in  the  alkali  cyanides,  as  is  frequently  the 
case,  silver  chloride  will  be  precipitated  in  preference  to  the  silver 
cyanide  because  of  the  greater  insolubility  of  the  former.  The 
silver  chloride  reacts  with  the  alkali  cyanide  as  follows: 

AgCl-f  2KCN  =  KCN.AgCN-}-  KCL 


352  VOLUMETRIC  METHODS. 

When  all  of  the  cyanide  has  been  converted  into  the  double  cyanide 
of  silver  and  potassium,  the  next  drop  of  silver  nitrate  produces  a 
permanent  white  precipitate.  If  chlorides  are  present,  this  precipi- 
tate will  be  silver  chloride,  thus  forming  a  more  delicate  indicator 
than  the  more  soluble  silver  cyanide.  The  amount  of  cyanide 
present  will  also  be  correctly  determined,  as  is  evident  from  the 
equation  given.  If  the  titration  is  carried  to  the  second  stage, 
the  presence  of  the  chloride  introduces  an  error,  as  all  the  chloride 
combines  with  silver  before  the  end-point  is  reached.  The  amount 
of  chloride  present  may  be  determined  by  noting  the  volume  of 
silver  nitrate  solution  required  for  the  completion  of  each  stage 
of  the  titration.  The  excess  of  the  silver  nitrate  required  for  the 
second  part  of  the  titration  is  that  necessary  to  precipitate  the 
chloride. 

512.  Strength  of    Solutions. — The  standard  solutions  used  in 
precipitation  methods  may  be  made  according  to  the  normal 
system,  a  gram  molecular  weight  of  silver  nitrate,  sodium  chloride, 
etc.,  being  dissolved  in  a  liter  of  a  normal  solution.     Such  solutions 
are  convenient  for  use  when  several  substances  must  be  titrated 
with  the  same  standard  solution.    When  a  solution  is  to  be  used 
exclusively  for  the  titration  of  a  given  substance,  it  is  more  con- 
venient to  make  it  of  such  strength  that  1  c.c.  is  equal  to  an  amount 
of  the  given  substance  which  can  be  represented  by  a  simple 
number.     If  a  silver  nitrate  solution  is  made  of  such  a  strength  that 
1  c.c.  is  equal  to  1  mg.  of  chlorine,  the  number  of  cubic  centi- 
meters of  silver  nitrate  solution  used  to  titrate  the  chlorine  in  1 
gram  of  a  given  substance  will  give  the  per  cent  of  chlorine  if 
divided  by  10. 

513.  Standard  Silver  Nitrate  Solutions  are  made  by  dissolving 
weighed  amounts  of  the  crystals  and  diluting  to  the  required 
volume.    The  salt  may  be  purchased   in  very  pure   condition, 
seldom  requiring  drying  even.     As  pure  metallic  silver  may  also 
be  readily  obtained  the  standard  solutions  are  sometimes  made 
by  weighing  out  the  pure  metal  and  dissolving  in  nitric  acid.     If 
a  neutral  solution  is  required,  the  excess  of  nitric  acid  may  be 
removed  by  evaporation  to  dryness  on  the  water-bath.     The  last 
traces  of  acid  are  removed  by  moistening  with  water  several  times 
and  evaporating  to  dryness  on  the  water-bath.     If  protected  from 


TITRATION  OF  CHLORIDES.  353 

the  sunlight  and  contact  with  organic  matter,  solutions  of  silver 
nitrate  retain  their  strength  for  a  considerable  time  unless  very 
dilute. 

514.  Standardization. — Solutions    of    silver    nitrate    may    be 
standardized  by  titration  against  weighed  amounts  of  pure  sodium 
chloride  prepared  as  directed  in  Exercise  6.     A  saturated  solution 
of  potassium  chromate  is  used  as  the  indicator.     4  to  5  drops  of 
fihe  chromate  solution  are  added  to  the  solution. to  be  titrated,  the 
volume  of  which  should  not  be  large.     The  detection  of  the  first 
faint  tinge  of  red  in  the  yellow  chromate  solution  offers  some 
difficulties.      It  is  more  readily  done  by  gaslight  than  by  sun- 
light because  of  the  yellow  tinge  of  the  former.     By  daylight  the 
red  tinge  is  more  readily  detected  by  a  comparison  of  the  color  of 
the  solution  being  titrated,  with  the  color  of  a  solution  previously 
titrated  from  which  the  red  color  has  been  removed  by  the  addi- 
tion of  a  pinch  of  sodium  chloride.     The  red  color  is  also  made 
more  pronounced  by  looking  at  the  solution  through  a  layer  of 
water  tinted  with  chromate  to  the  same  color  as  the  solution  to  be 
titrated.     It  is  also  advantageous  to  carry  out  the  titration  in  a 
porcelain  dish,  thus  giving  a  white  surface  for  comparison. 

515.  Indirect    Determinations    by    Means    of    Standard    Silver 
Nitrate. — Besides  the  determination  of  chlorides,  this  method  may 
be  used  to  determine  indirectly  all  substances  which  can  be  con- 
verted into  neutral  chlorides  by  treatment  with  hydrochloric  acid 
and  evaporation  to  dryness  to  expel  the  excess  of  hydrochloric 
acid.     The  carbonates  of  the  alkalies  and  the  alkaline  earths  may 
be  treated  in  this  manner  to  determine  the  base,  and  in  cases  where 
only  the  normal  carbonate  can  be  present,  carbon  dioxide  may  be 
determined  in  this  manner.     The  nitrates  also  of  these  metals  may 
be  converted  into  chlorides  by  a  similar  treatment  and  determined. 

516.  Determination   of  Sodium   and   Potassium   in   the   Mixed 
Chlorides. — The  amount  of  potassium  and  sodium  present  in  a 
mixture  of  the  chlorides  of  these  metals  may  be  ascertained  by  a 
determination  of  the  amount  of  chlorine  present,  the  weight  of 
the  mixed  chlorides  having  first  been  found.     An  indirect  deter- 
mination of  this  kind  gives  good  results  only  when  there  is  a  con- 
siderable difference  in  the  atomic  weights  of  the  elements,  and  a 
large  precipitate  is  weighed.    The  method  is  not  accurate  for  a 


354  VOLUMETRIC  METHODS. 

small  amount  of  one  element  in  the  presence  of  a  large  amount  of  the 
other.  The  weight  of  sodium  chloride  in  the  mixture  is  calculated 
by  multiplying  the  amount  of  chlorine  found  by  2.1032, 

/Molecular  weight  KC1\ 
\    Atomic  weight  Cl     /' 

deducting  from  the  product  the  combined  weight  of  the  chlorides 
and  multiplying  the  remainder  by  3.6426: 

Mol.  wt.  Nad 


Mol.  wt.  KCl-Mol.  wt.  NaCl 

In  multiplying  the  weight  of  chlorine  found  by  2.1032,  the  amount 
of  potassium  chloride  is  found  which  would  be  obtained  if  this 
salt  alone  were  present.  If  this  product  is  the  same  as  the  weight 
of  the  mixed  chlorides,  only  potassium  can  be  present. 


EXERCISE  68. 
Determination  of  Cyanogen  in  Potassium  Cyanide. 

In  order  to  secure  a  fair  sample  of  the  material,  weigh  out  5  grams  of 
the  potassium  cyanide  selected  from  various  parts  of  the  stock  to  be  exam- 
ined. Dissolve  in  water  and  dilute  to  500  c.c.  Withdraw  50-c.c.  portions 
for  titration.  If  a  pipette  is  used  for  this  purpose,  a  loose  plug  of  cotton 
moistened  with  silver  nitrate  solution  should  be  placed  in  the  upper  part  of 
the  stem,  and  great  care  should  be  taken  not  to  draw  any  of  the  solution 
into  the  mouth. 

The  silver  nitrate  solution  is  made  by  weighing  out  17  grams  of  the  pure 
dry  crystals,  dissolving  in  water,  and  diluting  to  1  liter.  It  is  advisable  to 
test  the  distilled  water  for  chloride  before  adding  it  to  the  silver  nitrate. 
Pour  the  solution  into  a  glass-stoppered  bottle  and  keep  in  the  dark.  The 
50-c.c.  portions  of  the  cyanide  solution  are  diluted  with  an  equal  volume  of 
water  and  the  silver-nitrate  solution  added  with  constant  stirring  until  a 
slight  permanent  opalescence  is  obtained.  1  c.c.  N/10  silver  nitrate  solu- 
tion is  equal  to  .01304  gram  of  potassium  cyanide.  As  commerical  potas- 
sium cyanide  is  frequently  contaminated  with  sodium  cyanide,  the  percentage 
of  potassium  cyanide  found  may  be  greater  than  100%.  This  will  not  occur 
if  the  result  is  given  in  percentage  of  cyanogen.  1  c.c.  of  N/10  silver  nitrate 
solution  is  equal  to  .005208  gram  of  cyanogen. 


DETERMINATION  OF  SILVER.  355 

517.  Determination  of  Silver  in  Alloys. — The  silver  in  alloys 
can  very  readily  be  titrated  by  the  VOLHARD  METHOD,  since  the 
presence  of  a  considerable  amount  of  free  nitric  acid  does  not  dis- 
solve the  silver  sulphocyanate.  Nitrous  acid  must  be  absent, 
however,  and  that  produced  by  dissolving  the  alloy  must  be  entirely 
expelled  by  boiling  the  solution.  The  amount  of  copper  present 
must  not  exceed  70%.  Alloys  in  which  the  percentage  of  copper 
is  higher,  may  be  analyzed  by  adding  an  accurately  measured 
volume  of  standard  silver  solution  or  an  accurately  weighed 
amount  of  the  pure  metal,  so  that  the  ratio  of  copper  to  silver 
shall  not  exceed  7  to  3.  Mercury  must  be  removed  by  ignition. 
Arsenic,  antimony,  cadmium,  lead,  bismuth,  tin,  and  zinc  do  not 
interfere  with  the  tit  rat  ion.  Cobalt  and  nickel  interfere  because 
of  the  color  of  their  nitrates. 


EXERCISE  69. 
Determination  of  Silver  in  a  Coin  by  Volhard's  Method. 

Weigh  out  7.5  to  8  grams  of  ammonium  sulphocyanate,  dissolve  in 
water,  and  dilute  to  1  liter.  Standardize  the  solution  by  titration  against 
the  tenth-normal  silver  solution  prepared  for  Exercise  68.  Measure  out 
into  a  beaker  30  to  40  c.c.  of  the  standard  silver  solution,  dilute  with 
100  to  200  c.c.  of  water,  and  add  5  c.c.  of  a  cold  saturated  solution  of 
ferric  alum.  The  ferric  iron  solution  must  be  free  from  chlorine,  and  if 
turbid,  a  few  cubic  centimeters  of  dilute  nitric  acid  may  be  added.  Place 
the  beaker  containing  the  silver  on  a  white  surface  and  add  the  sulpho- 
cyanate solution  with  constant  stirring  until  a  faint  permanent  reddish 
tinge  is  given  to  the  solution.  Repeat  the  titration  until  duplicates  are 
obtained.  As  the  sulphocyanate  solution  is  permanent,  it  is  advisable  to 
dilute  it  to  exact  strength.  When  it  is  used  to  determine  silver,  it  is  most 
convenient  to  dilute  it  to  such  a  strength  that  1  c.c.  will  be  equal  to  exactly 
.010  gram  of  silver.  Having  found,  for  instance,  that  30  c.c.  of  the  sulpho- 
cyanate solution  is  equal  to  32  c.c.  of  the  N/10  silver  nitrate  solution,  the 
amount  of  silver  equivalent  to  the  30  c.c.  of  the  sulphocyanate  solution  is 
found  by  multiplying  32  by  .010793,  which  gives  .3454  gram  of  silver.  As 
it  is  desired  to  dilute  the  solution  so  that  30  c.c.  shall  be  equal  to  .300  gram, 
the  volume  to  be  diluted  to  1000  c.c.  is  found  by  the  proportion 

.300  :  .3454  :  :  x  :  1000. 


356  VOLUMETRIC  METHODS. 

As  x  =  868.58,  this  number  of  cubic  centimeters  is  diluted  to  1  liter.  The 
strength  of  the  solution  after  dilution  is  verified  by  again  titrating  against 
the  standard  silver  solution. 

Thoroughly  clean  a  dime  or  other  silver  coin  by  rubbing  with  alcohol 
ammonia,  etc.  Weigh  it  carefully  and  dissolve  in  20  c.c.  of  a  mixture  of 
equal  parts  of  nitric  acid  and  water.  For  this  purpose  place  the  coin  in  a 
beaker,  cover  with  a  watch-crystal,  and  warm  on  the  water-bath  until  it  is 
entirely  dissolved.  Remove  the  watch-crystal  after  rinsing  it  with  water 
and  continue  warming  the  solution  until  the  nitrous  fumes  are  entirely 
expelled.  Transfer  the  solution  to  a  250-c.c.  flask,  rinsing  the  beaker  thor- 
oughly. Dilute  the  solution  to  the  mark  and  shake  thoroughly.  Take 
out  50-c.c.  portions  with  a  pipette,  dilute  with  100  to  200  c.c.  of  water, 
add  5  c.c.  of  the  ferric  iron  solution,  and  titrate  with  the  sulphocyanate. 
Repeat  the  titration  until  duplicates  are  obtained.  Calculate  the  per  cent 
of  silver  in  the  coin. 


PROBLEMS. 

Problem  14.  The  combined  weight  of  sodium  and  potassium  chlorides 
was  2.5685  grams.  1.432  grams  of  chlorine  was  present.  Calculate  the 
percentage  of  potassium  and  sodium  chloride  present. 

Problem  15.  The  combined  weight  of  the  sodium  and  potassium  chlorides 
was  2.032  grams.  When  converted  into  sulphates,  the  weight  was  2.385 
grams.  Calculate  the  percentage  of  sodium  and  potassium  present. 


CHAPTER  XXVII. 

DETERMINATION  OF  PHOSPHORIC  ACID. 

A  GOOD  many  methods  of  determining  phosphoric  acid  volu- 
me trically  have  been  devised.  None  of  these  methods  give  as 
reliable  and  accurate  results  as  the  gravimetric  method.  The 
great  amount  of  labor  necessary  and  the  great  length  of  time 
which  must  elapse  from  the  beginning  to  the  end  of  the  gravimetric 
determination  render  the  use  of  the  volumetric  methods  very 
desirable.  Three  of  the  best  of  these  methods  will  be  described. 

THE  URANIUM  METHOD. 

This  method  is  applicable  to  the  titration  of  the  phosphoric 
acid  in  the  alkali  and  alkaline  earth  phosphates.  IRON,  and  espe- 
cially ALUMINIUM,  if  present  in  considerable  amount,  interfere 
seriously  with  the  titration.  The  method  is  applicable  to  most 
of  the  material  used  for  fertilizers,  such  as  bones,  bone-ash,  soluble 
phosphates,  phosphorite,  etc.  The  method  is  based  on  the  fact 
that  when  uranium  nitrate  or  acetate  is  added  to  a  neutral  solution 
of  a  phosphate  all  of  the  phosphoric  acid  is  thrown  down  as  yellow 
uranyl  phosphate,  U02HP04.  Free  mineral  acid  must  be  absent, 
but  the  solution  may  be  moderately  acid  with  acetic  acid. 

A  very  delicate  INDICATOR  for  the  excess  of  the  uranium  is 
found  in  the  reaction  with  potassium  ferrocyanide,  which  gives  a 
decided  brown  tinge  with  very  small  amounts  of  uranium.  Unfor- 
tunately, the  method  is  not  an  absolute  one,  but  gives  results 
which  vary  with  the  volume  of  the  solution,  the  salts  present,  etc., 
so  that  the  standardization  of  the  solution  must  be  made  under 
as  nearly  as  possible  the  same  conditions  as  the  determination  to  be 
carried  out.  The  standardization  and  determination  should  be 

357 


358  VOLUMETRIC  METHODS. 

carried  out  by  the  same  individual.  To  complete  the  precipita- 
tion of  the  phosphoric  acid,  the  solution  must  be  boiled  after  each 
addition  of  the  standard  solution.  Some  calcium  phosphate  is 
carried  down  with  the  uranium  phosphate,  so  that  if  a  solution 
containing  calcium  is  to  be  titrated,  the  uranium  solution  must  be 
standardized  by  calcium  phosphate,  as  the  alkali  phosphates  are 
not  carried  down  in  this  manner.  Several  titrations  must  also 
be  made,  in  the  first  adding  the  uranium  solution  in  small  amounts 
until  the  end-point  is  reached.  In  the  second  tit  ration  nearly 
all  of  the  uranium  solution  is  added  at  once. 

518.  Standard    Solution. — The  uranium  solution  is  made  by 
dissolving  about  35  grams  of  uranium  nitrate  or  acetate  in  water 
and  diluting  to  a  liter.     3J  grams  of  ammonium  or  sodium  acetate 
are  added  to  the  nitrate  solution  to  neutralize  any  free  acid  present. 
The  addition  of  50  c.c.  of  pure  glacial  acetic  acid  to  either  the 
nitrate  or  the  acetate  solution  renders  it  more  stable  on  exposure 
to  light.     The  nitrate  may  usually  be  obtained  in  a  purer  condi- 
tion than  the  acetate,  while  the  addition  of  an  alkali  acetate  to 
the  latter  is  unnecessary.     The  solution  is  allowed  to  stand  for 
several  days  and  is  then  filtered,  or  the  clear  liquid  drawn  off. 

519.  Standardization. — For  standardizing  the   solution  a  pure 
salt  or  a  solution  of  a  phosphate  in  which  the  phosphoric  acid  has 
been  accurately  determined  must  be  available.     DISODIUM  PHOS- 
PHATE and  MICROCOSMIC  SALT  may  be  recrystallized  until  absolutely 
pure,  although  some  uncertainty  always  exists  as  to  the  amount 
of  water  present   in   the   dried   crystals.     DIHYDRIC   POTASSIUM 
PHOSPHATE  has  the  advantage  that  it  contains  no  water  of  crystal- 
lization.    If  these  pure  salts  are  not  at  hand,  a  solution  may  be 
made  containing  approximately  0.1  gram  P205  in  50  c.c.     100-c.c. 
portions  of  this  solution  may  be  measured  out  and  the  phosphoric 
acid  present  determined  by  precipitation  with  magnesia  mixture 
and  weighing  as  magnesium  pyrophosphate. 

520.  Titration. — 50  c.c.  of  the  standard  phosphate  solution  is 
measured  out  into  a  beaker  and  heated  to  90°  or  100°.     The 
uranium  solution  is  added  until  a  drop  taken  out  and  placed  on  a 
white  porcelain  surface  gives  a  brown  ring  when  the   centre  of 
the  drop  is  touched  with  a  glass  rod  which  has  been  dipped  in  a 
freshly  made   solution   of  potassium  ferrocyanide.     This  solution 


DETERMINATION  OF  PHOSPHORIC  ACID.  359 

is  made  by  dissolving  about  J  gram  of  the  salt  in  20  c.c.  of  water. 
After  each  addition  of  uranium  the  solution  is  brought  to  a  boil. 
The  color  of  the  spot  test  increases  slowly  on  standing.  The  worker 
must  therefore  note  the  depth  of  color  after  a  fixed  interval  of 
time.  Uniformity  in  this  respect  is  most  readily  secured  by  always 
observing  the  color  after  an  additional  drop  has  been  added  to 
the  solution  being  titrated  and  the  solution  brought  to  a  boil. 
When  the  end-point  is  reached,  this  drop  will  be  in  excess,  but  a  cor- 
rection may  easily  be  made  to  the  reading  after  noting  the  change 
in  the  reading  of  the  burette  produced  by  letting  one  drop  fall 
from  the  burette.  A  second  and  third  tit  rat  ion  are  made,  adding 
nearly  the  entire  amount  of  uranium  solution  at  once.  The  ura- 
nium solution  is  then  diluted  so  that  1  c.c.  is  equal  to  .005  gram 
of  P205. 

521.  Standardization  with  Calcium  Phosphate. — When  the  ura- 
nium solution  is  to  be  used  for  the  titration  of  phosphoric  acid  in 
the  presence  of  calcium,  it  must  be  standardized  by  means  of  a 
solution  of  tricalcium  phosphate.     About  5.5  grams  of  the  pure 
material  is  weighed  out  and  dissolved  in  a  little  dilute  nitric  or 
hydrochloric  acid.     It  is  precipitated  by  adding  a  slight  excess  of 
ammonia   and  redissolved  in  a  moderate  excess  of   acetic  acid. 
The  solution  is  then  diluted  to  a  liter.     The  phosphoric  acid  in  50- 
c.c.  portions  of  this  solution  is  determined  by  precipitation  as 
molybdate,  dissolving  in  ammonia,  and  reprecipitating  with  mag- 
nesia mixture  and  weighing  as  magnesium  pyrophosphate.      The 
titration  of  50-c.c.  portions  of  this   solution  with  the   uranium 
solution  is  conducted  as  already  given. 

522.  Titration    of    Phosphates. — The    analysis   of    phosphates 
which  are  free  from  more  than  traces  of  iron  and  aluminium  is 
made  by  dissolving  weighed  portions,  so  that  50  c.c.  shall  con- 
tain about  0.1  gram  P205.   If  the  material  is  dissolved  in  the  strong 
mineral  acids,  the  solution  must  be  neutralized  with  ammonia  and 
then  acidified  with  acetic  acid.     50-c.c.  portions  are  then  titrated 
exactly  as  directed  for  the  standardization. 

When  considerable  iron  or  aluminium  is  present,  the  phosphoric 
acid  must  be  separated  from  these  elements  by  precipitation  as 
phosphomolybdate,  which  is  dissolved  in  ammonia,  and  the  phos- 
phoric acid  again  precipitated  with  magnesia  mixture.  The  mag- 


360  VOLUMETRIC  METHODS. 

nesia  precipitate  is  then  dissolved  in  a  little  nitric  or  hydrochloric 
acid,  the  solution  neutralized  with  ammonia  and  then  acidified 
with  acetic  acid.  The  phosphoric  acid  is  then  titrated  with  the 
uranium  solution. 


TITRATION  OF  THE  PHOSPHOMOLYBDATE. 

Two  volumetric  methods  of  determining  phosphoric  acid  have 
been  based  on  the  properties  of  the  phosphomolybdate  precipi- 
tate. By  one  of  these  methods,  that  of  Pemberton,*  the  molybdic 
acid  in  this  precipitate  is  titrated  by  means  of  standard  caustic  soda 
or  potash  with  phenolphthalein  as  the  indicator  according  to  the 
following  equation : 

2(NH4)3P04.12Mo03+46KOH= 

2(NH4)2HP04+  (NH4)2Mo04+  23K2Mo04+  22H20. 

By  the  other  methodf  the  molybdic  acid  is  reduced  by  means  of 
zinc  to  molybdic  oxide  according  to  the  equation: 

2Mo03+  6H=Mo203+  3H20. 

The  molybdic  oxide  is  then  oxidized  by  means  of  standard  potas- 
sium permanganate  solution. 

523.  Pemberton's  Alkalimetric  Method. — This  method  is  appli- 
cable to  the  titration  of  the  phosphates  of  the  alkalies  and  the  alka- 
line earths,  but  the  results  are  not  so  good  when  considerable 
quantities  of  iron  or  aluminium  are  present.     In  the  presence  of 
large  amounts  of  these  metals,  it  is  advisable  to  redissolve  and 
reprecipitate  the  phosphomolybdate.    As  sulphuric  acid  may  be 
carried  down  with  the  precipitate,  it  should  be  absent  from  the 
solution.     If  present  it  may  be  removed  by  means  of  barium 
chloride. 

524.  Standard  Solutions. — A  standard  solution  of  nitric  acid 
and  one  of  caustic  soda  or  potash  free  from  carbon  dioxide  are 
required.    The  solutions  are  made  of  such  a  strength  that  1  c.c.  is 
equal  to  .001  gram  of  P205.    Since  46  molecules  of  caustic  soda  or 
potash  are  equal  to  1  molecule  of  phosphorus  pentoxide,  a  liter  of 

*  Jour.  Am.  Chem.  Soc.,  1894,  278. 

t  Blair,  The  Chemical  Analysis  of  Iron,  1901,  p.  104. 


DETERMINATION  OF  PHOSPHORIC  ACID.  361 

normal  solution  will  be  equal  to  l/46th  of  the  molecular  weight  of 
P205,  or  3.087  grains.  A  solution  of  convenient  strength  will  be 
.3239  normal,  1  liter  of  such  a  solution  being  equal  to  1  gram  of 
P205.  The  nitric  acid  may  be  made  by  diluting  21  c.c.  strong 
nitric  acid  (sp.  gr.  1.40)  to  1  liter.  The  caustic  soda  solution  may 
be  made  by  adding  to  100  grams  of  pure  caustic  soda  an  amount  of 
water  just  insufficient  to  dissolve  it.  After  allowing  the  solution 
to  settle  in  a  tall  covered  vessel,  17  c.c.  of  the  clear  liquid  is  with- 
drawn with  a  pipette  and  diluted  to  1  liter.  Most  of  the  sodium 
carbonate  remains  undissolved.  If  caustic  potash  is  used,  about 
20  grams  are  weighed  out  and  diluted  to  a  liter.  The  carbon 
dioxide  present  must  be  precipitated  by  means  of  barium  hydroxide 
or  chloride.  Both  the  caustic  soda  and  caustic  potash  solutions 
must  be  protected  from  the  carbon  dioxide  of  the  air. 

525.  Standardization. — The  solutions  may  be  standardized  by 
any  of  the  methods  given  in  Chapters  XXI  and  XXII.     The  acid 
must  be  compared  with  the  alkali,  using  phenolphtha'ein  as  the 
indicator.     A  much  better  method  of  standardization  consists  in 
precipitating  a  known  amount  of  phosphoric   acid  by  means  of 
the  molybdate  solution  and  titrating  the  precipitate  in  exactly 
the  same  manner  as  in  the  analysis  of  an  unknown  phosphate. 

526.  Precipitation    of   the    Phosphoric   Acid. — The  molybdate 
solution  is  made  according  to  the  directions  given  on  p.  507.    For 
washing  the  precipitate,  a  dilute  solution  of  nitric  acid  is  used,  a 
solution  made  by  adding  about  15  c.c.  concentrated  acid  to  1  liter 
of  water  being  suitable.     The  acid  is  washed  out  by  means  of  a 
potassium  nitrate  solution  made  by  dissolving  1  gram  of  potassium 
nitrate  in  1  liter  of  water. 

The  phosphate  must  be  free  from  organic  matter.  If  present, 
it  is  destroyed  by  ignition  of  the  material  in  a  platinum  dish  or 
crucible.  If  the  percentage  of  phosphoric  acid  is  large,  a  solution 
should  be  made  by  dissolving  a  weighed  amount  in  nitric  acid  and 
diluting  to  a  known  volume.  Portions  are  then  measured  out 
containing  not  more  than  .050  gram  P205.  The  solution  is  nearly 
neutralized  with  ammonia,  warmed  to  about  40°,  and  40  c.c.  of 
the  molybdate  solution  added.  The  precipitation  is  best  made  in 
an  Erlenmeyer  flask  of  about  250  c.c.  The  solution  is  allowed  to 
digest  for  ten  to  fifteen  minutes,  the  temperature  of  about  40° 


362  VOLUMETRIC  METHODS. 

being  maintained  by  warming  on  the  water-bath.  By  closing  the 
flask  with  a  rubber  stopper  and  shaking  about  five  minutes,  com- 
plete precipitation  will  be  insured  <  Allow  the  precipitate  to  settle 
for  a  few  minutes  and  decant  the  solution  through  a  small  filter- 
paper.  Transfer  the  precipitate  and  wash  with  dilute  nitric  acid 
until  free  from  molybdenum  as  indicated  by  the  absence  of  a  pre- 
cipitate with  ammonium  sulphide.  The  precipitate  is  then  washed 
with  the  potassium  nitrate  solution  until  free  from  acid.  The 
washing  must  be  thorough,  but  as  little  wash-water  as  possible 
must  be  used,  because  of  the  slight  solubility  of  the  precipitate. 
If  properly  done,  200  c.c.  will  be  found  sufficient. 

527.  Titration. — The  paper  and  the  precipitate  are  placed  in  a 
beaker  and  a  measured  amount  of  the  standard  caustic  soda  or 
potash  solution  sufficient  to  dissolve  the  precipitate  is  added.  10- 
c.c.  portions  may  be  added,  and  if  on  agitation  the  precipitate  does 
not  dissolve,  another  10-c.c.  portion  is  added.  Continue  the  addi- 
tion of  10-c.c.  portions  until  the  precipitate  is  completely  dissolved. 
Add  a  few  drops  of  phenolphthalein  and  titrate  the  excess  of  alkali 
with  the  standard  nitric  acid.  The  amount  of  phosphoric  acid 
present  is  calculated  from  the  number  of  cubic  centimeters  of 
caustic  soda  used. 


REDUCTION  OF  THE  MOLYBDATE  PRECIPITATE  WITH 
ZINC  AND  TITRATION  WITH  PERMANGANATE  SO- 
LUTION. 

528.  Standard  Solution. — A  standard  solution  of  potassium 
permanganate  will  be  required  for  this  method.  As  1  molecule  of 
P205  is  combined  with  24  molecules  of  Mo03,  which  will  require  36 
atoms  of  oxygen  to  reoxidize  after  being  reduced  to  Mo203,  we  have 
the  relation  Mol.  wt.  P205(142)  =Mol.  wt.  36  0(576).  A  liter  of  a 
normal  solution  of  potassium  permanganate  will  therefore  be  equal 
to  1.9722  gram  of  P205.  From  this  figure  the  value  of  a  fifth- 
or  tenth-normal  solution  may  be  computed.  If  standardized  by 
means  of  iron,  the  value  of  the  solution  in*  phosphorus  pentoxide 
may  be  calculated  directly  from  the  factor  .03528,  which  is  the 
equivalent  of  1  gram  of  iron  in  phosphorus  pentoxide.  The  best 
method  consists  in  reducing  and  titrating  the  molybdate  precipitate 
from  a  known  amount  of  phosphoric  acid. 


DETERMINATION  OF  PHOSPHORIC  ACID.  363 

529.  Titration. — The  precipitation  of  the  phosphate  with  the 
molybdate  solution  is  carried  out  exactly  as  given  for  the  precedirg 
method.  Instead  of  washing  with  dilute  nitric  acid  and  potassium 
nitrate,  a  solution  of  acid  ammonium  sulphate  is  used.  This  is 
made  by  adding  to  1  liter  of  water  15  c.c.  of  strong  ammonia  (sp.  gr. 
0.90)  and  25  c.c.  of  concentrated  sulphuric  acid  (sp.  gr.  1.84).  After 
washing  the  precipitate  until  it  is  free  from  the  molybdenum  solu- 
tion, as  indicated  by  the  test  with  ammonium  sulphide,  it  is  dis- 
solved in  dilute  ammonia.  During  the  washing,  the  phospho- 
molybdate  need  not  be  entirely  removed  from  the  flask  in  which 
it  was  precipitated.  The  ammonia  solution  of  the  precipitate  is 
allowed  to  flow  into  the  flask  together  with  the  water  used  to  wash 
the  paper.  5  grams  of  pulverized  zinc  and  15  c.c.  of  concentrated 
sulphuric  acid  are  now  added.  The  flask  is  closed  with  a  stopper 
through  which  passes  a  glass  tube  bent  at  a  slightly  acute  angle 
and  dipping  into  a  saturated  solution  of  sodium  bicarbonate. 
When  all  of  the  zinc  is  dissolved  the  solution  is  titrated  with  the 
standard  potassium  permanganate  solution.  The  solution  before 
titration  should  be  green  in  color.  If  it  is  brown  the  determi- 
nation should  be  rejected.  A  blank  should  be  made  in  which  the 
same  amount  of  ammonia,  zinc,  and  sulphuric  acid  is  added  to 
enough  distilled  water  to  make  the  volume  of  the  solution  the 
same  as  that  in  the  titration  of  the  molybdate  precipitate.  After 
the  zinc  is  dissolved,  the  solution  is  titrated  with  the  standard 
permanganate  and  the  amount  added  is  subtracted  from  the  vol- 
ume used  in  the  determination  of  phosphorus. 


TECHNICAL  ANALYSIS. 

CHAPTER  XXVIII. 

ANALYSIS  OF  IRON  AND  STEEL. 

IRON  and  steel  as  manufactured  to-day  are  quite  complex 
substances,  containing  almost  invariably,  besides  IRON,  the  ele- 
ments CARBON,  SILICON,  SULPHUR,  PHOSPHORUS,  and  MANGANESE, 

while  special  varieties  may  contain  NICKEL,  CHROMIUM,  MANGANESE, 
etc. 

530.  The  Metals  are  undoubtedly  present  in  the  conditions  in 
which  they  usually  exist  in  alloys.     The  analysis  for  the  determina- 
tion of  the  amount  of  the  metals  present  other  than  iron  is  carried 
out  by  the  methods  already  given  for  the  analysis  of  alloys. 

531.  The  Acid  Elements  are  for  the  most  part  combined  directly 
with  the  iron  as   carbide,  silicide,  sulphide,  and  phosphide.    As 
these  elements  are  usually  met  with  in  combination  with  oxygen, 
the  ordinary  methods  of  analysis  are  not  always  suitable  for  their 
determination  in  iron  and  steel.     The  modification  usually  consists 
in  adopting  a  method  of  dissolving  the  metal  by  which  the  acid 
element  is  certain  to  be  thoroughly  oxidized.     Solution  of  iron  or 
steel  in  a  non-oxidizing  acid  like  hydrochloric  results  in  the  evo- 
lution of  more  or  less  of  the  acid  elements  present  as  hydrogen 
compounds,  such  as  hydrogen  sulphide,  phosphine,  and  various  hy- 
drocarbons.    Silicon  even  is  liable  to  be  volatilized  as  silicon  tetra- 
chloride.    When  solution  is  effected  with  nitric  acid,  silicon,  phos- 
phorus, and  sulphur  are  converted  into  non-volatile  oxygen  com- 
pounds, while  more  or  less  of  the  carbon  is  lost  as  carbon  dioxide. 
For  the  determination  of  carbon,  the  iron  is  dissolved  in  solutions 
of  cupric  salts  or  the  carbon  dioxide  evolved  on  dissolving  the  metal 
in  an  oxidizing  solution  is  collected  and  determined. 

The  determination  of  these  elements  must  be  carried  out 
with  unusual  care,  since  a  very  slight  variation  in  the  percentage 

364 


SILICON.  365 

produces  a  very  considerable  variation  in  the  properties  of  the 
metal. 

532.  To  Obtain  a  Sample  for  Analysis  a  perfectly  clean  drill  is 
taken,  and  by  means  of  a  drill-press  or  an  ordinary  brace,  holes  are 
bored  in  various  parts  of  the  casting  or  bar  of  iron  or  steel.     Loose 
dirt,  sand,  etc.,  must  be  removed  from  the  surface  so  as  not  to  fall 
into  the  drillings.     If  the  surface  cannot  be  cleaned  perfectly  the 
first  drillings  should  be  discarded.      The  remainder  of  the  drillings 
may  then  be  collected  on  a  piece  of  clean  glazed  paper.     The  iron 
may  also  be  separated  from  sand  and  dirt  by  means  of  a  magnet. 
Oil  may  be  removed  by  washing  a  few  times  with  ether.     The  drill- 
ings are  usually  fine  enough  for  most  purposes.     If  not,  they  may 
be  crushed  in  an  agate  or  hardened-steel  mortar.    In  no  case  may 
the  fine  portions  be  taken  and  the  large  pieces  discarded.     A  fair 
sample  of  both  portions  must  be  taken  and  further  pulverized  if 
necessary. 

DETERMINATION  OF  SILICON. 

533.  Solution  of  the  Iron  in  Nitric  Acid. — One  to  five  grams 
of   the   drillings   are  placed  in  a  beaker  of  200-  to  300-c.c.  ca- 
pacity and  40  c.c.  of  a  mixture  of  20   c.c.  concentrated  nitric 
acid  and  20  c.c.  water  are  added.     The  action  is  generally  quite 
violent  accompanied  by  frothing,  so  that  to  prevent  loss  it  should 
be  moderated  by  placing  the  beaker  in  cold  water.      If  action 
does  not  begin  as  soon  as  the  acid  is  added,  the  beaker  may  be 
gently  warmed  and  cooled  when  the  action  becomes  violent.     It 
may  also  be  moderated  by  adding  the  acid  in  several  portions. 
When  the  action  has  ceased,  the  beaker  is  warmed  on  the  hot-plate 
to  dissolve  the  few  remaining  portions  of  metal.     Solution  may  be 
hastened  by  the  addition  of  a  little  hydrochloric  acid.     The  solu- 
tion is  evaporated  to  dryness  and  the  residue  heated  in  the  air-bath 
until  nitrous  fumes  are  no  longer  evolved.     The  residue  is  then 
dissolved  in  30  c.c.  hydrochloric  acid  by  warming  gently.     The 
solution  is  diluted  somewhat,  warmed,  and  the  silica  filtered  off 
on  an  ashless  paper  and  washed.     Dilute  hydrochloric  acid  is  first 
used  and  then  water. 

534.  Volatilization  of  Silica. — The  moist  paper  is  transferred 
to  a  weighed  platinum  crucible,  the  paper  burned,  and  the  precipj" 


366  ANALYSIS  OF  IRON  AND  STEEL. 

tate  ignited,  finally  over  the  blast-lamp,  and  weighed.  Even  if 
the  silica  is  perfectly  white,  it  is  necessary  to  test  its  purity  by  add- 
ing a  few  drops  of  concentrated  sulphuric  acid  and  then  enough 
pure  hydrofluoric  acid  to  dissolve  the  silica.  The  solution  is  then 
evaporated  to  dryness  and  the  residue  ignited  and  weighed.  The 
weight  of  the  residue  is  subtracted  from  the  weight  of  the  impure 
silica  and  the  amount  of  silicon  calculated. 

535.  Drown's  Method. — A   more  rapid  method  which  is  very 
much  used  for  pig  iron  and  which  gives  excellent  results  is  based  on 
the  fact  that  the  silicic  acid  may  be  dehydrated  by  heating  with 
concentrated  sulphuric  acid.     The  drillings  are  dissolved  in  nitric 
acid  as  already  described.     20  c.c.  of  sulphuric  acid  consisting  of 
equal  parts  of  concentrated  acid  and  water  are  then  added  for  each 
gram  of  drillings  taken.     The  solution  is  heated  until  copious 
fumes  of  sulphuric  acid  are  given  off.     The  lumps  of  ferric  sulphate 
must  be  broken  up  with  a  rod  and  the  solution  stirred  to  prevent 
bumping  and  spattering  of  the  material.     The  concentrated  acid 
is  allowed  to  cool  somewhat,  cautiously  diluted  with  water,  and 
the  solution  warmed  gently  until  the  ferric  sulphate  is  completely 
dissolved.      The  hot  solution  is  filtered  immediately,  as  the  silica 
dehydrated  in  this  manner  gradually  goes  into  solution  again. 
The  silica  is  washed  first  with  water  and  then  with  dilute  hydro- 
chloric acid  and  finally  with  water.      It  is  ignited  and  weighed, 
and  then  volatilized  as  directed  in  the  first  method.     This  method  is 
used  when  silica  alone  is  to  be  determined. 

536.  Solution  of  the  Iron  in  Hydrochloric  Acid. — The  silica  may 
also  be  determined  after  solution  of  the  iron  in  hydrochloric  acid 
and  potassium  chlorate.     The  evaporation  of  the  solution  to  dry- 
ness  may  then  be  accomplished  more  rapidly,  but  a  slight  loss  of 
silicon  as  SiCl4  is  almost  inevitable. 

537.  Fusion  of  Silica  Precipitate. — Instead  of  ascertaining  the 
amount  of  impurity  in  the  silica  by  treatment  with  hydrofluoric 
and  sulphuric  acids,  it  may  be  fused  with  four  or  five  times  its 
weight  of  sodium  carbonate.    The  fused  mass  is  dissolved  in  water, 
acidified  with  hydrochloric  acid,  evaporated  to  dryness,  redissolved 
in  hydrochloric  acid  and  water,  and  the  silica  filtered  off,  washed, 
ignited,  and  weighed.     The  purification  of  the  silica  by  volatiliza- 


SULPHUR.  367 

tion  with  hydrofluoric  acid  is  more  convenient  if  a  pure  acid  is  at 
hand. 


DETERMINATION  OF  SULPHUR. 

538.  Solution  of  the  Iron  in  Nitric  Acid. — Sulphur  may  be  deter- 
mined in  the  filtrate  from  the  silica  by  precipitation  with  barium 
chloride  and  weighing  as  barium  sulphate.  The  iron  must  be  dis- 
solved by  the  first  method  given,  but  concentrated  nitric  acid  must 
be  used  in  place  of  the  acid  of  sp.  gr.  1.2  in  order  to  insure  com- 
plete oxidation  of  the  sulphur.  If  the  metal  dissolves  slowly,  a 
few  drops  of  hydrochloric  acid  may  be  added.  As  the  percentage 
of  sulphur  is  usually  small,  the  large  amount  (5  grams)  should  b<i 
used.  After  solution  has  been  effected  a  little  sodium  carbonate 
is  added  to  prevent  loss  of  sulphur  trioxide,  and  the  solution  is 
evaporated  to  dryness.  The  residue  is  dissolved  in  hydrochloric 
acid,  and  again  evaporated  to  dryness  to  completely  remove  the 
nitric  acid  and  dehydrate  the  silicic  acid.  The  residue  is  again 
dissolved  in  hydrochloric  acid,  and  the  excess  expelled  by  evapora- 
tion. The  solution  must  not  be  allowed  to  go  to  dryness,  as  a  crust 
of  insoluble  basic  ferric  chloride  would  be  produced.  The  solution 
is  diluted  and  the  silica  and  graphite  filtered  off  and  washed. 

The  filtrate  is  heated  to  boiling,  and  10  c.c.  of  a  10%  solution 
of  barium  chloride  added  slowly  and  with  constant  stirring.  The 
solution  is  allowed  to  stand  overnight  in  the  cold.  The  precipitate 
is  then  filtered  off  and  washed  with  very  dilute  hydrochloric  acid 
until  free  from  iron,  and  then  with  water.  The  barium  sulphate 
is  ignited  and  weighed  in  the  usual  manner. 

539.  Errors. — This  method  tends  to  give  a  low  result  on  account 
of  the  solubility  of  barium  sulphate  in  hydrochloric  acid  and  ferric 
chloride.  The  result  is  apt  to  be  high  on  account  of  the  great  diffi- 
culty of  obtaining  a  precipitate  of  barium  sulphate  which  is  free 
from  iron.  The  iron  carried  down  seems  to  be  present  as  a  basic 
ferric  sulphate  which  loses  S03  on  ignition  and  leaves  the  barium 
sulphate  colored  red  by  the  ferric  oxide.  Fusing  the  precipitate 
with  sodium  carbonate,  dissolving  in  water,  and  filtering  effects  a 
separation  from  the  iron,  but  the  barium  sulphate  obtained  from 
the  filtrate  will  not  represent  the  sulphur  present  in  the  iron  because 


368  ANALYSIS  OF  IRON  AND  STEEL. 

of  loss  of  S03  during  the  first  ignition.  The  presence  of  considerable 
amounts  of  hydrochloric  acid  in  the  iron  solution  tends  to  prevent 
the  precipitation  of  the  iron  with  the  barium  sulphate,  but  another 
error  is  introduced  because  of  the  marked  solubility  of  the  latter 
in  the  dilute  hydrochloric  acid  and  the  ferric  chloride.  The  large 
excess  of  barium  chloride  used  tends  to  produce  complete  precipi- 
tation of  the  sulphuric  acid.  The  success  of  the  method  therefore 
depends  on  regulating  the  amount  of  hydrochloric  acid  so  as  to 
prevent  the  precipitation  of  iron,  but  not  that  of  the  barium  sul- 
phate. The  so-called  evolution  methods  are  therefore  generally 
preferred  when  applicable.  In  steel  the  sulphur  is  generally  so 
combined  that  the  evolution  method  may  be  used,  but  the  evolution 
of  the  sulphur  is  not  complete  in  all  pig  and  cast  irons. 

540.  For  Pi£  Irons  the  following  method  is  used:   After  solu- 
tion of  the  pig  iron  in  strong  nitric  acid,  10  grams  of  sodium  car- 
bonate  are   added,  and  the  solution  transferred  to  a   platinum 
dish,  evaporated  to  dryness,  and  the  residue  ignited.     It  is  then 
digested  with  water,  a  little  sodium  carbonate  added,  and  the 
solution  filtered.     The  insoluble  material  is  washed  with  water 
containing  sodium  carbonate.     The  filtrate  is  acidified  with  hydro- 
chloric acid  and  evaporated  to  dryness  to  dehydrate  the    silica, 
whJch  is  filtered  off  and  washed.     The  sulphuric  acid  in  the  filtrate 
is  precipitated  with  barium  chloride  and  weighed  as  barium  sul- 
phate. 

541.  Determination   of   Sulphur  in  the  Reagents. — All  of  the 
reagents  used  in  sulphur  determinations  must  be  free  from  sulphur. 
Their  purity  is  most  easily  assured  by  making  a  blank  determina- 
tion as  follows:  The  total  amount  of  nitric  and  hydrochloric  acid 
used  is  measured  out  into  a  beaker,  a  little  sodium  carbonate  added, 
and  the  solution  evaporated  to  dryness.     The  residue  is  dissolved 
in  a  little  water,  a  few  drops  of  hydrochloric  acid  added,  and  after 
heating   to  boiling  a  few  cubic  centimeters  of  barium  chloride 
solution  are  added.     Any  precipitate  formed  is  filtered  off,  washed, 
ignited,  and  weighed,  and  the  amount  of  barium  sulphate  obtained 
deducted  from  the  amount  obtained  from  the  solution  of  iron. 


SULPHUR.  369 


EVOLUTION  METHODS   FOR  SULPHUR. 

The  evolution  methods  are  now  more  largely  used  than  the 
oxidation  methods.  In  the  evolution  methods  advantage  is  taken 
of  the  fact  that  when  iron  or  steel  is  dissolved  in  hydrochloric  acid 
the  sulphur  is  evolved  as  hydrogen  sulphide.  Air  as  well  as  all 
oxidizing  agents  must  be  excluded  from  the  evolution-flask. 

542.  For   the    Absorption  of    the    Hydrogen  Sulphide  a  large 
number  of  solutions  have  been  suggested.    Formerly  a  hydrochloric- 
acicl  solution  of  BROMINE  was  largely  used.     The  sulphur  was  oxi- 
dized by  this  solution  to  sulphuric  acid,  which  was  precipitated 
and  weighed  as  barium  sulphate.     The  evolution  of  the  disagreeable 
bromine  fumes  may  be  obviated  by  the  substitution  of  an  ammo- 
niacal  solution  of  hydrogen  peroxide  or  a  3%  solution  of  sodium 
peroxide.     These  reagents  are,  however,  almost  never  free  from 
sulphur,  necessitating  a  separate  determination  of  the  amount 
present. 

Another  class  of  absorption  liquids  consists  of  solutions  of 
metallic  salts.  The  hydrogen  sulphide  is  precipitated  as  a  metallic 
sulphide,  which  in  some  cases  is  filtered  off,  washed,  and  weighed, 
and  in  other  cases  is  oxidized  by  means  of  an  appropriate  solution 
to  sulphuric  acid  which  is  precipitated  as  barium  sulphate.  The 
use  of  solutions  of  silver  and  copper  for  this  purpose  seems  to  be 
objectionable  because  of  the  reduction  of  the  former  by  the  hydro- 
carbons evolved,  and  the  formation  of  an  insoluble  phosphorus 
compound  in  the  latter.  Alkaline  solutions  of  lead,  zinc,  and 
cadmium  seem  to  be  very  efficient  absorbents.  Very  good  results 
are  obtained  by  washing  the  cadmium  sulphide,  drying  at  100°, 
and  weighing. 

543.  lodometric  Determination  of  the  Sulphur. — The  sulphur 
in  the  cadmium  and  zinc  sulphides  may  also  be  determined  by 
treatment  with  a  measured  excess  of  standard  iodine  solution, 
acidulating,  and  titrating  the  excess  with  standard  sodium  thio- 
sulphate.    The  following  reactions  then  take  place: 

CdS+2HCl=CdCl2-fH2S, 
H2S+I2=2HI  +  S. 


370 


ANALYSIS  OF  IRON  AND  STEEL. 


The  hydrogen  sulphide  may  also  be  absorbed  by  passing  the 
gas  directly  through  a  measured  amount  of  standard  iodine  solu- 
tion and  titrating  the  excess.  The  volatility  of  the  iodine  intro- 
duces an  error  in  this  determination.  This  may  be  obviated  by 
absorbing  the  hydrogen  sulphide  in  a  solution  of  caustic  soda 
and  determining  the  sulphur  with  iodine  solution  in  the  same 
manner  as  with  cadmium  sulphide. 

544.  All  of  the  Sulphur  is  not  Evolved  as  Hydrogen  Sulphide 
when  a  metal  of  high  carbon  content  is  analyzed.     Organic  sul- 
phides  are   formed  among  which  methyl   sulphide  (CH3)2S   has 
been  identified.*    These  sulphur  compounds  are  expelled  from 
the  evolution-flask  with  difficulty,  and  are  not  taken  up  so  readily 
by  the  absorption  solution.     By  passing  the  gases  through  a  red- 
hot  glass  tube  these  compounds  may  be  decomposed  so  that  the 
sulphur  is  absorbed. 

545.  A  Convenient  Form  of  Apparatus  for  carrying  out  the  deter- 
mination is  shown  in  Fig.  59.    The  Kipp  generator  is  charged 


FIG.  59. 

with  zinc  and  acid  for  the  generation  of  hydrogen.  The  wash- 
bottle  A  contains  a  solution  suitable  for  the  absorption  of  hydro- 
gen sulphide. 

A  solution  of  LEAD  NITRATE  in  CAUSTIC  POTASH  is  very  efficient. 
It  is  made  by  dissolving  one  part  of  caustic  potash  in  two  parts  of 
water  and  pouring  in  a  solution  of  lead  nitrate  with  constant 
stirring  until  a  permanent  precipitate  is  produced.  After  being 

*  Philips,  Stahl  und  Eisen,  16,  633. 


SULPHUR.  371 

allowed  to  settle,  the  clear  liquid  is  siphoned  off  for  use.  It  should 
be  kept  in  a  glass-stoppered  bottle.  The  stopper  should  be  coated 
with  a  little  paraffine  to  prevent  its  sticking.  This  solution  may 
also  be  used  in  the  absorption-flasks  C  and  E.  20  to  30  c.c.  in 
each  flask  will  be  sufficient,  water  being  added  until  the  flasks  are 
about  half-full.  The  flask  B  should  have  a  capacity  of  about  \ 
liter. 

546.  Solution  of  the  Iron. — Ten  grams  of  filings  are  placed  in  the 
flask  B.     If  a  portion  weighing  13.732  grams  is  used,  the  number  of 
milligrams  of  barium  sulphate  obtained  will  be  equal  to  the  number 
of  one-thousandths  per  cent  of  sulphur.     If,  for  instance,  .0156 
gram  of  barium  sulphate  is  obtained,  the  percentage  of  sulphur 
present  will  be  .0156.      The  air  in  the  apparatus  is  displaced  with  a 
stream  of  hydrogen.     The  glass  tube  D  is  then  heated  to  redness, 
the   dropping-funncl  filled    with    dilute    hydrochloric    acid,   and 
connection  made  immediately  with  the  Kipp  generator.     About 
20  c.c.  of  dilute  acid  should  be  used  for  each  gram  of  iron.     Some 
workers  use  for  each  gram  of  iron  10  c.c.  of  a  dilute  hydrochloric 
acid  composed  of  equal  volumes  of  concentrated  acid  and  water, 
while  others  recommend  18  c.c.  of  a  mixture  of  two-thirds  dilute 
hydrochloric  acid  and  one-third  dilute  sulphuric  acid. 

When  the  glass  tube  has  been  heated  to  dull  redness  the  acid 
is  allowed  to  flow  into  the  flask  B  by  turning  the  stop-cock  of  the 
dropping-funnel.  The  flask  is  heated  with  a  Bunsen  burner  until 
the  solution  boils,  a  rapid  stream  of  gas  being  allowed  to  pass 
through  the  apparatus.  After  boiling  for  about  fifteen  minutes  the 
iron  is  usually  dissolved. 

547.  Absorption  of  the  Sulphur  by  Lead  Solution. — The  Bun- 
sen  burner  is  removed  and  the  stream  of  hydrogen  allowed  to  flow 
for  about  fifteen  minutes  more  to  sweep  all  of  the.  hydrogen  sul- 
phide out  of  the  apparatus.    The  precipitate  in  the  flask  C  is  filtered 
off  on  a  small  paper.     If  a  precipitate  has  formed  in  the  flask  D, 
it  is  filtered  off  on  the  same  paper  and  washed  once  or  twice  with 
hot  water.     The  moist  paper  with  the  precipitate  is  thrown  into  a 
beaker,  a  little  potassium  chlorate  added,  and  5  to  20  c.c.  of  con- 
centrated hydrochloric  acid  added,  the  amount  used  depending  on 
the  amount  of  lead  sulphide  present.     Allow  the  beaker  to  stand  in 
a  warm  place  until  chlorine  is  no  longer  evolved,  dilute  the  strong 


372  ANALYSIS  OF  IRON  AND  STEEL. 

acid  and  filter.  After  washing  the  paper  with  hot  water  the  fil- 
trate is  neutralized  with  ammonia  and  then  acidified  with  2  or  3 
drops  of  hydrochloric  acid.  The  sulphuric  acid  is  then  precipi- 
tated with  barium  chloride  and  weighed  as  barium  sulphate. 

548.  Absorption  of  the  Sulphur  by  a  Cadmium  Solution. — 
Instead  of  using  an  alkaline  solution  of  lead  nitrate  in  the  absorp- 
tion-flasks C  and  D,  an  alkaline  solution  of  cadmium  may  be 
used  as  proposed  by  T.  T.  MorreL* 

The  cadmium  sulphide  is  filtered  off  on  a  Gooch  crucible  and 
washed  with  water  containing  a  little  ammonia.  The  precipitate 
is  dried  at  100°  and  weighed. 

The  cadmium  sulphide  together  with  the  paper  may  also  be 
placed  in  a  beaker,  200  or  300  c.c.  of  cold  water  added,  and  then 
dilute  hydrochloric  acid  until  the  precipitate  is  dissolved.  A  few 
cubic  centimeters  of  freshly  prepared  starch  solution  are  then  intro- 
duced, and  standard  iodine  solution  added  from  a  burette  until  a 
permanent  blue  color  is  produced. 


DETERMINATION   OF    PHOSPHORUS. 

The  determination  of  phosphorus  in  iron  or  steel  is  invariably 
preceded  by  solution  of  the  iron  and  oxidation  of  the  phosphorus 
to  phosphoric  acid,  which  is  then  separated  from  the  iron  by  pre- 
cipitation as  ammonium  phospho-molybdate.  The  estimation  of 
the  amount  of  phosphoric  acid  may  then  be  carried  out  gravi- 
metrically  or  volumetrically  by  any  of  the  methods  given  in 
Chapters  IX  and  XXVII. 

549.  Oxidation  with  Nitric  Acid  and  Separation  of  Silica. — 
From  1  to  5  grams  of  the  iron  or  steel,  according  to  the  amount  of 
phosphorus  present,  are  dissolved  in  nitric  acid,  sp.  gr.  1.2,  about 
15  c.c.  being  used  for  each  gram  of  iron.  Evaporate  to  dryness, 
and  heat  in  the  air-bath  at  200°  for  one  hour.  At  this  tempera- 
ture the  oxidation  of  the  phosphorus  is  completed.  The  residue 
is  treated  with  30  c.c.  concentrated  hydrochloric  acid,  and  warmed 
until  the  oxide  of  iron  is  dissolved.  The  solution  is  evaporated  to 
dryness  again  to  dehydrate  the  silica.  Dissolve  once  more  in 

*  Chem.  News.  XXVIII.  229. 


PHOSPHORUS.  373 

30  c.c.  hydrochloric  acid.  Evaporate  the  solution  to  a  sirupy 
consistency,  dilute,  and  filter.  The  insoluble  residue  may  be 
used  for  the  determination  of  silicon,  if  that  is  desired. 

550.  Separation  of  Titanium. — If  titanium  is  present  it  will  form 
an   insoluble   phosphate  which  will  remain  with  the  silica  and 
cannot  be  dissolved  by  means  of  hydrochloric  acid.     The  paper 
containing  the  washed  silica  and  graphite  is  transferred  to  a  plati- 
num crucible  and  burned.     The  silica  is  volatilized  by  treatment 
with  hydrofluoric  and  a  few  drops  of  concentrated  sulphuric  acid 
and  heating  until  the  sulphuric  acid  is  volatilized.     The  residue, 
consisting  of  Ti02  and  P205,  is  fused  for  one  half  hour  with  2  or  3 
grams  of  sodium  carbonate,  the  melt  dissolved  in  water,  and  the 
solution  containing  the  P205  as  sodium  phosphate  is  filtered.     The 
filtrate  from  the  insoluble  sodium  titanate  is  acidified  with  nitric 
acid  and  added  to  the  first  filtrate  from  the  impure  silica.      The 
hydrochloric  acid  is  then  expelled  by  addition  of  30  c.c.  of  concen- 
trated nitric  acid  and  evaporating  nearly  to  dryness. 

551.  The  Phosphoric  Acid  may   be  Precipitated,  after    nearly 
neutralizing  the  nitric  acid  with  ammonia,  by  means  of  molybdate 
solution  and  determined  gravimetrically  as  directed  in  Chapter  IX, 
or  volumetrically  as  directed  in  Chapter  XXVII. 

552.  Oxidation  with  Potassium   Permanganate.  —  The  destruc- 
tion of  the  organic  matter  as  well  as  the  complete  oxidation  of  the 
phosphorus  in  the  solution  of  the  iron  in  nitric  acid  may  be  advan- 
tageously accomplished  by  means  of  potassium  permanganate  as 
well  as  by  heating  the  dry  nitrate  at  200°.     After  dissolving  the 
iron  in  nitric  acid,  10  c.c.  of  a  1.5%  solution  of  potassium  perman- 
ganate is  added  and  the  solution  boiled  until  the  pink  color  is 
destroyed  and    the  manganese  is  reduced  to  the  dioxide.    This 
precipitate  is  dissolved  by  the  addition  of  small  amounts  of  sul- 
phurous acid,  ferrous  sulphate,  or  sodium  thiosulphate.     When 
the  solution   has    cooled   to   about   40°,   it    is    neutralized    with 
ammonia,  the  precipitate  dissolved  with  a  little  nitric  acid,  and 
the  phosphoric  acid  precipitated  by  the  addition  of  molybdate 
solution. 

553.  The  Molybdate    Precipitate  may   be  directly  weighed  as 
directed  in  Chapter  IX.     It  may  also  be  titrated  with  standard 
caustic  potash  or  soda  as  directed  in  Chapter  XXVII,  or  with 


374  ANALYSIS  OF  IRON  AND  STEEL. 

standard  potassium  permanganate  solution  after  reduction  with 
zinc  as  directed  in  the  same  chapter. 

DETERMINATION  OF  CARBON. 

Carbon  exists  in  iron  and  steel  in  several  conditions,  and  the 
qualities  of  the  metal  are  considerably  influenced  by  the  condition 
in  which  the  carbon  exists.  Analytical  methods  are  in  use  for 
determining  the  amount  of  carbon  existing  in  two  conditions.  The 
so-called  FREE  or  GRAPHITIC  CARBON  as  well  as  the  total  amount  of 
carbon  are  determined  directly.  The  COMBINED  CARBON  is  usually 
determined  by  difference. 

TOTAL  CARBON. 

a.  SOLUTION  OF  IRON  AND  OXIDATION  OF  CARBON  BY 
CHROMIC  ACID. 

This  is  a  very  accurate  as  well  as  a  rapid  and  largely  used 
method  of  determining  carbon  in  all  grades  of  iron  and  steel,  with 
the  exception  of  ferric  chrome  and  irons  high  in  silicon.  The 
carbon  is  oxidized  by  the  chromic  acid  to  carbon  dioxide,  which 
may  be  absorbed  in  an  appropriate  apparatus  and  weighed  or  the 
volume  of  the  gas  may  be  measured  and  the  weight  calculated. 

554.  The  Following  Solutions  are  prepared  for  the  combustion 
of  the  carbon- 

1.  Two  hundred  grams  of  copper  sulphate  are  dissolved  in  a 
liter  of  water. 

2.  A  saturated  solution  of  chromic  acid. 

3.  A  solution  made  by  taking  35  c.c.  of  solution  2,  190  c.c.  of 
water,  750  c.c.  of  concentrated  sulphuric  acid,  and  340  c.c.  of 
phosphoric  acid,  sp.  gr.  1.4. 

After  adding  a  few  cubic  centimeters  of  sulphuric  acid,  solution  2 
should  be  heated  to  boiling  to  destroy  any  organic  matter  that 
might  be  present.  Solution  3  should  also  be  heated  to  boiling  before 
use. 

555.  The  Apparatus  is  set  up  as  shown  in  Fig.  60.    The  round- 
bottomed  flask  A  should  have  a  capacity  of  about  250  c.c.     The 
short  piece  of  combustion  tubing  B  is  filled  with  lumps  of  copper 
oxide  which  are  kept  in  place  by  short  coils  of  copper  gauze.    The 


TOTAL  CARBON. 


375 


tube  is  placed  in  a  short  tube  furnace  or  is  heated  by  several 
Bunsen  burners.  This  tube  is  heated  to  redness  during  the  com- 
bustion of  the  carbon  in  order  to  decompose  the  hydrocarbons 
evolved.  If  it  is  omitted,  the  result  must  be  increased  2%  for 
malleable  iron  and  steel  and  3J%  for  pig  iron.  The  presence  of 
copper  sulphate  in  the  combustion-flask  reduces  the  amount  of 
the  hydrocarbons  evolved.  The  U-tube  C  is  filled  with  calcium 
chloride  for  drying  the  carbon  dioxide  which  is  absorbed  in  the 
caustic  potash  bulb  D.  If  this  bulb  is  not  fitted  with  a  small 
calcium  chloride  tube,  the  U-tube  E  must  be  weighed  as  well  as 


FIG.  60. 

the  caustic  potash  bulb  D.  Another  calcium  chloride  tube  must 
then  be  inserted  between  the  U-tube  E  and  the  aspirator.  The 
U-tube  attached  to  the  dropping-funnel  is  filled  with  soda-lime 
and  serves  to  remove  carbon  dioxide  from  the  air  drawn  through 
the  apparatus.  The  apparatus  is  set  up  and  tested  for  leaks  as 
already  directed  for  the  determination  of  carbon  dioxide  in  Chap- 
ters IX  and  XV. 

556.  Process. — From  1  to  5  grams  of  the  iron,  depending  on  the 
amount  of  carbon  present,  is  weighed  out  and  placed  in  the  flask  A. 
15  c.c.  of  copper  sulphate  solution  is  introduced  through  the  drop- 
ping^funnel.  After  one  or  two  minutes  introduce  15  c.c.  of  the 
chromic  acid  solution  and  135  c.c.  of  solution  3.  Allow  a  slow 
stream  of  air  to  pass  through  the  apparatus,  and  heat  the  solution 
gently  with  the  Bunsen  burner,  bringing  it  to  a  boil  in  fifteen  to 
twenty  minutes.  Continue  the  boiling  for  about  two  hours. 


376  ANALYSIS  OF  IRON  AND  STEEL. 

Aspirate  about  2  liters  of  air  through  the  apparatus,  disconnect 
the  absorption-tubes,  and  weigh  after  allowing  them  to  remain 
in  the  balance-case  for  about  fifteen  minutes. 

b.  WEIGHING  OF  CARBON  AFTER  SEPARATION  OF  THE  IRON  BY 
MEANS  OF  POTASSIUM  CUPRIC  CHLORIDE  SOLUTION. 

557.  Action  of  Copper  Solutions.  —  On  treating  iron  or  steel 
with  a  solution  of  copper,  the  copper  is  displaced  by  the  iron  which 
unites  with  the  acid,  forming  a  ferrous  salt.  The  combined  carbon 
as  well  as  the  graphitic  carbon  remains  undissolved,  no  hydrogen  or 
hydrocarbons  being  evolved. 

Formerly  a  neutral  solution  of  COPPER  SULPHATE  was  employed. 
This  has  largely  been  replaced  by  a  solution  of  CUPRIC  AMMONIUM 
CHLORIDE,  by  which  the  iron  is  dissolved  much  more  rapidly,  the 
precipitated  copper  also  going  into  solution.  On  account  of  the 
difficulty  of  obtaining  the  double  ammonium  salt  free  from  organic 
material,  the  use  of  the  double  chloride  of  potassium  and  copper 
has  arisen.  It  has  also  been  recently  shown  by  the  American 
members  of  the  International  Steel  Standards  Committee  that 
more  accurate  results  are  obtained  by  the  use  of  a  copper  solution 
containing  about  10  per  cent  of  hydrochloric  acid.  The  reactions 
taking  place  may  be  represented  as  follows: 


2  =  FeCl2+Cu; 
Cu+CuCl2  =  2CuCl. 

The  cuprous  chloride  produced  in  this  manner  may  be  recon- 
verted into  cupric  chloride  by  passing  chlorine  through  the  solu- 
tion until  it  smells  strongly  of  chlorine.  Blair*  states  that  a  solu- 
tion regenerated  in  this  manner  is  more  active  than  a  fresh  solution. 
The  same  solution  may  be  used  repeatedly  until  the  concentration 
of  the  iron  salts  is  too  great.  Blair  has  used  the  same  solution 
eleven  times  with  good  results. 

558.  The  Solution  of  the  Iron  is  effected  by  treating  1  gram  of 
pig  iron,  Spiegel,  or  ferromanganese  with  100  c.c.  of  a  saturated 
solution  of  the  potassium  cupric  chloride  and  7.5  c.c.  of  concen- 
trated hydrochloric  acid.  3  grams  of  steel  or  puddled  iron  are 

*  Chemical  Analysis  of  Iron,  1901,  p.  165. 


TOTAL  CARBON.  377 

taken  and  treated  with  200  c.c.  of  the  copper  solution  and  15  c.c. 
strong  hydrochloric  acid.  The  iron  is  dissolved  very  much  more 
rapidly  if  the  solution  is  stirred.  After  digestion  for  several 
minutes  at  the  ordinary  temperature  the  solution  may  be  warmed 
gently,  but  not  higher  than  60°  or  70°.  If  many  determinations 
are  to  be  made,  a  mechanical  device  for  stirring  should  be  used. 

559.  Direct  Weighing  of  Carbon. — When  the  iron  and  the 
precipitated  copper  are  completely  dissolved*  the  carbon  is  fil- 
tered off  and  washed  with  hot  water  and  a  little  hydrochloric  acid 
to  dissolve  the  copper  and  iron  salts.  If  the  iron  or  steel  contains 
only  combined  carbon  and  is  free  from  graphite  the  carbon  may 
be  weighed  directly,  after  being  filtered  off  on  a  Gooch  crucible 
and  dried  at  100°.  The  carbon  is  then  burned  off  and  the  residue 
weighed.  The  difference  is  carbonaceous  matter  which  contains 
about  70%  of  carbon. 


c.  COMBUSTION  OF  SEPARATED  CARBON  IN  A  STREAM  OF  OXYGEN. 

560.  Filtration  of  Carbon  on  a  Platinum  Boat. — A  more  common 
and  generally  applicable  method  consists  in  burning  the  separated 
carbon  in  oxygen  or  chromic  acid  and  collecting  and  weighing  or 
measuring  the  carbon  dioxide  produced.  If  the  carbon  is  to  be 
burned  in  a  stream  of  oxygen,  the  most  suitable  apparatus  on  which 
to  filter  it  off  and  wash  it  is  a  platinum  boat  with  perforated  bottom 
which  is  of  such  a  size  that  it  may  be  placed  in  the  glass  or  porcelain 
tube  used  for  the  combustion.  A  suitable  holder  is  provided  for  the 
boat  so  that  it  may  be  used  in  the  same  manner  as  a  Gooch  crucible. 
Asbestos  which  has  been  ignited  so  as  to  free  it  from  organic  matter 
is  made  into  a  pulp  by  mixing  with  water,  floated  on  the  bottom 
of  the  boat  and  sucked  into  a  smooth  compact  layer  with  the 
filter-pump.  After  filtering  off  the  carbon  on  the  boat  and  wash- 
ing it,  the  whole  is  dried  at  100°  and  transferred  to  the  combus 
tion-tube.  Carbonaceous  matter  adhering  to  the  sides  of  the  beaker 
in  which  the  iron  was  dissolved  is  wiped  off  with  a  wad  of  ignited 
asbestos  held  by  a  pair  of  forceps.  The  asbestos  is  then  trans- 
ferred to  the  boat. 

*  The  metallic  copper  may  easily  be  seen  because  of  its  bright-red  color.      : .»* 


378  ANALYSIS  OF  IRON  AND  STEEL. 

561.  Filtration  of  Carbon  on  a  Carbon  Filter. — Instead  of  the 
perforated  boat  and  holder  a  small  platinum  tube  fitting  into  the 
combustion-tube  may  be  used  for  filtering  off  the  asbestos.     The 
asbestos  is  held  on  a  perforated  platinum  disk  resting  on  a  shoulder 
hi  the  tube.     After  the  carbon  has  been  filtered  off  and  washed, 
the  disk  is  loosened  by  pushing  it  further  into  the  tube  by  means 
of  a  glass  or  platinum  rod.     This  is  done  to  provide  a  passageway 
for  the  oxygen  used  in  the  combustion.     After  drying,  the  tube  con- 
taming  the  asbestos  and  the  carbon  is  inserted  into  the  combustion- 
tube.    An  ordinary  glass  carbon  filter  may  also  be  used  in  which 
the  asbestos  rests  on  a  perforated  platinum  or  procelain  disk  or  a 
Spiral  of  platinum  wire.    After  filtering  off  the  carbon,  the  disk 
with  the  asbestos  and  the  carbon  is  pushed  out  of  the  tube  and 
transferred  to  a  platinum  boat.     The  particles  of  asbestos  and 
carbon  still  adhering  to  the  glass  tube  are  wiped  out  by  wads  of 
ignited  asbestos  held  in  a  pair  of  forceps.     The  boat  and  contents 
are  then  dried  at  100°  and  transferred  to  the  combustion-tube. 

562.  A  Platinum,  Glass,  or  Porcelain  Combustion-tube  may  be 
used.     It  should  be  long  enough  to  extend  several  centimeters 
beyond  the  furnace,  so  that  the  rubber  stoppers  are  not  softened 
by  the  heat.    The  forward  half  of  the  portion  of  the  tube  which  is 
in  the  furnace  is  filled  with  granulated  copper  oxide  which  is  held 
in  place  by  short  rolls  of  copper  gauze.      Another  roll  of  copper 
gauze,  to  which  a  hook  made  of  copper  wire  is  fastened,  is  pro- 
vided for  insertion  into  the  tube  after  the  boat  has  been  placed 
in  the  empty  rear  end. 

563.  A  Stream  of  Oxygen  is  obtained  from  a  steel  tank  which 
has  been  filled  under  pressure  or  from  an  ordinary  gasometer  which 
has  been  filled  with  oxygen  by  heating  gently  in  a  retort  an  intimate 
mixture  of  1  part  of  powdered  manganese  dioxide  and  20  parts  of 
potassium  chlorate.     When  the  oxygen  begins  to  be  given  off  from 
this  mixture  the  heating  with  the  Bunsen  burner  must  be  discon- 
tinued or  the  evolution  of  gas  will  be  too  violent.     The  oxygen  is 
purified  by  passing  through  a  large  U-tube  or  tower  filled  with 
soda-lime  and  pieces  of  fused  caustic  potash.     A  similar  apparatus 
should  be  provided  for  purifying  air. 

564.  Absorption   Apparatus. — The    carbon  obtained  from  iron 
and  steel  is  almost  never  free  from  sulphur.     Some  chlorides  are 


TOTAL  CARBON.  379 

also  generally  present  which  cannot  be  washed  out.  Sulphur 
dioxide,  hydrochloric  acid,  and  chlorine  are  therefore  given  off 
on  burning  the  carbon,  and  these  gases  must  be  absorbed  before 
the  carbon  dioxide  is  allowed  to  pass  into  the  caustic  potash.  A 
spiral  of  silver  foil  should  be  placed  in  the  end  of  the  combustion- 
tube  for  the  absorption  of  the  chlorine.  A  U-tube  filled  with 
pieces  of  pumice-stone  which  have  been  saturated  with  copper- 
sulphate  solution  and  thoroughly  dried  at  a  little  over  200°  should 
be  connected  with  the  combustion-tube  for  the  absorption  of 
hydrochloric  acid.  For  the  absorption  of  sulphur  dioxide  a  layer 
of  lead  peroxide  is  placed  in  the  bottom  of  this  tube  and  separated 
from  the  copper  sulphate  by  means  of  wads  of  asbestos  or  cotton 
wool. 

A  U-tube  filled  with  calcium  chloride,  a  caustic  potash  bulb 
for  the  absorption  of  carbon  dioxide,  and  a  guard  tube  containing 
soda-lime  and  calcium  chloride  are  attached  in  the  order  mentioned. 
After  the  apparatus  has  been  tested  for  leaks  in  the  usual  manner, 
the  combustion-tube  is  heated  to  redness  and  oxygen  and  air 
passed  through  to  burn  out  any  organic  matter  which  may  be 
present. 

565.  The  Combustion. — The  caustic  potash  bulb  is  weighed 
and  replaced  in  the  series  of  U-tubes.  The  boat  containing  the 
carbon  is  inserted  in  the  combustion-tube  and  pushed  in  until  it 
is  close  to  the  layer  of  copper  oxide.  The  roll  of  copper  gauze  is 
inserted  in  the  end  of  the  combustion-tube,  which  is  closed  with 
the  rubber  stopper  connecting  with  the  supply  of  oxygen.  The 
apparatus  is  tested  for  leaks  by  sucking  out  a  little  air  so  that  the 
liquid  rises  in  one  limb  of  the  caustic  potash  bulb.  If  this  column 
of  liquid  remains  at  a  constant  height  for  at  least  five  minutes  the 
apparatus  is  tight.  A  slow  stream  of  oxygen  is  now  admitted, 
and  two  burners  are  lit  under  the  forward  end  and  one  under  the 
copper  gauze  in  the  rear  end  of  the  combustion-tube.  When 
these  portions  of  the  tube  are  red-hot,  the  third  burner  from  the 
forward  end  is  lighted  and  then  the  other  burners  in  order  with  a 
few  minutes '  interval.  If  a  glass  tube  is  used,  the  combustion  may 
be  observed  by  the  glowing  of  the  carbon.  The  combustion  is 
usually  complete  fifteen  minutes  after  the  tube  has  become  red- 
hot.  The  asbestos  in  the  boat  then  appears  nearly  pure  white. 


380  ANALYSIS  OF  IRON  AND  STEEL. 

The  burners  are  then  turned  down  and,  after  a  few  minutes,  extin- 
guished so  as  to  cool  the  tube  gradually.  The  stream  of  oxygen 
is  replaced  by  a  stream  of  air  of  which  from  1  to  2  liters  should  be 
passed  through  the  apparatus  to  sweep  out  the  carbon  dioxide. 
Neither  the  oxygen  nor  the  air  should  pass  faster  than  three  bub- 
bles per  second.  The  absorption  apparatus  is  then  disconnected, 
wiped  perfectly  clean  with  a  linen  handkerchief,  and  after  stand- 
ing in  the  balance-case  for  not  more  than  one-half  hour  is  weighed. 
Until  experience  has  been  attained  in  conducting  the  combustion, 
it  is  advisable  to  insert  the  absorption  apparatus  and  repeat  the 
combustion  to  ascertain  if  any  of  the  carbon  remains  unburned. 

566.  If  Several  Determinations  are  to  be  made  a  duplicate 
absorption  apparatus  should  be  prepared  and  weighed  while  the 
first  combustion  is  in  progress.  When  the  first  absorption  appara- 
tus is  disconnected,  the  second  is  inserted,  and  a  boat  contain- 
ing the  carbon  from  another  sample  of  iron  is  inserted  in  place  of 
the  first  boat.  During  the  progress  of  the  second  combustion  the 
absorption  apparatus  of  the  first  is  weighed  and  may  then  be  used 
for  the  third  determination.  When  work  is  discontinued  the 
entrance  of  air  or  other  gases  into  the  apparatus  is  prevented  by 
closing  all  openings  with  glass  plugs. 

The  carbon  from  which  the  iron  has  been  separated  may  also 
be  burned  with  chromic  and  sulphuric  acids. 


d.  COMBUSTION  OF  THE  SEPARATED  CARBON  WITH  CHROMIC  ACID. 

567.  -An  Apparatus  similar  to  that  shown  in  Fig.  61   is  used. 
The  flask  A  has  a  capacity  of  about  250  c.c.     The  U-tube  B  con- 
tains a  sufficient  amount  of  a  solution  of  silver  sulphate  in  con- 
centrated sulphuric  acid  to  fill  the  curved  part  of  the  tube.     An 
empty  TJ-tube  C  is   placed  next  and  then  a  U-tube  D  containing 
pumice-stone  which  has  been  saturated  with  copper  sulphate  solu- 
tion and  dried.      The  usual  U-tube  filled  with  calcium  chloride  for 
drying  the  carbon  dioxide,  the  caustic  potash  bulbs,  and  the  guard 
tube  are  then  connected  in  the  order  mentioned. 

568.  The    Combustion. — The   asbestos   containing  the   carbon 
is  transferred  to  the  flask  and,  after  inserting  the  stopper,  10  c.c. 


TOTAL  CARBON 


381 


of  a  saturated  solution  of  chromic  acid  is  introduced  through  the 
funnel,  followed  by  100  c.c.  of  concentrated  sulphuric  acid  which 
has  been  brought  to  the  boiling-point  after  the  addition  of  a  little 
chromic  acid.  A  slow  current  of  air  is  passed  through  the  apparatus 
and  the  flask  is  gently  heated  with  the  Bunsen  burner  and  gradu- 
ally brought  nearly  to  the  boiling-point.  The  heating  is  con- 


FIG.  61. 

tinued  until  no  more  gas  is  evolved  and  the  oxidation  is  complete. 
The  current  of  air  is  continued  until  from  1  to  2  liters  have  passed 
through  the  apparatus,  when  the  absorption-bulbs  are  discon- 
nected and  weighed. 

e.   DETERMINATION  OF  CARBON  AFTER  VOLATILIZATION  OF  THE 
IRON  IN  A  STREAM  OF  CHLORINE. 

The  carbon  in  irons  containing  chromium  and  a  large  percentage 
of  silicon  can  be  determined  only  after  volatilization  of  the  iron 
in  a  stream  of  chlorine. 

569.  Separations  Effected. — The  iron  is  converted  into  ferric 
chloride,  which  can  be  completely  expelled  by  heating  the  tube. 
The  silica  is  also  volatilized  as  silicon  tetrachloride,  SiQ4,  and 
may  be  absorbed  in  water  which  will  also  contain  any  titanium 
present  in  the  iron,  being  volatilized  as  titanium  tetrachloride, 
TiCl4.  These  elements  may  be  separated  by  adding  to  the  solu- 
tion concentrated  sulphuric  acid  and  evaporating  to  fumes.  The 
silicic  acid  is  dehydrated  and  may  be  filtered  off  and  weighed 
while  the  titanium  remains  in  solution.  Any  metals  present 


382  ANALYSIS  OF  IRON  AND  STEEL. 

which  form  volatile  chlorides  will  also  be  present  in  this  solution. 
In  the  boat  hi  which  the  iron  was  placed  will  remain  all  of  the 
carbon,  most  of  the  manganese,  chromic  chloride,  and  the  slag 
which  was  present  in  the  iron. 

570.  Process. — The  chlorine  is  generated  in  a  large  flask  of 
about  l.V  liters  capacity,  charged  with  190  grams  of  manganese 
dioxide  intimately  mixed  with  280  grams  of    sodium  chloride. 
The  chlorine  is  evolved  on  adding  from  a  dropping-funnel  350  c.c. 
of  a  mixture  of  equal  parts  of  concentrated  sulphuric  acid  and 
water.      The  chlorine  is  washed  by  passing  through  water  and 
dried  by  passing  through  concentrated  sulphuric  acid.     The  iron 
is  spread  over  the  bottom  of  a  porcelain  boat  which  is  placed  in  a 
glass  combustion-tube  which  has  been  drawn  out  and  bent  at  right 
angles  where  it  emerges  from  the  furnace  so  that  it  can  dip  under 
water.     All  connections  must  be  made  of  glass  as  far  as  possible, 
the  inner  surface  of  rubber  stoppers  being  coated  with  paraffine. 
The  air  is  displaced  from  the  apparatus  by  the  stream  of  chlorine 
and  the  combustion-tube  heated  just  sufficiently  to  expel  the  ferric 
chloride  from  the  boat.     This  is  usually  accomplished  by  heating 
the  tube  to  dull  redness.     The  chlorides  are  dissolved  out  with 
water  and  the  carbon  filtered  off  on  ignited  asbestos,  which  is  re- 
turned to  the  boat  and  dried.     The  carbon  may  then  be  burned 
in  a  combustion-tube  in  a  stream  of  oxygen,  or  it  may  be  burned 
with  chromic  and  sulphuric  acids  as  already  directed. 

DETERMINATION  OF  GRAPHITIC  CARBON. 

571.  On  dissolving  iron  and  steel  in  acid,  part  of  the  carbon 
remains  undissolved  while  part  goes  into  solution  or  is  evolved  in 
combination  with  hydrogen.     As  graphite  is  insoluble  in  dilute 
acid,  all    of   the   carbon   present    in   this   form   remains   undis- 
solved.     If  dilute  hydrochloric  acid  is  used  a  part  also  of  the 
combined  carbon  remains  undissolved.      If  nitric  acid  of  specific 
gravity  1.2    is  used,   only  the  graphitic  carbon  remains  undis- 
solved.     For  this  determination   1   gram   of  pig  iron  is  taken, 
while  as  much  as  10  grams  of  steel  containing  a  very  small  per 
cent  of  graphitic  carbon  is  taken.     15  c.c.  of  the  nitric  acid  is 
taken  for  each  gram  of  iron.      The  first  violent  action  of  the  acid 


COMBINED  CARBON.  383 

is  moderated  by  cooling  the  beaker  with  water.  When  all  of  the 
iron  is  dissolved,  dilute  the  acid,  filter  off  the  carbon  on  ignited 
asbestos,  and  wash  with  water  until  free  from  iron.  Treat  the 
residue  on  the  filter  with  a  hot  solution  of  potassium  hydroxide 
of  sp.  gr.  1.1  to  remove  silica,  and  wash  with  water,  then  with 
dilute  hydrochloric  acid,  and  finally  with  water.  Determine  the 
amount  of  carbon  present  by  burning  in  oxygen  or  in  chromic 
acid  as  already  directed. 

DETERMINATION  OF  COMBINED  CARBON. 

The  amount  of  combined  carbon  may  be  found  indirectly  by 
subtracting  the  amount  of  graphitic  carbon  from  the  total  amount 
of  carbon  found. 

572.  Colorometric  Method  of  Eggertz. — This  method  is  founded 
on  the  fact  that  when  steel  is  dissolved  in  nitric  acid  of  1.2 
specific  gravity  a  brown  color  is  given  to  the  solution  which  is 
proportional  to  the  amount  of  combined  carbon  present.     Two 
specimens  of  steel  which  give  the  same  depth  of  color  contain  the 
same  percentage  of  combined  carbon.     If  the  colors  differ,  the 
darker  solution  may  be  diluted  until  the  depth  of  color  is  identi- 
cal.    The  ratio  of  the  percentage  of  combined  carbon  in  the  two 
samples  of  steel  will  then  be  the  same  as  the  ratio  of  the  volumes 
of  the  two  solutions.     If  the  percentage  of  combined  carbon  in 
one  of  the  steels  is  known,  the  amount  present  in  the  other  may 
be  calculated.     A  so-called  standard  steel  must  therefore  be  se- 
lected and  the  percentage  of  combined  carbon  determined  by  one 
of  the  methods  already  given. 

573.  Standards. — It  has  been  found,  however,  that  two  samples 
of  steel  containing  exactly  the  same  percentage  of  combined  carbon 
give  different  colors  if  one  sample  has  been  treated  differently,  so 
far  as  tempering,  hammering,  etc.,  or  if  it  has  been  made  by  a 
different  process  of  manufacture.     This  peculiarity  is  believed  to 
be  due  to  the  fact  that  the  carbon  exists  combined  in  two  different 
conditions,  and  when  existing  in  one  condition  does  not  impart  the 
same  depth  of  color  to  the  acid  as  when  it  exists  in  the  other  condi- 
tion.    Standards  should  therefore  be  at  hand  for  each  kind  of 
steel  to  be  tested,  such  as  Bessemer,  crucible,  open-hearth,  etc,, 


384  ANALYSIS  OF  IRON  AND  STEEL. 

and  the  rolling,  hammering,  tempering,  etc.,  of  the  standard  should 
as  far  as  possible  be  the  same  as  that  of  the  sample  to  be  analyzed. 
The  percentage  of  carbon  should  also  be  as  nearly  as  possible  the 
same  in  the  standard  as  in  the  steel  to  be  analyzed. 

574-  Solution  of  the  Steel. — A  sufficient  number  of  test-tubes 
6  inches  long  and  f  inch  in  diameter  are  taken  and  2  grams  of  each 
sample,  including  the  standard,  are  weighed  out  and  placed  in  a 
test-tube.  The  test-tubes  are  carefully  numbered  to  correspond 
to  the  numbers  of  the  samples,  and  are  placed  in  beakers  or  bottles 
containing  cold  water.  Nitric  acid  of  1.2  specific  gravity  is  then 
added  to  the  test-tubes.  Acid  of  this  strength  is  made  by  diluting 
concentrated  nitric  acid  (sp.  gr.  1.4)  with  an  equal  volume  of 
water.  The  acid  is  added  from  a  burette,  3  c.c.  being  used  for  steels 
containing  .3%  of  carbon;  4  c.c.  for  .3  to  .5%  of  carbon;  5  c.c. 
for  .5  to  .8%;  6  c.c.  for  .8  to  1%;  and  7  c.c.  for  over  1%.  If  the 
amount  of  carbon  present  is  unknown,  3  c.c.  of  the  acid  should  be 
added  at  first,  and  then  successive  portions  of  1  c.c.  as  the  increase 
in  the  depth  of  color  or  the  presence  of  undissolved  carbonaceous 
matter  indicates  the  necessity  for  more  acid.  The  nitric  acid 
used  must  be  absolutely  free  from  chlorine.  A  small  funnel  or  a 
glass  bulb  of  suitable  size  should  be  placed  in  the  mouth  of  each 
test-tube.  The  test-tubes  are  now  immersed  in  the  water  cf  a 
water-bath  while  held  upright  by  some  suitable  device,  such  as  a 
metallic  disk  in  which  holes  of  the  proper  size  to  admit  the  tubes 
have  been  cut.  The  water  is  heated  to  boiling  and  the  tubes 
shaken  at  intervals  until  the  carbonaceous  matter  is  completely 
dissolved,  which  will  require  from  fifteen  to  forty-five  minutes.  As 
soon  as  solution  is  complete  each  tube  is  removed  from  the  water- 
bath  and  placed  in  cold  water,  the  light  being  excluded  as  much 
as  possible.  Direct  sunlight  bleaches  the  color  very  quickly. 

575.  For  Comparing  the  Color  cf  the  Solutions  Eggertz  used 
graduated  tubes.  •  These  tubes  are  of  uniform  bore,  made  of  clear, 
colorless  glass,  and  are  carefully  graduated  from  the  bottom  up- 
wards, having  a  capacity  of  30,  or  more,  cubic  centimeters.  Trans- 
fer the  solution  having  the  lightest  color  to  one  of  the  tubes,  using 
the  same  amount  of  water  to  rinse  the  test-tube  as  the  volume  of 
the  solution.  Mix  the  resulting  solution  thoroughly.  In  every 
case  the  acid  solution  of  the  steel  must  be  diluted  with  at  least  as 


MANGANESE.  385 

much  water  as  the  amount  of  acid  used  to  dissolve  the  steel  in 
order  to  destroy  the  color  due  to  the  ferric  nitrate. 

The  solution  of  the  steel  to  be  compared  with  the  one  first 
diluted  is  transferred  to  another  tube  in  the  same  manner,  and 
the  color  of  the  two  solutions  is  compared  by  holding  the  tubes 
vertically  and  side  by  side  in  front  of  a  piece  of  white  paper  between 
the  observer  and  the  light.  The  darker  solution  is  diluted  by  the 
addition  of  water  until  the  depth  of  color  in  the  two  tubes  is  iden- 
tical. The  percentage  of  combined  carbon  known  to  be  present 
in  the  standard  steel  is  divided  by  the  volume  of  the  solution  of  this 
steel.  The  quotient  gives  the  amount  of  carbon  represented  by 
each  cubic  centimeter  of  the  solution.  This  number  multiplied 
by  the  number  of  cubic  centimeters  to  which  the  solution  of  the 
steel  to  be  analyzed  was  diluted  gives  the  percentage  of  combined 
carbon  present. 


DETERMINATION   OF   MANGANESE. 

The  difficulties  attending  the  determination  of  manganese  in 
iron  and  steel  arise  mainly  from  the  presence  of  the  large  amounts 
of  iron.  The  iron  may  be  precipitated  as  basic  acetate,  but  the 
precipitation  must  be  repeated  several  times,  and  if  the  percentage 
of  manganese  is  small,  so  that  a  large  amount  of  iron  must  be 
taken  for  the  analysis,  the  basic  acetate  precipitate  becomes  very 
large  and  unwieldy.  The  separation  of  the  iron  by  solution  of 
the  ferric  chloride  in  ether  is  a  more  rapid  and  convenient  method, 
which  has  already  been  described  in  Chapter  XIII,  p.  160. 


a.  FORD'S  METHOD. 

Equally  as  efficient  is  a  method  known  in  this  country  as 
Ford's  method.*  Advantage  is  taken  of  the  fact  that  potassium 
chlorate  converts  the  managnese  present  in  strong  nitric  acid  solu- 
tion into  manganese  dioxide,  which  can  be  filtered  off  from  the 
iron  which  remains  in  solution.  The  separated  manganese  may 

*  Trans.  Inst.  Min.  Engineers,  IX,  397,  X,  100. 


386  ANALYSIS  OF  IRON  AND  STEEL. 

then  be  determined  gravimetrically  or  volumetrically.     The  reac- 
tions taking  place  may  be  represented  as  follows: 

KC103 + HN03  =  KN03 + HC103  ; 

2HC103+ Mn(N03)2  =  Mn02+  2C102+ 2HN03. 

576.  Solution  of  the  Steel. — 5  to  10  grams  of  steel  or  pig  iron 
are  taken,  while  only  1  gram  of  spiegel  or  ferro-manganese  contain- 
ing 20  to  40%  of  manganese,  and  \  gram  if  the  per  cent  of  manga- 
nese is  still  higher.     If  less  than  0.2%  of  silicon  is  present,  the 
weighed  portion  is  dissolved  in  60  c.c.  of  nitric  acid  of  1.2  specific 
gravity  and  the  solution  evaporated  to  small  bulk.     A  large  beaker 
should  be  used  and  the  acid  added  cautiously.     If  the  percentage 
of  silicon  is  high,  the  steel  should  be  dissolved  in  hydrochloric 
acid   and  the  solution  evaporated  to  dryness  to  dehydrate  the 
silica,  which  otherwise  forms  a  gelatinous  mass  and  interferes  with 
the  subsequent  filtration.     Pig  iron  is  also  dissolved  in  hydro- 
chloric acid,  the  solution  filtered,  and  the  filtrate  evaporated  to 
dryness.     The  hydrochloric   acid   is  removed   in  either  case  by 
evaporation  with  50  c.c.  of  strong  nitric  acid  to  a  small  bulk. 
The  silica  may  also  be  removed  by  dissolving  the  iron  in  nitric 
acid  to  which  a  few  drops  of  hydrofluoric  acid  have  been  added 
and  evaporating  to  a  small  bulk. 

577.  Precipitation  of  the  Manganese. — This  solution,  as  well  as 
that  obtained  from  the  steel  low  in  silica,  is  treated  with  100  c.c. 
of  concentrated  nitric  acid  and  5  grams  of  potassium  chlorate 
added.     The  solution  is  boiled  for  fifteen  minutes,  50  c.c.  strong 
nitric  acid  are  added,  and  5  grams  of  potassium  chlorate,  and  the 
solution  again  boiled  for  fifteen  minutes.     When  no  more  chlorine 
fumes  are  evolved  the  solution  is  rapidly  cooled  by  standing  the 
beaker  in  cold  water  and  the  precipitate  filtered  off  on  asbestos. 
It  is  washed  with  strong  nitric  acid  and  then  with  water.      It  is 
advisable  to  keep  the  strong  nitric  acid  filtrate  separate  from  the 
wash-water  and  to  test  it  for  manganese  by  adding  a  little  more 
potassium  chlorate  and  again  boiling. 

578.  If  the   Manganese   is  to   be   Determined   Gravimetrically 
the  manganese  dioxide  is  dissolved  in  sulphurous  acid  or  hydrogen 
peroxide  and  hydrochloric  acid.     The  excess  of  the  sulphur  dioxide 


MANGANESE.  387 

or  hydrogen  peroxide  is  expelled  by  boiling  the  solution.  The 
small  amount  of  iron  present  is  oxidized  by  the  addition  of  a  little 
bromine  water  the  excess  of  which  must  be  boiled  out.  It  is  precipi- 
tated by  adding  an  excess  of  ammonia  to  the  hot  solution.  The 
precipitate  is  filtered  off,  washed  once  or  twice,  dissolved  in  hydro- 
chloric acid,  and  the  solution  boiled  to  expel  a  trace  of  chlorine 
from  the  manganese  dioxide  and  reprecipitated  with  ammonia,  fil- 
tered off,  and  washed  thoroughly.  The  combined  filtrates  con- 
tain all  of  the  manganese  and  a  little  cobalt  if  that  element  was 
present  in  the  iron.  The  cobalt  may  be  precipitated  by  passing 
hydrogen  sulphide  through  the  solution  after  acidifying  with  acetic 
acid  and  heating  to  boiling.  After  filtering  off  the  precipitate 
and  boiling  the  solution  to  expel  the  excess  of  hydrogen  sulphide, 
the  manganese  may  be  determined  either  as  sulphide  or  as  phos- 
phate as  directed  in  Chapters  VI  and  VII. 

579.  The  Manganese  may  also  be  Determined  Volumetrically 
by  dissolving  the  manganese  dioxide  in  excess  of  oxalic  acid  or  a 
pure  ferrous  salt  and  titrating  the  excess  with  a  standard  solution 
of  potassium  permanganate  (see  pp.  322,  325).  As  the  per  cent 
of  oxygen  in  the  precipitate  may  not  be  exactly  that  correspond- 
ing to  the  formula  Mn02,  it  is  advisable  to  standardize  the  potas- 
sium permanganate  solution  by  using  the  precipitate  obtained 
from  a  sample  of  steel  or  iron  in  which  the  manganese  has  been 
determined  gravimetrically.  The  manganese  dioxide  obtained 
from  a  weighed  amount  of  this  iron  is  dissolved  in  excess  of  a 
reducing  agent  and  the  excess  determined  by  means  of  the  per- 
manganate solution. 


6.  VOLHARD'S  METHOD. 

580.  Solution  of  the  Metal. — This  very  accurate  method  of 
determining  manganese  may  be  used  for  determining  this  element 
in  all  samples  of  iron  and  steel  except  those  containing  very 
minute  amounts  of  manganese.  Weigh  out  5  grams  of  ferro- 
manganese  and  from  1  to  2.5  grams  of  spiegeleisen,  white  pig  iron, 
steel,  etc.  Dissolve  *  in  a  mixture  of  275  c.c.  of  water  125  c.c.  of 

*  Reis,  Zeit.  f.  angew.  Ch.,  1892,  pp.  604,  672. 


388  ANALYSIS  OF  IRON  AND  STEEL. 

concentrated  nitric  acid  (sp.  gr.  1.4)  and  100  c.c.  concentrated 
sulphuric  acid.  Use  from  25  to  50  c.c.  of  this  acid  mixture,  accord- 
ing to  the  amount  of  iron  taken.  After  the  iron  is  dissolved,  evapo- 
rate the  solution  until  fumes  of  sulphuric  acid  are  evolved.  After 
cooling,  add  100  c.c.  of  water  and  10  c.c.  of  the  acid  mixture  and 
warm  until  solution  is  complete.  Transfer  to  a  flask  of  750-  to 
1000-c.c.  capacity  and  add  3  grams  of  barium  peroxide  and  5  c.c. 
concentrated  nitric  acid  to  oxidize  the  hydrocarbons  present. 
Boil  several  minutes  to  decompose  the  excess  of  barium  peroxide. 

581.  Titration. — Dilute  the  solution  until  the  flask  is  about  one- 
half  full,  add  zinc  oxide  suspended  in  water,  shaking  the  solution 
after  each  addition  until  the  solution  is  neutral  and  the  iron  is  pre- 
cipitated, leaving  it  colorless  or  slightly  milky  with  the  excess  of 
zinc  oxide.      Warm  the  solution  to  about  90°,  but  not  to  boiling, 
and  titrate  with   standard   potassium  permanganate  solution  as 
directed  in  Chapter  XXIV,  page  323. 

582.  If  the  Zinc  Oxide  contains  manganese  it  may  be  purified 
as  follows:  Dissolve  the  impure  oxide  in  hydrochloric  acid,  finally 
adding  an  excess  of  the  zinc  oxide  so  that  part  of  it  remains  undis- 
solved.     Add  a  little  bromine  and  heat  the  solution  to  boiling  and 
filter.     Precipitate  the  zinc  oxide  by  means  of  ammonia,  carefully 
avoiding  an  excess.     Wash  the  oxide  thoroughly  and  then  transfer 
to  a  bottle  with  water.     Shake  the  bottle  well  before  withdrawing 
any  of  the  oxide  for  use. 


c.  DESHAY'S  METHOD.* 

This  method  is  based  on  the  fact  that  manganese  nitrate 
when  boiled  with  nitric  acid  and  lead  peroxide  is  converted  into 
permanganic  acid,  which  may  then  be  titrated  with  a  standard 
solution  of  sodium  arsenite.  The  method  is  especially  applicable  to 
iron  and  steel  containing  less  than  2%  of  manganese.  It  cannot 
be  used  when  chromium  is  present,  as  chromic  acid  is  produced. 

583.  Process. — 0.5  gram  of  steel  or  pig  iron  is  dissolved  in  a 
small  beaker  or  test-tube  in  30  c.c.  nitric  acid  of  1.2  specific  gravity. 
The  solution  is  boiled  until  the  nitrous  fumes  are  completely 

*  Bull.  Soc  Chim  de  Paris.  June  20,  1878. 


MANGANESE.  389 

expelled.  From  1  to  3  grams  of  lead  peroxide  or  red  lead  free 
from  manganese  are  then  added  and  the  solution  diluted  with 
hot  water  to  about  60  c.c.  It  is  boiled  for  three  or  four  minutes 
and  after  allowing  the  peroxide  of  lead  to  settle,  the  solution  is 
decanted  and  the  residue  again  boiled  after  the  addition  of  50  c.c. 
of  dilute  nitric  acid.  If  the  solution  again  becomes  colored  with 
permanganic  acid,  it  must  be  decanted  and  the  boiling  repeated 
after  the  addition  of  dilute  nitric  acid.  The  solution  is  filtered 
through  asbestos  which  is  free  from  organic  matter.  The  filtrate  is 
titrated  with  a  standard  solution  of  sodium  arsenite. 

584.  The  Sodium  Arsenite  Solution  may  be  made  by  dissolving 
4.5  grams  of  pure  arsenious  oxide  in  water  to  which  25  grams  of 
sodium  carbonate  has  been  added  and  diluting  the  solution  to  2 
liters.     This  solution  should  then  be  standardized  by  comparison 
with  a  standard  potassium  permanganate  solution,  or  a  still  better 
plan  is  to  standardize  it  with  a  sample  of  steel  in  which  the  man- 
ganese has  been  determined  gravimetrically.     This  is  done  by 
weighing  out  \  gram  of  the  steel,  dissolving  in  nitric  acid,  and 
oxidizing  with  peroxide  of  lead.     The  permanganic  acid  produced 
in  this  manner  is  titrated  with  the  arsenite  solution  to  be  standard- 
ized.    The  amount  of  manganese  corresponding  to  1  c.c.  of  the 
arsenite  solution  may  then  be  calculated  from  the  known  amount 
of  manganese  in  the  steel  used. 

585.  Separation  of  Lead  Peroxide  by  a  Centrifugal  Machine. — 
Instead  of  filtering  the  permanganic  acid  from  the  lead  peroxide, 
the  separation  may  be  effected  by  means  of  a  centrifugal  machine. 
In  order  to  secure  complete  oxidation  of  the  manganese  by  boiling 
once,  a  smaller  amount  of  the  steel  is  taken,  from  .05  to  .10  gram 
being  used.     10  c.c.  of  the  nitric  acid  of  sp.  gr.  1.2  are  added  to 
dissolve  the  iron  and  after  the  nitrous  fumes  have  been  expelled 
by  boiling,  10  c.c.  of  hot  water  and  \  gram  of  lead  peroxide  are 
added  and  the  solution  boiled  for  three  minutes.     The  solution 
is  washed  into  a  wide-mouthed  two-ounce  bottle,  enough  water 
being  used  to  make  50  c.c.     The  bottle  is  then  placed  in  the  cen- 
trifugal machine  and  rotated  for  two  minutes.      The  liquid  will 
then  be  perfectly  clear  and  may  be  poured  out  without  disturbing 
the  lead  peroxide,  which  forms  a  compact  layer  on  the  bottom  of 
the  bottle.    The  permanganic  acid  is  then  titrated  with  a  solution 


390  ANALYSIS  OF  IRON  AND  STEEL. 

of  sodium  arsenite  which  is  equal  to  about  .0001  gram  of  man- 
ganese per  cubic  centimeter. 


PROXIMATE  ANALYSIS  OF  COAL. 

The  methods  used  in  the  analysis  of  coal  do  not  give  the 
amount  of  any  definite  chemical  compound  which  is  present  in  the 
coal.  The  determination  of  MOISTURE  gives  very  nearly  the  amount 
of  water  present,  but  as  this  constituent  is  obtained  by  loss  in 
weight  after  drying  the  coal  in  the  air,  an  error  is  introduced 
because  SULPHUR  and  other  constituents  present  are  oxidized  at 
the  temperature  used  for  drying  the  coal,  and  therefore  the  coal 
loses  weight  for  a  time,  and  then  begins  to  increase  in  weight. 
The  conditions  under  which  this  as  well  as  other  determinations 
are  made  are  somewhat  arbitrarily  fixed  so  that  the  results  obtained 
by  various  chemists  may  be  comparable  with  each  other  and  form 
a  basis  for  the  comparison  of  the  coals  analyzed.  The  methods 
given  in  this  chapter  are  those  adopted  by  a  committee  of  the 
American  Chemical  Society,  consisting  of  Wm.  A.  Noyes,  W.  F. 
Hillebrand,  and  C.  B.  Dudley.* 

586.  The  Determination  of  Moisture  is  made  by  heating  1  gram 
for  one  hour  at  104°  to  107°.  This  temperature  may  most  readily 
be  maintained  by  using  a  double-walled  air-bath  in  which  pure 
toluene  is  boiled.  By  attaching  a  reflux  condenser  to  the  usual 
steam  vent,  loss  of  toluene  may  be  obviated.  As  considerable 
moisture  is  lost  in  powdering  the  coal,  the  committee  recommends 
the  making  of  two  determinations  of  moisture,  one  on  the  coal  as 
powdered  for  the  determination  of  sulphur,  ash,  etc.,  and  another 
on  the  material  broken  up  only  sufficiently  fine  for  sampling.  As 
it  is  necessary  that  the  percentages  found  shall  apply  to  the  unpow- 
dered  coal  as  it  is  actually  used,  a  correction  must  be  made  on  the 
percentages  found  by  using  the  powdered  material.  The  differ- 
ence in  the  percentages  of  moisture  found  is  divided  by  the  sum 
of  the  percentages  of  the  other  constituents  in  the  powdered 
coal.  The  quotient  multiplied  by  100  is  the  percentage  by  which 

*  Jour.  Am.  Chem.  Soc.,  XXI,  1116. 


VOLATILE  MATTER.  391 

the  percentages  of  the  other  constituents  are  reduced.     The  illus- 
tration given  in  the  report  mentioned  is  as  follows: 

Coarsely  Ground  Powdered 

Coal.  Coal. 

Moisture 12.07  10.39 

Volatile  combustible  matter.  .  34.25 


The  correction  factor  will  be 

12.07-10.39      1.68 


=  .0187. 


100-10.39      89.61 
The  true  per  cent  of  volatile  combustible  matter  will  be 

34.25-  (34.25  X  .0187)  =33.61. 

587.  The  Volatile  Combustible  Matter  is  determined  by  heating 
a  fresh  portion  of  the  powdered  coal  in  a  tightly  closed  platinum 
crucible  with  the  full  flame  of  a  Bunsen  burner  for  seven  minutes. 
The  loss  in  weight  less  the  moisture  is  the  volatile  combustible 
matter,  while  the  residue  in  the  crucible  is  the  coke. 

This  determination  serves  to  distinguish  the  different  kinds  of 
coal  which  are  mined  for  various  industrial  purposes.  The  ANTHRA- 
CITE COALS  give  only  a  small  per  cent  of  volatile  combustible 
matter,  leaving  a  residue  in  the  crucible  which  can  hardly  be 
distinguished  from  the  powdered  coal.  The  BITUMINOUS  and 
CANNEL  COALS  melt  on  being  heated  in  the  crucible,  and  the  vola- 
tile matter  in  escaping  produces  a  porous  mass  which  is  much 
more  bulky  than  the  original  coal.  This  material  is  what  is  known 
in  commerce  as  "COKE."  A  good  bituminous  coal  gives  about 
60%  of  coke.  Cannel  coals,  on  the  other  hand,  give  60%  or  70% 
of  volatile  combustible  matter  with  a  corresponding  smaller  per 
cent  of  coke.  Between  these  extremes  coals  may  be  found, 
giving  nearly  all  percentages  of  coke  and  volatile  combustible 
matter. 

588.  The  Percentage  of  Ash  is  found  by  burning  a  weighed 
portion  of  the  coal  in  a  platinum  crucible.     The  percentage  of  ash 
subtracted  from  the  percentage  of  coke  gives  the  percentage  of 
FIXED  CARBON,  the  volatile  combustible  matter  being  composed  of 
carbon  which  has  united  with  the  hydrogen  present,  forming  vola- 
tile combustible  hydrocarbons. 


392  ANALYSIS  OF  COAL. 

It  is  evident  that  a  low  percentage  of  ash  in  coal  is  very  desir- 
able, as  this  constituent  not  only  has  no  calorific  value,  but  forms 
a  waste  product  which  must  be  disposed  of. 

589.  For  Determining  Sulphur  various  methods  have  been  pro- 
posed and  used,  the   difference  being  found  in  the  methods  of 
oxidizing  the  carbon  and  the  sulphur,  which  are  ultimately  precipi- 
tated by  means  of  barium  chloride  and  weighed  as  barium  sul- 
phate.    ESCHKA'S  METHOD,  by  which  the  coal  is  burned  by  the 
oxygen  of  the  air  after  mixing  with  magnesium  oxide  and  sodium 
carbonate,  and  the  sulphur  oxidized  by  means  of  bromine  water, 
is  one  of  the  most  rapid  as  well  as  a  very  accurate  method.     As 
the  sulphuric  acid  is  extracted  from  the  alkaline  flux  with  hot 
water,  some  of  the  silica  in  the  coal  is  also  dissolved,  and  this  is 
subsequently    carried    down    with    the    barium    sulphate.     More 
accurate  results  may  be  obtained  by  evaporating  the  solution  to 
dryness  after  acidifying  with  hydrochloric  acid  in  order  to  dehydrate 
and  remove  this  silica. 

590.  The  Sulphur  Exists  in  Coal  in  at  least  three  conditions: 
as  a  METALLIC  SULPHIDE,  especially  PYRITES,  as  CALCIUM  or  BARIUM 
SULPHATE,  and  as  a  SULPHURETTED  HYDROCARBON.     In  a  proximate 
analysis  of  coal  about  one-half  of  the  sulphur  existing  as  pyrites 
and  all  of  the  organic  sulphur  is  probably  volatilized  with  the 
volatile  combustible  matter.     The  remainder  of  the  sulphur  exist- 
ing as  pyrites  is  expelled  during  the  combustion  of  the  fixed  carbon, 
only  that  existing  as  barium  or  calcium  sulphate  remaining  with 
the  ash  unless  calcium  or  barium  carbonate  was  present  in  the  coal. 


EXERCISE  70. 
Proximate  Analysis  of  Coal. 

The  carefully  selected  sample  is  coarsely  ground  in  a  porcelain  mortar 
and  well  mixed.  A  few  grams  are  taken  and  powdered  and  placed  in  a 
well-stoppered  bottle.  The  main  portion  of  the  sample  is  preserved  in  the 
same  manner. 

591.  Determination  of  Moisture. — The  temperature  of  an  air-bath  is 
brought  to  104°-107°  by  adjusting  the  flame  of  the  Bunsen  burner.  Two 
platinum  or  porcelain  crucibles  are  cleaned  and,  after  drying  for  a  few 
minutes  in  the  air-bath,  are  cooled  in  desiccators  and  weighed.  One  gram 


VOLATILE  MATTER.  393 

of  the  coarsely  ground  coal  is  weighed  and  placed  in  one  of  the  crucibles, 
while  one  gram  of  the  powdered  material  is  weighed  out  and  placed  in  the 
other.  The  uncovered  crucibles  are  then  placed  in  the  air-bath  and  heated 
for  one  hour.  They  are  then  cooled  in  the  desiccators  after  replacing  the 
covers,  and  weighed.  If  porcelain  crucibles  have  been  used,  the  coal  is 
carefully  brushed  out  and  the  crucibles  weighed,  as  the  latter  may  have  lost 
weight  during  the  heating.  The  loss  in  weight  in  each  case  in  centigrams 
gives  the  percentage  of  moisture.  The  percentage  found  in  the  coarsely 
ground  material  is  reported,  while  the  difference  between  this  percentage 
and  that  found  in  the  powdered  material  is  used  to  calculate  the  correction 
on  the  percentage  of  the  other  constituents. 

592.  Determination  of  Volatile  Combustible  Matter- — A  20-  to  30-gram 
platinum  crucible  is  cleaned,  placed  on  a  platinum  or  pipe-stem  triangle, 
and  ignited  with  the  Bunsen  burner.  It  is  cooled  in  a  desiccator  and  weighed. 
One  gram  of  the  powdered  coal  is  weighed  out  and  transferred  to  the 
crucible,  which  is  placed  on  a  large  ring  of  the  iron  ring-stand,  and  the 
height  of  the  ring  so  adjusted  that  the  bottom  of  the  crucible  is  6  to  8  cm. 
above  the  top  of  the  Bunsen  burner,  which  must  have  a  clear  blue  flame 
fully  20  cm.  high  when  burning  free.  The  cover  of  the  crucible,  which  must 
fit  tight,  is  adjusted,  and  the  lighted  burner  placed  under  the  crucible,  which 
is  heated  seven  minutes.  The  crucible  must  be  protected  from  draughts 
during  the  heating,  and  the  upper  surface  of  the  cover  must  burn  clear.  The 
hot  crucible  is  transferred  to  the  desiccator,  and  after  cooling  for  fifteen  to 
twenty  minutes  is  weighed.  The  loss  in  weight  less  the  moisture  in  the 
powdered  coal  is  the  volatile  combustible  matter.  Compute  the  percentage, 
making  the  correction  for  difference  in  percentage  of  moisture  in  the  coarsely 
ground  and  powdered  coal. 

COKE. — The  residue  in  the  crucible  is  COKE.  The  percentage  is  cor- 
rected as  already  directed. 

593-  Ash  is  determined  by  burning  one  gram  of  the  coal  placed  in  a 
platinum  crucible.  The  portion  used  for  the  determination  of  moisture 
may  be  taken,  or  a  fresh  portion  of  the  powdered  material  may  be  weighed 
out.  The  coke  left  after  expelling  the  volatile  matter  burns  with  difficulty. 
The  crucible  is  placed  on  its  side  on  a  pipe-stem  or  platinum  triangle,  and 
is  heated  at  first  with  a  very  low  flame.  The  burner  must  not  be  placed 
so  that  the  flame  passes  in  front  of  the  crucible,  thus  preventing  access  of 
air.  The  combustion  is  accelerated  by  bringing  fresh  portions  of  coal  to 
the  surface  by  turning  the  crucible  when  the  carbon  has  burned  out,  leaving 
the  nearly  white  ash  on  the  surface.  When  no  more  black  particles  are  visi- 
ble which  burn  on  exposure  to  the  air,  the  crucible  is  cooled  in  the  desiccator 
and  weighed,  and  the  percent  of  ash  calculated.  The  per  cent  of  coke  minus 
the  per  cent  of  ash  gives  the  per  cent  of  FIXED  CARBON. 

594.  Determination  of  Sulphur  by  Eschka's  Method.— An  intimate  mix- 
ture of  1  part  of  dry  sodium  carbonate  and  2  parts  of  magnesium  oxide  is 
made.  The  magnesium  oxide  must  be  light  and  porous,  not  compact  and 


394  ANALYSIS  OF  COAL. 

heavy.  One  and  one-half  grams  of  this  mixture  are  weighed  and  placed  in 
a  large  platinum  crucible,  or,  still  better,  in  a  platinum  dish  having  a  capac- 
ity of  75  to  100  c.c.  One  gram  of  the  finely  powdered  coal  is  weighed  out 
and  placed  on  the  magnesia  mixture,  and  the  two  thoroughly  mixed  by 
stirring  with  a  platinum  or  glass  rod.  The  dish  is  heated  very  cautiously 
with  the  Bunsen  burner,  which  is  at  first  held  in  the  hand.  The  heat  13 
raised  very  slowly,  especially  with  soft  coals.  When  the  strong  glowing  has 
ceased  the  heat  is  gradually  increased,  until  in  fifteen  minutes  the  bottom 
of  the  dish  is  at  a  low  red  heat. 

When  the  carbon  is  completely  burned  the  residue  is  transferred  to  a 
beaker,  the  dish  is  rinsed  with  about  50  c.c.  of  water,  45  c.c.  of  bromine- 
water  is  added,  and  the  solution  is  boiled  for  five  minutes.  After  allowing 
the  insoluble  matter  to  settle  the  solution  is  decanted  through  a  filter. 
The  residue  is  treated  twice  with  30  c.c.  of  water,  which  is  brought  to  a  boil, 
and  decanted  through  the  filter,  which  is  then  washed  until  the  filtrate 
gives  only  a  slight  opalescence  with  silver  nitrate  and  nitric  acid.  To  the 
filtrate,  which  should  have  a  volume  of  about  200  c.c.,  li  c.c.  of  concen- 
trated hydrochloric  acid  or  3  c.c.  of  dilute  acid  are  added.  The  solution  is 
boiled  until  the  bromine  is  expelled,  and  10  c.c.  of  a  10  per  cent  solution  of 
barium  chloride  is  added  drop  by  drop  with  constant  stirring,  especially  at 
first.  Digest  on  the  hot  plate  until  the  solution  is  clear,  filter  oft0  the  barium 
sulphate,  and  wash  free  from  chlorides  with  hot  water.  Transfer  the  moist 
precipitate  to  a  weighed  platinum  crucible  which  is  heated  with  a  small 
flame  until  the  paper  is  burned.  Finally  heat  to  redness,  cool  in  the  desicca- 
tor, and  weigh. 

A  BLANK  DETERMINATION  should  be  made  to  ascertain  if  the  fusion  mix- 
ture is  free  from  sulphur.  This  determination  is  carried  out  exactly  as 
directed  for  the  determination  of  sulphur  in  the  coal.  The  amount  of 
barium  sulphate  obtained  is  subtracted  from  that  obtained  from  the  coal. 
The  percentage  of  sulphur  is  then  calculated. 

If  the  coal  contains  much  pyrite  or  calcium  sulphate,  the  residue  of 
magnesium  oxide  must  be  dissolved  in  hydrochloric  acid  and  barium  chloride 
added  to  the  hot  solution.  Any  barium  sulphate  precipitated  is  filtered 
and  weighed  as  already  directed. 

595.  Determination  of  Sulphur  with  the  Parr  Calorimeter. — If  a  determina- 
tion of  the  calorific  value  of  the  coal  is  made  by  means  of  the  Parr  calorim- 
eter, the  sulphur  may  be  conveniently  determined  in  the  same  sample. 
The  sodium  peroxide  oxidizes  the  sulphur  to  sodium  sulphate.  The  cartridge 
is  placed  in  a  porcelain  dish  and  the  fused  mass  dissolved  out  with  hot 
water.  After  rinsing  the  cartridge  thoroughly,  the  solution  is  acidified 
with  hydrochloric  acid  and  evaporated  to  dryness  on  the  water-bath  to 
dehydrate  the  silica.  The  residue  is  taken  up  with  water  and  a  little 
hydrochloric  acid,  and  filtered.  After  washing  the  paper  free  from  chlorides, 
the  solution  is  heated  to  boiling  and  the  sulphuric  acid  precipitated  by 
means  of  barium  chloride,  which  is  added  slowly  and  with  constant  stirring. 
The  precipitate  is  filtered  off  on  a  quantitative  paper,  washed  free  from 
chlorides,  and  ignited  and  weighed  in  the  usual  manner. 


THERMAL   UNITS.  395 


DETERMINATION  OF  THE   CALORIFIC  VALUE 
OF   FUELS. 

596.  Effect  of  Composition  on  Calorific  Value. — The  results  of  a 
proximate  analysis  of  a  fuel  give  only  a  general  idea  of  its  calorific 
or  heating  value  by  indicating  the  class  in  which  the  fuel  belongs. 
Anthracite  coals  are  characterized  by  a  small  percentage  of  vola- 
tile combustible  matter  (usually  less  than  10%),  while  bituminous 
coals  contain  a  large  amount  of  this  constituent  (usually  30-40%), 
and  in  canriel  coals  this  is  the  main  constituent.     The  proportion 
of  this  constituent  is  dependent  on  the  amount  of  hydrogen  pres- 
ent, as  this  element  forms  volatile  compounds  with  the  carbon. 
When  hydrogen  burns,  it  evolves  a  much  larger  amount  of  heat 
than  is  produced  by  the  combustion  of  the  same  amount  of  carbon. 
Coals  containing  a  large  proportion  of  volatile  combustible  matter 
have  therefore  generally  a  high  calorific  value.     On  the  other  hand 
the  calorific  value  is  reduced  by  the  presence  of  ash  which  has  no 
calorific  value,  and  by  water  which  absorbs  heat  when  it  is  con- 
verted into  steam.    It  is  evident  that  the  calorific  value  of  a 
fuel   could  be   calculated   from   the   percentage    of    carbon   and 
hydrogen  present.    This  method  is  subject  to  the  error  arising 
from  the  fact  that  the  heat  of  combustion  of  hydrogen  and  car- 
bon varies  with  the  nature  of  their  chemical  combination  with 
each  other.     Direct  methods  for  determining  calorific  values  are 
therefore  more  reliable.    The  instruments  devised  for  this  pur- 
pose are  called  calorimeters. 

597.  Thermal  Units. — Various  units  for  measuring  heat  are  in 
use.    They  are  all  based  upon  the  fact  that  a  definite  amount  of 
heat  will  raise  the  temperature  of  a  definite  amount  of  water  a 
definite  number  of  degrees.    Those  based  on  the  metric  system  are 
the  large  and  the  small  calorie.    A  small  calorie  is  defined  as  the 
amount  of  heat  necessary  to  raise  1  gram  of  water  from  4°  to  5° 
Centigrade.    The  large  calorie  is  the  amount  of  heat  necessary  to 
raise  1  kilogram  of  water  from  4°  to  5°  C.,  and  is  equal  to  1000 
small  calories,  and  is  denoted  by  the  capital  C.    A  unit  of  heat  equal 
to  100  small  calories  has  also  been  used,  arid  is  designated  by  K. 


396  CALORIFIC  VALUE  OF  FUELS. 

As  the  specific  heat  *  of  water  changes  but  slightly  with  ordinary 
changes  of  temperature,  only  a  very  slight  error  will  be  intro- 
duced by  using  other  temperatures  than  4°  to  5°  C.  when  meas- 
uring the  rise  in  temperature.  For  practical  purposes,  there- 
fore, a  calorie  may  be  defined  as  the  heat  necessary  to  raise  one 
gram  of  water  one  degree  Centigrade.  Another  unit  of  heat  in 
common  use  is  the  British  Thermal  Unit,  designated  by  the  initial 
letters  B.  T.  U.  This  unit  is  the  amount  of  heat  necessary  to 
raise  the  temperature  of  1  pound  of  water  from  39°  to  40°  Fahren- 
heit, or,  for  ordinary  purposes,  one  degree  F.  at  ordinary  tem- 
peratures. 

The  heat  of  combustion  of  a  fuel  may  therefore  be  expressed 
in  calories  or  in  B.  T.  U.,  the  relation  between  these  units  being 
easily  calculated  from  the  known  equivalents  of  grams  and 
pounds  and  degrees  Centigrade  and  degrees  Fahrenheit  (large 
calorie  =  3.96832  B.  T.  U.).  However,  when  the  heat  of  com- 
bustion of  a  substance  is  given  in  calories,  1  gram  or  1  kilo- 
gram is  the  amount  of  the  substance  for  which  the  heat  is  given, 
while  if  the  heat  is  given  in  B.  T.  U.,  the  number  given  refers 
to  the  amount  of  heat  obtained  from  1  pound  of  combustible 
substance.  As  1  kilogram  =  2,20462  pounds,!  to  convert  the 
heat  of  combustion  in  calories  to  the  heat  of  combustion  in  B.  T.  U., 
multiply  the  number  of  calories  by  1.8  (3. 96732-:-  2. 20462). 

598.  Calorimeters  embody  three  essential  features :    (a)  A  com- 
bustion chamber;  (b)  a  tank  of  water  with  a  delicate  thermometer 
for  indicating  the  amount  of  heat  absorbed;  and  (c)  an  insulating 
device  to  prevent  the  external  heat  from  reaching  the  water  and 
the  thermometer,  and  also  to  prevent  the  heat  generated  in  the 
combustion  chamber  from  escaping  from  the  apparatus. 

599.  The  Bomb  Calorimeter  of  Berthelot  is  the  most  accurate  cal- 
orimeter which  has  been  devised.    It  is  adapted  for  the  combustion 
of  any  solid  or  liquid  combustible  substance.     The  bomb  is  the 

*  SPECIFIC  HEAT  OF  WATER  ACCORDING  TO  ROWLAND  AND  PERUET. 

5  1.0054  9  1.0026  13  1.0009  17  0.9993  21 '   0.9977 

6  1.0047  10  1.0019  14  1.0005  18  0.9988  22     0.9974 

7  1.0040  11  1.0014  15  1.0000  19  0.9984  23     0.9974 

8  1.0033  12  1.0012  16  0.9995  20  0.9979  24     0.9972 

t  For  these  and  other  equivalents  see  Van  Nostrand's  Chemical  Annual. 


THE  PARR  CALORIMETER.  397 

characteristic  part  of  this  instrument.  It  consists  of  a  brass  or 
steel  receptacle  of  400  to  600  cubic  centimeters  capacity,  which 
can  be  tightly  closed  and  can  sustain  a  pressure  of  at  least  25 
atmospheres.  A  small  platinum  cup  is  suspended  from  the  lid 
of  the  bomb.  The  substance  to  be  burned  is  placed  in  this  cup. 
Two  insulated  wires  are  led  through  the  lid  and  extend  to  within 
a  short  distance  of  the  cup.  These  wires  are  connected  with  a 
short  piece  of  fine  iron  or  platinum  wrire  which  can  be  fused  by 
means  of  an  electric  current  and  thus  ignite  the  combustible 
material  at  any  desired  moment.  Pure  oxygen  under  pressure 
is  led  into  the  bomb  by  means  of  a  tube  passing  through  the  lid 
and  containing  a  close-fitting  valve.  The  oxygen  is  forced  in 
until  a  pressure  of  25  atmospheres  is  produced.  In  this  atmos- 
phere of  pure  oxygen  under  pressure,  all  substances  may  te  burned. 
The  bomb  is  immersed  in  a  tank  of  water  in  which  a  delicate 
thermometer  is  suspended  to  register  the  increase  in  temperature, 
the  water  being  agitated  by  means  of  an  appropriate  stirring 
devise  to  insure  uniform  distribution  of  the  heat.  The  tank  of 
water  is  enclosed  in  a  vessel  made  of  some  good  insulating  material. 

THE  PARR  CALORIMETER. 

600.  Chemical  Reactions  Involved.— This  instrument  differs  from 
the  bomb  calorimeter  in  that  sodium  peroxide  or  other  strong 
oxidizing  substances  are  mixed  with  the  fuel  to  produce  the  com- 
bustion. This  results  in  two  decided  advantages.  In  the  first 
place  the  combustion  chamber  may  be  very  materially  reduced  in 
size  (from  300  to  600  c.c.  in  the  bomb  calorimeter  to  25  c.c.  in  the 
Parr  calorimeter).  In  the  second  place  the  combustion  chamber  is 
not  required  to  stand  high  pressure  because  the  oxygen  is  not 
present  as  a  gas  under  pressure  and  the  products  of  combustion 
(C02,  S03,  H20,  etc.)  are  not  given  off  in  the  gaseous  form.  The 
alkali  resulting  from  the  decomposition  of  the  sodium  peroxide  com- 
bines with  the  carbon  dioxide,  sulphur  trioxide,  and  water,  forming 
sodium  carbonate,  sodium  sulphate,  and  sodium  hydroxide.  Veiy 
little  gas  pressure  is  therefore  produced  in  the  combustion  chamber. 
An  additional  error  is  introduced,  however,  because  heat  is  evolved 
by  the  combination  of  the  products  of  combustion  with  the  alkali. 


398 


CALORIFIC  VALUE  OF  FUELS. 


These  are  mainly  carbon  dioxide  and  water.  The  former  reacts 
with  the  sodium  oxide  to  form  sodium  carbonate,  while  the  latter 
forms  sodium  hydroxide.  The  chemical  reactions  which  take 
place  may  therefore  be  represented  in  simple  form  by  the  follow- 
ing equations: 


2NaA  +  C  =  Na2CO,  +  Na20  ; 
Na202+2H=2NaOH. 


FIG.  62. 


It  has  been  found  by  experience  as  well  as  by  calculation  from 
the  heats  of  formation  of  the  compounds  given  in  the  equations, 
that  very  nearly  73  per  cent  of  the  heat  produced  in  the  reac- 
tion is  due  to  the  oxidation  of  the  carbon  to  carbon  dioxide  and 
of  the  hydrogen  to  water.  The  factor  0.73  is,  therefore,  used  as  a 
correction  constant  in  the  final  calculation. 

601.  The  Sodium  Peroxide. — Another  source  of  error  is  met  with 
in  the  fact  that  sodium  peroxide  readily  absorbs  water,  forming  the 
compound  Na202.2H20.  When  heated  this  compound  decomposes 


THE  PARR  CALORIMETER.  399 

into  sodium  hydroxide,  oxygen,  and  water,  with  evolution  of  heat. 
In  one  experiment  10  grams  of  sodium  peroxide  absorbed  0.5  gram 
of  water  during  an  exposure  of  1  hour  to  the  air.  The  presence 
of  this  amount  of  water  caused  an  increase  of  0.194  degree  dur- 
ing the  combustion. 

A  special  grade  of  sodium  peroxide  is,  therefore,  manufactured  for 
the  Parr  calorimeter.  It  is  carefully  protected  from  exposure  to  the 
air  and  is  sold  in  hermetically  sealed  tin  cans.  When  desired 
for  use,  the  contents  of  the  can  are  transferred  to  a  glass  jar  pro- 
vided with  a  tightly  fitting  lid.  Glass-stoppered  bottles  are  not 
satisfactory.  The  " Lightening"  or  "Putnam"  jar,  which  closes 
with  a  lever  clamp,  has  been  found  well  adapted  for  this  purpose. 
The  sodium  peroxide  must  also  be  finely  divided.  It  is  not  advis- 
able to  attempt  to  grind  ordinary  samples  of  the  peroxide  because 
of  the  unavoidable  exposure  to  the  air. 

602.  Construction  of  the  Instrument. — Fig.  63  shows  the  con- 
struction of  the  cartridge  which  is  made  of  brass.  Both  top  and 
bottom  are  held  in  place  by  means  of  the  cylinders  or  caps  F  and 
E  which  are  screwed  to  the  body  of  the  cartridge.  The  joints  are 
made  air-tight  by  means  of  rubber  washers.  An  insulated  wire  / 
passes  down  through  the  stem  and  at  /  is  connected  to  the  wire 
H  by  means  of  a  piece  of  soft  iron  wire  which  may  be  fused  by 
means  of  an  electric  current  and  thus  ignite  the  combustible  mix- 
ture in  the  bottom  of  the  cylinder. 

Fig.  64  shows  the  cartridge  D  in  place  in  the  instrument. 
It  rests  on  a  pivot  at  F  and  can  be  rotated  by  means  of  the  pulley 
P.  The  blades  attached  to  the  center  of  the  cartridge  tend  to 
force  the  water  downwards  within  the  cylinder  E,  and  produce 
an  upward  current  outside  of  this  cylinder,  thus  keeping  the  tem- 
perature of  the  water  uniform.  Both  the  small  cylinder  E  and 
the  metallic  vessel  A  are  nickel-plated  and  highly  polished.  Such 
surfaces  neither  radiate  nor  absorb  heat  readily.  Loss  or  gain 
of  heat  is  still  further  prevented  by  the  two  vessels  C  and  B  of 
indurated  fiber,  as  well  as  by  the  air  spaces  between  them.  A 
double  close-fitting  lid  of  the  same  material  is  also  provided. 
Two  liters  of  water  for  the  absorption  of  the  heat  are  placed  in 
the  vessel  A,  the  temperature  being  measured  by  the  thermometer 
T.  Radiation  of  heat  cannot  be  wholly  prevented,  however,  so 


400 


CALORIFIC  VALUE  OF  FUELS. 


that  if  the  temperature  of  the  water  is  below  that  of  the  sur- 
rounding objects,  it  will  gain  heat,  while  if  it  is  above  that  of  the 
surrounding  objects,  it  will  lose  heat.  Error  from  this  source  is 
compensated  for  by  starting  a  determination  with  the  water  as 
much  below  atmospheric  temperature  as  it  will  rise  above  this 


j.  FIG.  64. 

temperature  after  the  combustion.  The  rate  at  which  the  water 
increases  or  decreases  in  temperature  may  also  be  determined, 
so  that  the  amount  of  heat  gained  or  lost  may  be  calculated. 

603.  Water  Equivalent  of  the  Instrument. — All  of  the  heat 
produced  by  the  combustion  of  the  fuel  does  not  enter  the  water. 
The  combustion  cartridge,  the  containing  vessel,  as  well  as  the 


THE  PARR  CALORIMETER.  401 

thermometer,  absorb  and  retain  a  portion  of  the  heat.  The 
amount  of  heat  lost  in  this  manner  may  be  easily  calculated. 
These  parts  of  the  instrument  are  weighed  and  the  water  equiva- 
lent calculated  by  multiplying  the  total  weight  by  the  specific 
heat  of  the  material  used.*  The  cartridge  and  containing  vessel 
in  the  Parr  calorimeter  are  made  of  brass,  specific  heat  0.092. 
The  thermometer  is  made  of  glass  and  mercury  and  only  a  por- 
tion is  immersed  in  the  water  and  is  affected  by  the  changes  in 
temperature  of  the  water.  In  order  to  ascertain  the  water 
equivalent  of  the  thermometer,  advantage  is  taken  of  the  fact 
that  the  water  equivalent  of  equal  volumes  of  mercury  and  glass 
is  the  same.  This  follows  from  the  fact  that  the  product  of  the 
specific  heat  and  specific  gravity  of  those  substances  is  the  same: 
2.5X0.19-0.47  for  glass  and  13.6X0.034=0.46  for  mercury. 
That  is,  one  cubic  centimeter  of  either  mercury  or  glass  is  raised 
one  degree  by  .46-.4T  calories  of  heat.  The  volume  of  that  por- 
tion of  the  thermometer  which  is  immersed  in  the  water  is  ascer- 
tained by  immersing  the  thermometer  in  a  graduated  cylinder 
partially  filled  with  water  to  the  same  point  to  which  it  is  immersed 
when  used  in  the  calorimeter.  The  volume  of  the  portion  of 
the  thermometer  which  is  immersed  is  then  read  directly  on  the 
cylinder. 

604.  Accelerators. — The  use  of  sodium  peroxide  alone  has  not 
been  found  entirely  satisfactory.  The  addition  of  small  amounts 
of  substances  like  potassium  chlorate  or  a  persulphate  is  advan- 
tageous because  the  oxygen  in  these  compounds  is  more  easily 
liberated,  and  this  assists  in  starting  the  reaction  which  proceeds 
easily  when  the  temperature  is  raised  sufficiently  to  decompose  the 
sodium  peroxide.  The  same  object  is  attained  by  the  addition  of 
substances  which  are  more  readily  oxidized  than  the  coal  or  other 
substance  to  be  burned.  Tartaric  acid  is  such  a  substance. 
Ammonium  compounds  are  also  very  easily  oxidized  on  account  of 
the  large  amount  of  hydrogen  present.  Ammonium  persulphate 
has  been  found  to  be  admirably  adapted  for  this  purpose  because 

*  SPECIFIC  HEATS. 

Brass,        0.092-0.094  Nickel,        0.110 

Glass,        0.190  Silver,         0.057 

Mercury,  0.034  Platinum,  0.032 


402  CALORIFIC  VALUE  OF  FUELS. 

it  contains  both  the  ammonium  radical  and  also  the  easily  liberated 
oxygen.  The  term  accelerator  is  used  to  designate  this  class  of 
substances. 

605.  Correction  Factors. — A  correction  factor  must  be  intro- 
duced into  the  calculation  when  accelerators  are  used.  rii,u  heat 
of  formation  of  potassium  chlorate  is  —11.9  calories  for  the  gram 
molecular  weight  when  formed  from  KC1  and  1J  02,  while  the 
same  constant  for  sodium  peroxide  is  119.8  when  formed  from 
Na20  and  0.  When  ammonium  persulphate  is  used,  heat  is  evolved 
by  the  combustion  of  the  hydrogen.  The  correction  for  J  grain  of 
potassium  chlorate  is  —  .040°  C.,  and  for  J  gram  of  a  mixture  of 
one  part  of  ammonium  persulphate  and  two  parts  of  potassium 
persulphate,  —  .197°  C.  This  persulphate  mixture  has  been  found 
especially  advantageous  for  use  with  anthracite  coal. 

No  appreciable  oxidation  of  nitrogen  takes  place  in  the  Parr 
calorimeter.  A  correction  must  be  introduced  for  sulphur  and 
for  ash.  When  using  J  gram  of  coal,  these  corrections  are 
-  .006°  C.  for  each  per  cent  of  sulphur,  and  -  .001°  C.  for  each 
per  cent  of  ash.  A  correction  of  —.008°  C.  must  be  made  for 
each  length  of  10  mm.  of  fine  iron  wire. 

Another  correction  should  also  be  introduced  when  bitu- 
minous coals  are  burned,  on  account  of  the  oxygen  or  hydroxyl 
compounds  present.  No  such  correction  is  necessary  for  anthra- 
cite coals.  The  corrections  may  be  summarized  as  follows: 

Corrections  for  average  anthracite  coals,  using  J  gram  of 
coal,  10  grams  of  Na202,  and  i  gram  of  persulphate  mixture: 

\  gram  persulphate  mixture  .  0.197 

1  per  cent  of  sulphur 0-006 

7     "      "      "   ash 0.007 

Fuse  wire..  .  0.008 


Total 0.218°C., 

or  0.392°F. 


Corrections  for  average  semi-bituminous  coal  (not  over  25 
per  cent  volatile  matter),  using  J  gram  of  coal,  10  grams  of 
Na202,  and  J  gram  KC103: 


THE  PARR  CALORIMETER.  403 

|  gram  KC103 0 .040 

1  per  cent  sulphur 0-006 

7    "      "    ash 0.007 

4     "       "     combined  water.  ..  0-013 
Fuse  wire..  .  0-008 


Total 0  -074°  C., 

or  0.133°F. 

Corrections  for  average  bituminous  coal    (over  25  per  cent 
volatile  matter) : 

\  gram  KC103 0.040 

2  per  cent  sulphur 0 .012 

10     "       "     ash 0.010 

10     "       "     combined  water...  0.033 
Fuse  wire..  .  0.008 


Total 0 . 103°  C., 

or  0.185°F. 


EXERCISE   71. 

Determination  of  the  Calorific  Value  of  Coal  by  Means  of  the 
Parr  Calorimeter. 

606.  Electric  Ignition. — Grind  the  coal  fine  enough  to  pass  through  a 
sieve  of  .25  mm.  mesh  or  100  to  the  square  inch.  Weigh  out  exactly 
\  grain  and  dry  for  1  hour  in  an  air-bath  heated  to  104°-107°  C.  If  the 
coal  contains  less  than  2£  per  cent  of  water,  the  drying  is  unnecessary.  By 
adding  cold  or  hot  water,  bring  the  temperature  of  two  liters  of  water  to 
about  2°  F.  below  room  temperature.  Measure  out  exactly  two  liters  of 
this  water  and  place  in  the  calorimeter.  Transfer  one  full  measure  (10 
grams)  of  sodium  peroxide  to  the  cartridge,  which  must  be  absolutely  dry. 
Transfer  the  coal  to  the  cartridge,  brushing  off  the  watch-crystal  carefully. 
If  the  coal  is  anthracite,  add  exactly  \  gram  of  the  persulphate  mixture.  If 
a  bituminous  coal  is  being  tested,  add  exactly  |  gram  of  potassium  chlorate. 
Mix  the  contents  of  the  cartridge  thoroughly  by  stirring  with  a  rod  which  is 
afterwards  well  brushed  off.  The  more  recent  instruments  are  supplied  with 
a  false  cap  with  burnished  face,  which  is  fastened  temporarily  in  place,  and 
the  contents  of  the  cartridge  mixed  by  shaking.  After  thorough  mixing  in 
this  manner,  the  provisional  cap  is  removed  and  the  cap  with  stem  and 
terminals  substituted.  In  this  way  no  chemical  can  come  in  contact  with 


404  CALORIFIC   VALUE  OF  FUELS. 

the  base  of  the  terminals  and  produce  a  short  circuit.  The  cartridge  is  then 
tapped  lightly  to  shake  all  the  material  from  the  upper  part  of  the  cylinder 
and  to  settle  the  contents.  When  the  electric  method  of  ignition  is  used,  the 
fine  iron  wire  (34  American  gauge)  is  wrapped  firmly  with  good  contact  around 
the  ends  of  the  terminals,  so  as  to  leave  a  U-shaped  loop  extending  about 
three-quarters  of  an  inch  below  the  terminals  and  well  into  the  chemical  mix- 
ture. If  the  loop  is  too  long,  it  is  not  readily  fused.  The  proper  length  of 
wire  may  be  ascertained  by  a  preliminary  test  without  screwing  the  top  on  the 
cartridge.  The  current  required  is  from  2  to  4  amperes  and  is  readily  obtained 
by  placing  in  parallel  four  to  eight  16-candle-power  lamps  in  an  ordinary 
lighting  circuit  of  110  volts.  The  current  may  also  be  obtained  from  6 
to  8  dry  cells  connected  in  series.  The  adjustment  of  the  wire  must  be  made 
before  the  sodium  peroxide  is  placed  in  the  cartridge,  as  it  rapidly  absorbs 
moisture.  When  the  top  has  been  pressed  firmly  in  place  and  screwed 
down,  the  vanes  are  adjusted  and  the  cartridge  placed  in  the  can  of  water. 
The  cover  is  adjusted  and  the  thermometer  inserted  so  that  the  bulb  is 
about  half-way  down  in  the  water.  The  pulley  on  the  stern  is  connected 
with  the  motor  and  the  cartridge  rotated  to  the  right,  or  as  the  hands  of  a 
watch  turn,  so  as  to  deflect  the  water  downwards  in  the  can.  The  ther- 
mometer is  read  at  three-minute  intervals  until  the  readings  are  constant 
or  the  rise  in  temperature  is  constant.  These  readings  must  be  made  with 
the  greatest  care  and  tenths  of  the  smallest  divisions  estimated  as  care- 
fully as  possible. 

After  igniting  the  charge  by  bringing  the  spring  in  contact  with  the 
revolving  stem  of  the  cartridge,  the  temperature  rises  rapidly  at  first, 
and  then  more  slowly,  until  the  maximum  temperature  is  reached.  The 
readings  are  made  at  three-minute  intervals  as  before,  until  the  tempera- 
ture is  constant  or  it  begins  to  fall.  The  cartridge  is  removed  and  the 
parts  unscrewed  and  placed  in  a  dish  of  hot  water  to  dissolve  the  melt. 
It  is  then  rinsed  thoroughly  with  distilled  water  and  dried,  when  it  is 
ready  for  another  determination. 

607.  Ignition  by  Hot  Wire. — If  the  ignition  is  made  by  means  of  a  hot 
wire,  the  method  of  filling  the  cartridge  and  rotating  it  until  the  tempera- 
ture is  constant  is  the  same  as  when  the  electric  ignition  is  used.     The  short 
piece  of  soft  iron  wire  is  placed  on  a  wire  gauze  and  heated  to  redness  in  a 
Bunsen-burner  flame  and  dropped  quickly  into  the  opening  at  the  upper  end 
of  the  valve.     With  the  pincers  the  valve  is  now  pressed  completely  down 
and  released  with  a  quick  motion,  so  as  to  prevent  the  escape  of  heated  air 
from  the  interior.     During  this  operation  the  cartridge  is  kept  revolving. 
The  remainder  of  the  operation  is  the  same  as  when  the  electric  method  of 
ignition  is  used. 

608.  Calculation.— The  various  correction  factors,  as  given  on  page  402, 
are  subtracted  from  the  total  rise  in  temperature.     The  remainder  is  multi- 
plied by  3117.     This  factor  is  obtained  from  the  following  formula: 


CALCULATION.  405 

Total  water  eqv.X  0.73  X  Corrected  rise    Calorific  vaiue. 
Weight  of  fuel  taken 

Substituting  the  known  values, 

2135  X  0.73  Xr 
~05" 

which  reduces  to  3117Xr.    The  product  will  be  the  calorific  power  of  the 
coal  in  B.  T.  U.  per  pound  of  coal. 


CHAPTER  XXIX. 
WATER   ANALYSIS. 

A  CHEMICAL  analysis  of  water  is  carried  on  by  very  different 
methods,  depending  on  the  use  to  which  the  water  is  to  be  put. 
These  methods  may  conveniently  be  divided  into  two  classes.  In 
the  first  class  are  placed  those  methods  by  which  the  POTABILITY 
of  a  water  is  determined,  while  in  the  second  class  are  included 
the  methods  by  which  the  suitability  of  a  water  for  INDUSTRIAL 
PURPOSES  is  determined.  As  the  requirements  of  the  various 
industries  are  somewhat  diverse,  the  methods  used  in  testing  a 
water  for  industrial  purposes  cannot  be  so  definitely  stated  as  those 
employed  when  the  water  is  to  be  used  for  domestic  consumption. 

In  the  latter  case  no  poisonous  substances  may  be  present  even 
in  minute  traces.  The  absence  of  lead,  copper,  etc.,  being  insured, 
we  may  conveniently  classify  the  methods  commonly  used  into 
those  by  which  the  amount  of  ORGANIC  impurities  and  those  by 
which  the  amount  of  INORGANIC  impurities  are  determined.  As 
the  commonly  occurring  inorganic  salts,  such  as  sulphates,  chlorides, 
and  carbonates  of  lime,  magnesia,  the  alkalies,  iron,  etc.,  are  not 
objectionable  unless  present  in  excessive  amounts,  only  the  total 
amount  of  the  calcium  and  magnesium  salts  present  is  determined 
and  reported  as  hardness  of  the  water.  If  the  water  is  classed  as  a 
MINERAL  WATER,  and  is  therefore  to  be  used  because  of  the  inor- 
ganic salts  present,  a  complete  analysis  of  these  must  be  made,  as 
well  as  determinations  of  the  organic  impurities.  The  most 
important  determinations  for  indicating  the  presence  of  organic 
impurities  are  the  estimation  of  the  amount  of  nitrogen  in  its 
various  forms  and  chlorine. 

SANITARY  ANALYSIS. 

609.  Bacteriological  Examination. — As  has  been  said,  the  sani- 
tary analysis  consists  in  the  determination  of  the  organic  and 
the  inorganic  impurities  present.  The  most  serious  ill  effect  of 

406 


SANITARY  ANALYSIS.  407 

using  impure  water  undoubtedly  arises  from  taking  into  the 
system  DISEASE  GERMS  which  exist  in  the  water.  It  would  seem, 
therefore,  that  the  examination  of  the  water  should  primarily  be 
entrusted  to  the  bacteriologist.  Unfortunately,  it  is  extremely 
difficult,  if  not  impossible,  in  the  present  condition  of  bacterio- 
logical science,  to  detect  and  identify  the  specific  disease  germs 
even  if  they  undoubtedly  exist  in  a  given  water.  This  is  largely 
due  to  the  fact  that  the  great  majority  of  the  germs  that  exist  in 
water  are  either  harmless  or  even  beneficial.  The  continual  drink- 
ing of  a  water  containing  relatively  few  disease  germs  by  a  person 
in  whose  system  the  germ  can  live,  ultimately  results  in  the  ac- 
cumulation of  the  germs  in  sufficient  quantity  to  produce 
disease. 

The  routine  bacteriological  tests  generally  made  on  water 
consist  of  a  count  of  the  total  number  of  bacteria  present  and 
a  test  for  the  presence  or  absence  of  B.  coli.  These  bacteria  are 
almost  invariably  present  in  the  intestines  of  human  beings  and 
other  warm-blooded  animals.  Their  presence  in  water  is  regarded 
as  evidence  of  contamination  with  sewage,  which  almost  always 
contains  disease  germs  excreted  from  the  body,  thus  rendering 
the  water  unwholesome  and  dangerous.  The  presence  of  an  ex- 
cessivr  number  of  bacteria  in  water  also  renders  it  deleterious  to 
health. 

610.  Source  of  Impurities. — The  chemical  examination  gives 
indirect  evidence  of  the  presence  of  disease  germs  by  revealing 
the  presence  and  the  amount  of  organic  matter  in  the  water,  as 
well  as  whether  it  is  of  animal  or  vegetable  origin,  and  the  extent 
to  which  it  has  undergone  decomposition.  As  SEWAGE  is  undoubt- 
edly the  most  common  source  of  contamination  of  water  with 
pathogenic  germs,  the  evidence  furnished  by  the  chemist  of  its 
presence  is  quite  sufficient  to  condemn  a  water  for  household  use, 
even  if  the  specific  disease  germ  is  not  found.  If  the  water  is  liable 
to  be  contaminated  with  sewage,  it  is  almost  certain  that  sooner 
or  later  it  will  be  rendered  poisonous  by  the  presence  of  the  germs 
of  disease.  It  is  evident  that  after  organic  impurities  have  been 
found  in  a  water  it  is  important  to  know  their  source  before  a  deci- 
sion can  be  reached  as  to  the  potability  of  a  water.  Whenever 
possible,  therefore,  the  chemical  examination  should  be  supplemented 


408  WATER  ANALYSIS. 

by  an  examination  of  the  source  of  the  water-supply;  that  is,  whether 
it  is  a  spring,  lake,  river,  well,  etc.  If  surface  or  underground 
drainage  can  mingle  with  the  water,  the  entire  region  drained 
should  be  examined,  if  possible,  for  sources  of  contamination. 

611.  The  Samples  must  be  taken  with  the  greatest  care.    As  the 
impurities  at  best  are  present  in  very  small  amount,  careless  wash- 
ing of  the  bottle,  or  collecting  water  contaminated  after  leaving 
the  source  of  supply,  may  easily  introduce  as  much  impurity  as 
was  originally  present.     The  bottles  used  should  be  thoroughly 
cleaned  by  means  of  strong  dichromate  solution  and  then  thor- 
oughly rinsed  out  with  distilled  water  and  drained.     They  should 
then  be  rinsed  out  with  the  water  to  be  examined. 

If  water  from  a  CITY  SUPPLY  is  to  be  examined,  it  must  be 
allowed  to  flow  from  the  pipes  a  considerable  time  before  filling 
the  bottle,  unless  the  water  is  to  be  examined  for  lead  or  other 
contamination  from  the  pipes. 

If  a  WELL-WATER  is  to  be  examined,  the  stagnant  water  in  the 
pump  must  first  be  completely  removed. 

A  sample  of  SPRING,  RIVER,  or  LAKE  WATER  is  best  taken  by  allow- 
ing the  bottle  to  fill  after  immersion  some  distance  under  the  sur- 
face of  the  water,  so  as  to  avoid  the  floating  surface  contamination. 

The  TIME  which  may  elapse  between  the  collection  of  a  sample 
and  the  beginning  of  its  analysis  varies  with  the  character  of  the 
sample  and  other  conditions.  The  following  limits  are  generally 
safe :  for  fairly  pure  surface-water,  24  to  48  hours ;  and  for  normal 
ground-water,  48  to  72  hours.  Polluted  water  requires  analysis 
within  12  hours.  A  bacteriological  examination  should  in  all  cases 
be  made  within  6  hours.  If  for  any  reason  the  examination  is 
delayed,  the  sample  must  be  preserved  on  ice. 

PHYSICAL  EXAMINATION. 

The  physical  examination  includes  observations  of  the  tem- 
perature, general  appearance,  color,  turbidity,  and  the  odor  in  hot 
and  cold  samples. 

612.  The  Temperature  should  be  taken  at  the  time  of  collec- 
tion to  the  nearest  0.5°  Centigrade. 

613.  Turbidity. — The  general  appearance  of  the  water  should 
be  determined  by  inspection  in  strong  light  after  standing  several 


PHYSICAL  EXAMINATION.  409 

hours.  Substances  remaining  in  suspension  are  then  classified 
as  turbidity  on  standing,  and  substances  settling  to  the  bottom  as 
sediment.  Instead  of  expressing  the  turbidity  as  none,  slight,  dis- 
tinct, decided,  etc.,  it  is  advisable  to  use  the  NUMERICAL  STANDARDS 
recently  introduced  by  which  the  turbidity  is  expressed  in  parts 
of  silica  per  one  million.  The  stock  suspension  of  silica  is  prepared 
as  follows :  Diatomaceous  earth,  as  free  from  amorphous  silica  and 
sponge  spicules  as  may  be  obtained,  is  washed  with  water  to  free 
it  from  any  soluble  salts,  and  ignited  to  remove  any  organic  matter. 
It  is  then  treated  with  warm  dilute  hydrochloric  acid,  and  washed 
with  successive  portions  of  distilled  water  until  free  from  acid. 
The  material,  now  composed  of  practically  pure  diatomaceous 
frustules,  is  ground  to  an  impalpable  powder  in  an  agate  mortar, 
sifted  through  a  sieve  that  has  200  meshes  to  the  inch,  and  dried 
in  a  desiccator.  One  gram  of  this  prepared  diatomaceous  earth 
in  1  liter  of  distilled  water  gives  the  stock  suspension  which  has 
a  turbidity  of  1000,  i.e.,  contains  1000  parts  of  silica  por  million. 

STANDARDS  for  comparison  should  be  prepared  from  this 
stock  suspension  by  dilution  with  distilled  water.  For  turbidity 
readings  below  20,  standards  of  0,  5,  10,  15,  and  20  are  kept  in 
gallon  bottles  made  of  clear  white  glass;  for  readings  above  20, 
standards  of  20,  30,  40,  50,  60,  70,  80,  90,  and  100  are  kept  in  100- 
c.c.  Nessler  jars,  approximately  20  mm.  in  diameter. 

COMPARISON  OF  THE  WATER  under  examination  with  the  stand- 
ards is  made  by  viewing  them  sidewise  towards  the  light,  looking 
at  some  object  and  noting  the  distinctness  with  which  the  margins 
of  the  object  can  be  seen.  The  standards  are  kept  stoppered, 
and  both  sample  and  standards  are  thoroughly  shaken  before 
making  the  comparison.  In  order  to  prevent  any  bacterial  or 
alga3  growths  from  appearing  in  the  standards,  a  small  amount  of 
bichloride  of  mercury  may  be  added  to  them. 

614.  The  Color  of  the  water  is  measured  by  comparison  with 
the  color  of  a  solution  of  platinum  and  cobalt  and  is  expressed  in 
parts  of  platinum  per  million.  A  standard  solution  which  has  a 
color  of  500  is  made  by  dissolving  1.246  grams  potassium  platinic 
chloride  (PtCl4.2KCl),  containing  0.5  gram  platinum,  and  1  gram 
of  crystallized  cobaltous  chloride  (CoCl2.6H20),  containing  0.25 
gram  of  cobalt  in  water,  with  100  c.c.  concentrated  hydrochloric 


410  WATER  ANALYSIS. 

acid,  and  making  up  to  1  liter  with  distilled  water.  By  diluting 
this  solution  standards  are  prepared  having  colors  of  0,  5,  10,  15, 
20,  25,  30,  35,  40,  50,  60,  and  70  parts  per  million.  These  are  kept 
in  100-c.c.  Nessler  jars  of  such  dkmeter  that  the  liquid  has  a  depth 
between  20  and  25  cm.  and  is  protected  from  dust. 

The  color  of  a  sample  is  observed  by  filling  a  similar  tube  with 
the  water  and  comparing  it  with  the  standards.  The  observation 
is  made  by  looking  vertically  downwards  through  the  tubes  upon 
a  white  surface  placed  at  such,  an  angle  that  light  is  reflected 
upwards  through  the  column  of  liquid.  The  reading  is  recorded 
to  the  nearest  unit.  Waters  that  have  a  color  darker  than  70  are 
diluted  before  making  the  comparison,  in  order  that  no  difficulties 
may  be  encountered  in  matching  the  hues.  Water  containing 
matter  in  suspension  is  filtered  until  no  visible  turbidity  remains. 

615.  The  Odor  of  the  water  both  hot  and  cold  is  observed.  To 
obtain  the  odor  of  the  cold  water  the  sample  in  the  large  collecting- 
bottle,  which  should  be  about  one-half  or  two-thirds  full,  is  shaken 
vigorously,  the  stopper  removed,  and  the  nose  immediately  placed 
to  the  mouth  of  the  bottle.  To  obtain  the  odor  of  the  hot  water 
it  is  poured  into  a  moderately  large  beaker  until  it  is  about  one- 
third  full.  The  beaker  is  covered  with  a  watch-crystal  and  the 
water  is  rapidly  brought  to  a  boil.  It  is  immediately  removed 
from  the  source  of  heat  and  allowed  to  cool  for  about  five  minutes. 
The  beaker  is  then  shaken  with  a  rotary  motion,  the  watch-crystal 
slipped  to  one  side,  and  the  odor  observed.  The  odor  is  expressed 
by  such  terms  as  the  following:  vegetable,  aromatic,  grassy,  fishy, 
earthy,  mouldy,  musty,  disagreeable,  peaty,  and  sweetish.  The 
intensity  of  the  odor  is  usually  indicated  by  the  numbers  1,  2,  3,  4, 
and  5,  1  indicating  very  faint;  2,  faint;  3,  distinct;  4,  decided; 
and  5,  strong. 

CHEMICAL  EXAMINATION. 

The  first  determination  carried  out  should  be  that  of  the 
amount  of  nitrogenous  organic  matter  present,  as  this  material  is 
continually  undergoing  changes,  and  the  condition  in  which  the 
nitrogen  exists  is  taken  as  an  index  of  the  source  of  the  contamina- 
tion, and  the  time  which  has  elapsed  since  it  has  been  introduced. 


NITROGEN.  411 

616.  The  Nitrogen  may  exist  as  so-called  ALBUMINOID  NITRO- 
GEN, AMMONIA,  NITRITES,  or  NITRATES.    The  albuminoid  nitrogen 
is  undoubtedly  present  as  part  of  a  class  of  complex  organic  com- 
pounds, called  proteins,  which  may  have  been  derived  from  plant, 
but  more  likely  animal,  tissues  which  are  present  in  the  water. 
These  tissues  undergo  a  more   or  less  rapid  decomposition   or 
putrefaction  by  which  the  nitrogen  assumes  for  the  most  part  the 
condition  of  FREE  AMMONIA,  which  may  then  be  oxidized  to  nitrites 
or  nitrates.    The  presence  of  the  unstable  nitrites  generally  indi- 
cates the  presence  of  sewage  or  other  organic  matter  in  a  state  of 
decomposition.    While  nitrates  may  be  produced  by  the  oxidation 
of  organic  nitrogen,  this  is  by  no  means  the  only  source,  since  the 
atmosphere  and  the  soil  contain  considerable  amounts  of  nitric 
acid  or  nitrates,  which  are  dissolved  by  the  ram.    The  amount  of 
nitrates  in  a  surface-water  increases  with  the  density  of   popula- 
tion in  the  region  drained. 

617.  Determination    of    Free    and    Albuminoid    Ammonia.  — 
Ammonia  is  determined  by  means  of  the  so-called  NESSLER  SOLU- 
TION, to  which  a  brown  color  is  given  by  very  small  amounts  of 
ammonia  or  ammonium  salts.     The  reagent  consists  of  a  strongly 
alkaline  solution  of  the  double  iodide  of  potassium  and  mercury, 
HgI2.2KI.     Ammonia  produces  in  this  solution  a  dark-brown  col- 
oration or  precipitate  of  dimercuric  ammonium  iodide,  NHg2I.H20, 
according  to  the  following  equation: 

2(HgI2.2KI)  +  NH3+ 3KOH  =  NHg2I.H20  +  7KI + 2H20. 

The  depth  of  the  color  produced  is  proportional  to  the  amount 
of  ammonia  present,  so  that  by  comparison  with  solutions  con- 
taining known  amounts  of  ammonia,  the  amount  of  ammonia 
present  in  a  given  solution  may  be  determined,  the  solution  con- 
taining the  known  amount  of  ammonia  being  so  adjusted  that 
its  color  exactly  matches  the  solution  containing  the  unknown 
amount. 

618.  Preparation  of  Nessler  Solution.* — Dissolve  61.75  grams 
of  potassium  iodide  in  250  c.c.  of  ammonia-free  water  and  add  a 
cold  solution  of  mercuric  chloride  which  has  been  saturated  by 

*  D.  D.  Jackson,  Tech.  Quar.,  XIII,  No.  4,  Dec.  1900,  p.  320. 


412  WATER  ANALYSIS. 

boiling  with  excess  of  the  salt.  Pour  in  the  mercury  solution 
cautiously  and  add  an  amount  just  sufficient  to  make  the  color  a 
permanent  bright  red,  which  will  require  about  400  c.c.  Dissolve 
the  red  precipitate  by  adding  exactly  .75  gram  potassium  iodide. 
Then  add  150  grams  of  potassium  hydrate  dissolved  in  250  c.c. 
of  water  and  dilute  the  solution  to  1  liter.  Mix  thoroughly  and 
allow  the  solution  to  stand  until  any  precipitate  formed  has  settled, 
leaving  a  pale  straw-colored  solution  which  is  siphoned  or  decanted 
off  into  another  bottle  for  use.  The  solution  improves  with  age1. 
Its  sensitiveness  is  increased  by  adding  mercuric  chloride  and 
decreased  by  adding  potassium  iodide.  The  2-c.c.  portions  used 
for  each  test  must  be  measured  quite  carefully  as  the  depth  of 
color  produced  with  a  given  amount  of  ammonia  depends  to  a 
certain  extent  upon  the  amount  of  Nessler  solution  used. 
The  following  solutions  will  also  be  required : 

619.  Alkaline    Potassium    Permanganate    Solution.  —  Dissolve 
200  grams  of  solid  caustic  potash*  and  8  grams  of  potassium 
permanganate  in  1250  c.c.  of  distilled  water.     Boil  the  solution 
down  to  1  liter  and  preserve  for  use. 

620.  Sodium   Carbonate    Solution. — Dissolve  50  grams  of  the 
pure  salt  in  300  c.c.  of  ammonia-free  water. 

621.  Standard  Ammonia   Solution. — Dissolve  1.5701  grams  of 
pure  dry  ammonium  chloride  in  ammonia-free  water  and  dilute  to 
500  c.c.     1  c.c.   of  this  solution  will  contain  1  mg.  of  ammonia. 
10  c.c.  should  be  diluted  to  1  liter  with  ammonia-free  water.     1  c.c. 
of  this  solution  will  contain  .01  mg.  of  ammonia  and  is  the  stand- 
ard solution  used. 

622.  Ammonia-free  Water. — Ordinary  distilled    water    gener- 
ally  contains   considerable   amounts   of   ammonia.     It   may   be 
tested  by  adding  2  c.c.  of  Nessler  solution  to  50  c.c.  of  the  water 
placed  in  a  NESSLER  TUBE.     These  tubes  are  made  of  colorless  glass 
and  contain  50-c.c.  when  filled,  to  a  mark  near  the  top.     A  tube  of 
suitable  proportions  has  a  diameter  of  1.7  cm.  and  a  height  of 
21  cm.  to  the  50-c.c.  mark.     If  after  standing  five  minutes  no  trace 
of  a  yellow  tint  is  developed  in  the  water  it  may  be  used  without 
purification.     Generally,  however,  it  must  be  freed  from  ammonia. 

*  If  caustic  potash  free  ir^m  carbonates  is  used,  the  bumping  so  apt  to  occur 
while  boiling  an  alkaline  solution  will  be  very  much  reduced. 


NITROGEN. 


413 


This  may  be  done  by  nearly  filling  a  large  glass  retort  or  a  suitable 
copper  or  tin  vessel  with  distilled  water  to  which  some  of  the  alka- 
line potassium  permanganate  solution  has  been  added.  A  glass 
or  block-tin  condenser  is  attached  and  the  water  is  distilled  until 
50-c.c.  portions  tested  with  the  Nessler  solution  are  found  free 
from  ammonia.  The  distillate  is  then  collected  in  a  clean  bottle 
for  use. 

The  distilled  water  may  also  be  freed  from  ammonia  by  adding 
enough  bromine  water  to  color  it  distinctly  and  then  boiling  until 
the  excess  of  bromine  is  expelled.  The  boiling  may  be  omitted 
if  a  drop  of  caustic  soda  is  added  to  the  water  tinted  with  bromine, 
and  after  standing  ten  minutes  the  undecomposed  hypobromite 
removed  by  adding  a  little  potassium  iodide. 

The  preparation  of  the  ammonia-free  water  and  all  ammonia 
determinations  must  be  carried  out  in  a  room  whose  atmosphere 
is  free  from  ammonia  and  ammonium  salts.  The  ordinary  labora- 
tory is,  therefore,  not  suited  to  the  purpose. 


DETERMINATION  OF  FREE  AMMONIA. 

623.  Distillation. — A  liter  fiask  is  connected  by  means  of  a 
short  piece  of  rubber  tubing  or  a  rubber  stopper  with  a  Liebig 
condenser.  A  tubulated  glass  retort  capable  of  holding  a  liter  of 
solution  is  also  very  convenient  for  this  purpose,  as  the  reagent 
can  be  introduced  through  the  tubulure.  The  stem  of  the  retort 
may  be  introduced  into  the  end  of  the  condenser-tube  and  a  tight 
joint  made  by  means  of  a  short  piece  of  rubber  tubing.  The  dis- 
tillate is  led  into  the  Nessler  tubes  by  means  of  an  adapter. 

Although  the  apparatus  is  thoroughly  cleaned  before  being  set 
up,  to  completely  remove  ammonia  about  200  c.c.  of  ammonia- 
free  water  is  introduced  into  the  retort  and  10  c.c.  cf  the  sodium 
carbonate  solution  added.  The  solution  is  boiled  and  the  first 
50  c.c.  of  the  distillate  discarded  while  the  second  50-c.c.  portion 
is  tested  for  ammonia  by  adding  2  c.c.  of  the  Nessler  solution.  If 
no  color  develops  after  standing  five  minutes  the  apparatus  may 
be  considered  free  from  ammonia  and  the  water  to  be  analyzed 
may  be  introduced  into  the  retort.  Unless  the  water  contains 
an  unusual  amount  of  ammonia  500  c.c.  will  be  found  to  be  a 


414 


WATER  ANALYSIS. 


convenient  amount.  A  preliminary  test  may  be  made  by  adding 
2  c.c.  of  Nessler  solution  to  50  c.c.  of  the  water  to  be  tested.  The 
color  developed  is  compared  with  the  color  produced  by  adding 
0.5  c.c.  of  the  standard  ammonia  solution  to  50  c.c.  of  ammonia- 
free  water.  If  the  color  produced  in  the  tube  containing  the  water 
to  be  analyzed  is  the  darker,  less  than  500  c.c.  of  the  water  should 
be  placed  in  the  retort  and  the  difference  made  up  by  ammonia- 
free  water. 

624.  Nesslerizing. — The  distillate  is  collected  in  the  Nesslerizing 
tubes,  which  are  placed  in  a  rack  in  order  after  being  filled  to  the 
50-c.c.  mark.  The  flame  of  the  Bunsen  burner  should  be  so 
adjusted  that  50  c.c.  are  distilled  in  about  eight  minutes.  As  soon 


FIG.  65. 

as  the  second  tube  is  filled  and  has  come  to  room  temperature  it 
is  Nesslerized.  2  c.c.  of  the  Nessler  solution  is  added.  Several 
clean  Nessler  tubes  are  taken  and  carefully  measured  amounts  of 
the  standard  ammonia  solution  added  from  a  burette,  such  as 
1  c.c.,  2  c.c.,  3  c.c.,  etc.  The  tubes  are  then  filled  with  ammonia- 
free  water  to  the  50-c.c.  mark,  and  2  c.c.  of  the  Nessler  solution 
added  to  each  tube.  After  a  little  practice  the  solutions  may 
readily  be  mixed  by  shaking  with  the  hand.  Long  clean  glass  rods 
may  also  be  used  for  stirring  the  contents  of  the  tubes. 

The  colors  are  compared  by  holding  two  tubes  in  the  hand 
and  looking  through  the  length  of  the  tubes,  which  are  held  over  a 
white  paper  or  piece  of  porcelain  in  a  good  light.  If  the  color  of  a 


FREE  AMMONIA.  415 

tube  does  not  match  that  of  any  of  the  standards  made,  others  must 
be  made  with  the  amount  of  standard  ammonia  solution  judged 
necessary  until  the  color  of  the  unknown  ammonia-tubes  have 
been  matched.  After  adding  the  Nessler  solution,  five  minutes 
must  elapse  before  comparing  the  color,  as  it  does  not  develop 
instantly.  After  being  fully  formed  the  color  is  quite  permanent, 
so  that  a  set  of  standard  tubes  may  be  used  during  a  day's  work 
without  error.  A  permanent  set  of  standard  tubes  has  been 
devised  by  D.  D.  Jackson.*  The  temperature  of  the  water  may 
vary  between  18°  and  25°  without  material  change  in  depth  of  color. 

If  the  ammonia  in  the  second  tube  corresponds  to  1  c.c.  or  less 
of  the  standard  solution,  further  distillation  may  be  dispensed  with. 
Otherwise  50-c.c.  portions  must  be  collected  until  this  limit  is 
reached.  Usually  three  50-c.c.  portions  are  collected.  If  the 
ammonia  in  the  second  tube  corresponds  to  2  c.c.  or  less  of  the 
standard  solution,  the  first  tube  may  be  Nesslerized  in  the  same 
manner  as  the  second.  If  the  second  tube  contains  a  greater 
amount  of  ammonia,  a  measured  portion  of  the  first  tube  must  be 
taken  and  diluted  with  ammonia-free  water  and  Nesslerized.  Or 
the  contents  of  the  first  and  third  tubes  may  be  mixed  and  50  c.c. 
of  the  mixture  Nesslerized.  In  no  case  should  the  color  of  a  tube 
be  read  which  responds  to  more  than  10  c.c.  of  the  standard  ammo- 
nia solution. 

625.  Calculation. — The  various  amounts  of  ammonia  found  in 
the  different  tubes  are  added  and  the  result  stated  in  parts  per 
million.  If,  for  instance,  the  first  tube  was  found  to  match  a  tube 
containing  9  c.c.  of  standard  ammonia  solution,  the  second  3  c.c., 
the  third  1  c.c.,  and  the  fourth  free  from  ammonia,  then,  as  1  c.c. 
is  equal  to  .01  mg.  of  ammonia,  the  total  amount  of  ammonia 
would  be  .09+. 03+. 01  =  .13  mg.  As  500  c.c.  of  the  water  was 
taken  and  as  1  mg.  is  the  millionth  part  by  weight  of  a  liter 
of  water,  the  amount  of  free  ammonia  present  in  the  water  would 
be  .26  parts  per  million. 

*Tech.  Quar.,  Vol.  XIII,  No.  4,  Dec.  1900,  p.  320. 


416  WATER  ANALYSIS. 


DETERMINATION  OF  ALBUMINOID  AMMONIA. 

626.  Distillation. — When  all  the  free  ammonia  has  been  dis- 
tilled off,  which  is  usually  the  case  when  150  c.c.  of  distillate  has 
been  collected,  50  c.c.  of  the  alkaline  permanganate  solution  is 
added  and  the  distillation  continued,  50-c.c.  portions  being  col- 
lected in  the  Nessler  tubes  and  the  ammonia  determined  and  cal- 
culated exactly  as  given  for  free  ammonia. 

The  albuminoid  ammonia  does  not  always  come  off  as  readily 
as  the  free  ammonia.  This  is  undoubtedly  due  to  the  greater  or 
less  difficulty  of  decomposing  organic  matter  from  different  sources. 
Organic  matter  derived  from  vegetables  is  generally  decomposed 
with  more  difficulty  than  the  contamination  derived  from  partially 
decomposed  animal  refuse  as  found  in  sewage.  Each  50-c.c.  por- 
tion of  the  distillate  for  albuminoid  ammonia  must  therefore  be 
tested  to  be  sure  that  all  of  the  ammonia  has  been  obtained,  the 
solution  in  the  retort  being  boiled  nearly  dry.  As  it  is  almost 
impossible  to  obtain  all  of  the  albuminoid  ammonia  from  many 
waters,  especially  highly  colored  surface-waters,  the  practice  is 
growing  of  collecting  five  50-c.c.  portions  of  the  distillate  for  the 
albuminoid  ammonia.  In  this  way  comparative  results  are  ob- 
tained which  are  of  almost  as  great  value  as  if  the  total  amount 
of  albuminoid  ammonia  were  obtained  in  each  case.  Much  time 
and  labor  are  also  saved.  About  one-half  of  the  total  organic 
nitrogen  will  generally  be  obtained  in  the  250  c.c.  of  distillate. 


DETERMINATION  OF  NITRITES. 

A  colorimetric  process  similar  to  that  used  to  determine 
ammonia  is  in  use  for  the  estimation  of  the  amount  of  nitrites 
present.  Of  the  several  methods  which  have  been  suggested, 
that  of  GRIESS  will  be  given.  It  depends  on  the  fact  that  a  red 
coloration  is  produced  when  a  solution  of  SULPHANILIC  ACID  and 
NAPHTHYLAMINE  HYDROCHLORiDE  are  added  to  an  acidified  solu- 
tion of  a  nitrite.  This  test  is  said  to  be  capable  of  showing  one 
part  of  nitrogen  as  nitrite  in  one  thousand  million  parts  of  water. 


NITRITES.  417 

The  following  reagents  must  be  prepared: 

627.  Sulphanilic  Acid. — Dissolve  0.8  gram  of  the  acid  in  100  c.c. 
of  hot  water. 

628.  Naphthylamine  Hydrochloride. — Dissolve  0.8  gram  of  the 
salt  in  100  c.c.  of  water  and  1  c.c.  of  strong  hydrochloric  acid. 
Decolorize   the   solution  by  adding  a  little   powdered   charcoal. 
Place  the  solution  in  a  glass-stoppered  bottle  with  the  charcoal 
and  filter  as  required  for  use. 

629.  Standard  Solution  of  Sodium  Nitrite. — Pure  silver  nitrite 
is  prepared  by  mixing  a  warm  concentrated  solution  of  8  parts  of 
sodium  nitrite  with  a  warm  concentrated  solution  of  16  parts  of 
silver  nitrate.     The  precipitate,  when  cold,  is  washed  with  cold 
water  and  quickly  dried  over  a  water-bath  with  as  little  exposure 
to  the  light  as  possible.     Weigh  .1091  gram  of  the  dry  nitrite  and 
dissolve   in  warm  water.     Add  a  slight  excess  of  pure  sodium 
chloride  and  dilute  to  1  liter.     After  allowing  the  silver  chloride 
to  settle,  draw  out  10  c.c.  and  dilute  to  1  liter.    This  will  give  a 
standard  solution,  1  c.c.  of  which  will  be  equal  to  .001  mg.  of 
nitrogen  as  nitrite 

630.  The  Determination  is  carried  out  as  follows:  100  c.c.  of 
the  water  to  be  examined  are  placed  in  a  cylinder  capable  of  hold- 
ing 100  c.c.,  which  is  made  of  the  same  kind  of  colorless  glass  as 
the  Nessler  tube,  although  a  shorter  and  wider  tube  is  found  con- 
venient, a  suitable  size  being  3  cm.  in  diameter  and  13.2  cm.  from 
the  bottom  to  the  100-c.c.  mark.  One  drop  of  concentrated  hydro- 
chloric acid  is  added  and  then  2  c.c.  of  the  sulphanilic  acid  solution 
and  2  c.c.  of  the  solution  of  hydrochloride  of  naphthylamine. 
After  mixing  well,  the  tube  is  tightly  stoppered  and  allowed  to 
stand  for  ten  minutes  until  the  color  is  fully  developed.  In  the 
meantime  tubes  are  prepared  containing  known  amounts  of 
nitrite,  1  c.c.,  2  c.c.,  3  c.c.,  etc.,  of  the  standard  nitrite  solution 
being  added  to  tubes  containing  100  c.c.  of  ordinary  distilled 
water,  or,  still  better,  water  redistilled  as  directed  for  ammonia- 
free  water.  After  the  addition  of  2  c.c.  of  each  reagent  to  each  of 
the  color  solutions  and  a  drop  of  hydrochloric  acid,  and  allowing 
the  tubes  to  stand  for  twenty  minutes,  the  colors  of  the  known  and 
the  unknown  solutions  are  compared  and  the  amount  of  nitrogen 
present  per  million  parts  is  computed.  If  100  c.c.  of  the  water 


418  WATER  ANALYSIS. 

is  taken  and  the  color  is  matched  by  the  tube  containing  3  c.c.  of 
the  standard  nitrite  solution,  the  water  contains  .03  part  per 
million  of  nitrogen  as  nitrite.  As  this  test  is  extremely  delicate, 
it  is  advisable  to  make  a  blank  determination  to  detect  accidental 
contamination  which  at  times  arises  from  nitrous  fumes  in  the  air. 


DETERMINATION  OF  NITRATES. 

This  determination  also  is  carried  out  by  a  colorimetric 
process,  the  yellow  AMMONIUM  SALT  OF  NITROPHENOL-SULPHONJC 
ACID  *  being  produced  when  the  water  to  be  examined  is  evap- 
orated to  dryness  and  treated  with  PHENOL-SULPHONIC  ACID  and 
ammonia.  The  following  reactions  take  place: 

C6H4.OH.S03H+ HN03  =  C6H3.OH.S03H.N02+ H20 ; 
C6H,.OH.SO,H.N08+  NH4OH  =  C6H3.ONH4.S03H.N02+ H20. 

The  intensity  of  the  color  developed  is  compared  with  the  color 
produced  by  the  same  process  in  water  containing  a  known  amount 
of  nitrate. 

The  following  solutions  are  required: 

631.  Phenol-sulphonic  Acid. — In  148  c.c.  of  pure  concentrated 
sulphuric  acid  24  grams  of  pure  phenol  are  dissolved  by  heating 
on  the  water-bath  and  12  c.c.  of  distilled  water  added. 

632.  Standard  Potassium  Nitrate  Solution. — 0.722  gram  of  pure 
dry  potassium  nitrate  is  weighed  out  and  dissolved  in  1  liter  of 
distilled  water.     10  c.c.  of  this  solution  are  evaporated  to  dryness, 
2  c.c.  of  the  phenol-sulphonic  acid  added,  and  quickly  and  thor- 
oughly mixed  with  the  residue  by  means  of  a  glass  rod.      The 
mixture  is  dissolved  in  distilled  water  and  the  solution  diluted  to 
1  liter. 

The  color-tubes  for  comparison  are  prepared  by  measuring  out 
portions  of  this  solution  into  100-c.c.  tubes  of  the  size  used  for 
the  nitrite  determination,  adding  5  c.c.  of  strong  ammonia  and 
diluting  to  100  c.c.  These  standards  are  permanent  for  several 
months  if  the  tubes  are  well  corked  with  stoppers  from  which  the 
coloring  matter  has  been  extracted  by  boiling  water.  1  c.c.  of  the 
dilute  nitrate  solution  is  equal  to  .001  mg.  of  nitrogen  as  nitrate. 

*  Jour.  Am.  Chem.  Soc.,  XXVI,  388. 


NITRATES.  419 

633.  The  Determination  is  carried  out  as  follows:  25  c.c.  of  the 
water  to  be  analyzed  are  taken,  or,  if  the  amount  of  nitrate  present 
is  small,  50  or  even  100  c.c.  are  taken  and  evaporated  to  dryness. 
The  residue  is  treated  with  1  c.c.  of  the  phenol-sulphonic  acid  and 
well  mixed  by  stirring  with  a  glass  rod.     10  c.c.  of  distilled  water 
are  added  and  then  5  c.c.  of  strong  ammonia.     The  solution  is 
washed  into  a  100-c.c.  tube  and  diluted  to  the  mark  with  distilled 
water.     The  depth  of  color  is  now  compared  with  that  of  the  stand- 
ard color-tubes  containing  various  amounts  of  the  standard  nitrate 
solution. 

When  the  color  has  been  exactly  matched,  the  parts  per  mil- 
lion of  nitrogen  as  nitrate  are  calculated.  If,  for  instance,  50  c.c. 
of  the  water  were  taken  and  the  color  was  matched  by  the  standard 
color-tube  containing  3  c.c.  of  the  dilute  nitrate  solution,  then  50 
c.c.  of  the  water  would  contain  .003  mg.  of  nitrogen  as  nitrate, 
which  would  be  .06  part  per  million.  If  25  c.c.  of  a  water  had  been 
taken,  the  result  would  be  .12  part  per  million  if  the  color  produced 
were  matched  by  a  color-tube  containing  3  c.c.  of  the  standard 
nitrate  solution. 

DETERMINATION  OF  THE  OXYGEN  REQUIRED  TO 
OXIDIZE  ORGANIC  MATTER. 

The  methods  already  given  have  shown  the  amount  of  nitro- 
gen present  in  a  given  water  in  the  four  forms:  ammonia, 
nitrous  and  nitric  acid,  and  albuminoid  substances.  A  very 
simple  method  is  in  use  for  the  determination  of  the  amount  of 
carbon  present.  It  consists  in  ascertaining  the  amount  of  per- 
manganate required  to  oxidize  the  organic  matter  present.  The 
water  is  boiled  for  ten  minutes  with  sulphuric  acid  and  an  excess 
of  standard  potassium  permanganate  solution.  The  excess  of  the 
permanganate  is  then  titrated  back  with  standard  oxalic  acid. 

634.  Errors. — It  is  evident  that  by  this  process  any  substance 
other  than  carbonaceous  material  capable  of  being  oxidized  under 
the  conditions  will  also  reduce  the  permanganate.     On  the  other 
hand,  the  oxidation  of  the  organic  matter  present  may  not  be 
complete.      It  has  indeed  been  shown  that  the  extent  to  which 
different  kinds  of  organic  matter  are  oxidized  varies  greatly.     As 
the  method,  therefore,  is  an  empirical  one,  the  conditions  of  the 
oxidation  must  be  rigidly  adhered  to.     If  this  is  done,  very  con- 


420  WATER  ANALYSIS. 

stant  results  are  obtained  and  the  method  has  shown  itself  of 
value  in  forming  a  judgment  of  the  amount  of  organic  matter  in 
the  water,  as  other  oxidizable  substances  are  rarely  present. 
Ferrous  iron  and  sodium  chloride  are  the  only  common  excep- 
tions. The  iron  is  quickly  precipitated  as  ferric  oxide  after  a  few 
hours'  exposure  of  the  water  to  the  air. 

If  a  considerable  amount  of  SODIUM  CHLORIDE  is  present,  a  cor- 
rection must  be  applied  as  found  by  a  blank  test.  Sodium  chloride 
is  added  to  distilled  water  until  there  is  present  the  amount  of 
chloride  which  has  been  found  in  the  water  being  examined.  The 
oxidation  with  the  permanganate  is  then  carried  out  in  the  same 
manner  as  when  the  water  was  tested.  The  amount  of  perman- 
ganate required  is  subtracted  from  that  used  in  testing  the  water. 

635.  The  Following  Solutions  are  required  and  are  made  with 
water  free  from  organic  matter.     This  is  prepared  by  adding  a 
little  potassium  permanganate  and  caustic  alkali  to  distilled  water 
and  redistilling  it.     The  first  portions  of  the  distillate  are  discarded. 
The  steam  or  distilled  water  should  not  come  in  contact  with  rubber 
or  other  organic  matter.     A  suitable  distilling  apparatus  may  be 
made  by  connecting  a  retort  with  a  Liebig  condenser  by  inserting 
the  tube  of  the  retort  into  the  inner  tube  of  the  condenser  as 
shown  in  Fig.  65,  page  414,  and  packing  the  joint  with  ignited 
asbestos  instead  of  the  rubber  connection. 

636.  Potassium  Permanganate  Solution  is  made  by  dissolving 
.3953  gram  of  the  pure  salt  in  1  liter  of  the  redistilled  water.     1  c.c. 
of  this  solution  will  contain  0.1  mg.  of  available  oxygen. 

637.  Oxalic  Acid  Solution  is   made  by  dissolving  .7875  gram 
of  the  pure  recrystallized  and  carefully  dried  salt  in  1  liter  of  water. 
If  this  salt  as  well  as  the  permanganate  are  pure,  these  solutions 
will  be  exactly  equal.     They  should  be  titrated  against  each  other 
by  measuring  out  10  c.c.  of  the  oxalic  acid,  adding  10  c.c.  of  dilute 
sulphuric  acid  and  200  c.c.  of  the  redistilled  water.     The  whole 
is  heated  to  boiling  and  the  potassium  permanganate  solution 
introduced  until  a  faint,  permanent  pink  coloration  is  produced. 
Neither  the  oxalic  acid  nor  the  permanganate  solutions  are  perma- 
nent and  must  be  made  up  fresh  or  standardized  from  time  to  time. 

Dilute  sulphuric  acid  is  made  by  diluting  1  part  of  concentrated 
acid  with  3  parts  of  the  redistilled  water. 


ORGANIC  MATTER.  421 

638.  The  Determination  is  carried  out  by  placing  100  c.c.  of 
the  water  to  be  tested  in  a  flask  and  adding  10  c.c.  of  the  dilute 
sulphuric  acid.  The  standard  potassium  permanganate  solution 
is  introduced  from  a  burette  until  the  water  is  colored.  It  is 
heated  to  boiling  and  more  permanganate  is  introduced  if  the 
color  fades.  If  brown  flakes  of  manganese  dioxide  are  formed, 
more  dilute  sulphuric  acid  must  be  added.  The  boiling  is  continued 
for  five  minutes.  An  excess  of  oxalic  acid  is  then  introduced  and 
titrated  back  with  the  permanganate  solution.  The  volume  of 
oxalic  acid  introduced  is  subtracted  from  the  total  amount  of  the 
permanganate  solution  used,  and  the  number  of  milligrams  of  oxy- 
gen consumed  is  calculated. 

The  result  is  stated  in  parts  of  oxygen  required  per  million 
parts  of  water. 

If  the  amount  of  organic  matter  in  the  water  is  considerable, 
less  than  100  c.c.  must  be  taken  for  this  determination.  The 
volume  should  in  each  case  be  made  up  to  100  c.c.  with  the  redis- 
tilled water.  As  the  success  of  this  determination  is  assured  only 
by  working  under  absolutely  fixed  conditions,  the  determination 
should  be  repeated  several  times  until  constant  results  are  obtained. 


DETERMINATION  OF  CHLORIDES. 

The  chlorine  in  natural  waters  exists  for  the  most  part  in 
the  form  of  sodium  chloride,  and  as  such  is  harmless  unless  present 
in  very  large  amounts.  As  considerable  amounts  of  salt  are  used 
very  generally  in  articles  of  food,  it  is  present  in  large  amounts  in 
urine,  and  therefore  in  sewage.  For  this  reason  the  amount  of 
chlorides  present  is  used  as  an  index  of  the  amount  of  contamina- 
tion of  the  water.  As  almost  all  natural  waters,  including  rain- 
water, contain  chlorides  in  greater  or  less  amount,  only  that  amount 
of  chlorides  which  is  above  the  average  of  similar  but  pure  waters 
can  be  used  as  an  argument  for  condemning  a  water  as  contami- 
nated. 

639.  Silver  Nitrate  Solution. — The  chlorine  is  determined  volu- 
metrically,  using  a  standard  silver  nitrate  solution  with  potassium 
chroma te  as  indicator,  as  described  in  Chapter  XXVI.  The  silver 


422  WATER  ANALYSIS. 

nitrate  solution  is  made  of  such  a  strength  that  1  c.c.  is  equal  to 
1  mg.  of  chlorine.  For  this  purpose  4.7938  grams  of  pure  silver 
nitrate  are  weighed  out  and  dissolved  in  a  liter  of  water.  The 
potassium  chromate  indicator  is  made  by  dissolving  5  grams  of  the 
pure  salt  in  100  c.c.  of  distilled  water. 

640.  The  Determination  is  carried  out  by  placing  100  c.c.  of  the 
water  to  be  examined  in  a  porcelain  dish  and  adding  1  c.c.  of  the 
indicator.     The  silver  nitrate  solution  is  added  until  a  faint  per- 
manent red  color  is  produced.     This  red  color  should  be  removed 
by  adding  a  pinch  of  sodium  chloride  or  a  drop  or  two  of  a  chloride 
solution  and  the  titration  repeated,  using  the  first  solution  for  com- 
parison.    This  solution  will  have  a  pure-yellow  color  with  a  white 
silver  chloride  precipitate  equal  to  that  obtained  in  the  second 
titration.     The  end-point  may  be  made  still  sharper  by  preparing 
a  solution  of  distilled  water  containing  1  c.c.  of  the  indicator, 
through  which  the  solution  being  titrated  may  be  observed. 

As  an  appreciable  amount  of  the  silver  nitrate  solution  may  be 
required  to  produce  the  color  indicating  the  end-point,  a  blank 
determination  should  be  made  by  adding  1  c.c.  of  the^ indicator  to 
100  c.c.  of  distilled  water  and  then  adding  silver  nitrate  solution 
until  the  red  color  is  observed.  The  volume  of  silver  nitrate  added 
should  be  subtracted  from  the  amount  added  to  the  water  being 
tested. 

When  the  amount  of  chlorides  present  is  very  small,  or  great 
accuracy  is  required,  an  amount  of  water  greater  than  100  c.c. 
should  be  taken  and  evaporated  down  to  a  bulk  of  about  100  c.c. 
after  the  addition  of  0.1  c.c.  of  a  saturated  solution  of  sodium 
carbonate.  This  solution  is  then  titrated  as  already  given.  If 
the  water  contains  considerable  coloring-matter,  so  that  after 
concentration  the  color  of  the  indicator  cannot  be  observed,  the 
coloring-matter  may  be  removed  before  concentration  by  shaking 
the  water  with  some  freshly  precipitated  aluminum  hydrate  and 
filtering  before  measuring  out  the  portion  for  evaporation.  If 
the  water  is  acid,  it  must  be  neutralized  with  sodium  carbonate. 

641.  The  Determination  of  Hardness  in  water,  both  temporary 
and  permanent,  has  already  been  given  in  Chapter  XXI,  page 
269. 

642.  Determination   of   Total   Solids. — This   determination   is 
conducted  by  evaporating  a  measured  volume  of  the  water,  100 


TOTAL  SOLIDS.  423 

to  500  c.c.  being  usually  taken,  to  dryness  in  a  weighed  platinum 
dish.  Various  temperatures  from  105°  to  180°  have  been  used 
for  drying  the  residue  before  weighing,  103°  having  now  been 
fixed  as  the  standard  temperature.  After  weighing,  the  organic 
matter  is  volatilized  by  gently  igniting  the  residue.  When  the 
carbon  has  been  burned  off,  the  alkaline-earth  metals  are  recon- 
verted to  carbonates  by  treatment  with  carbonic-acid  water,  and 
after  evaporating  and  gently  igniting,  the  residue  is  again  weighed. 
The  loss  in  weight  was  formerly  reported  as  ORGANIC  MATTER. 

It  is  generally  recognized  that  the  results  of  this  determination 
are  at  best  but  roughly  approximate.  During  the  evaporation 
considerable  organic  matter  may  be  lost,  still  more  being  volatilized 
if  the  residue  is  dried  at  180°;  while  if  the  residue  is  dried  at 
105°,  much  water  will  be  retained  if  hygroscopic  salts,  such  as  cal- 
cium chloride,  are  present.  During  the  ignition  this  water  will  be 
expelled  together  with  considerable  amounts  of  sodium  chloride, 
if  this  commonly  occurring  salt  is  present.  The  LOSS  ON  IGNITION 
is  therefore  considered  of  very  little  value,  the  chief  reason  for 
igniting  the  residue  being  to  observe  the  amount  of  carbonization 
which  takes  place.  The  blackening  of  the  residue  by  the  forma- 
tion of  carbon,  which  subsequently  burns  off,  is  taken  as  an  indica- 
tion of  the  presence  of  organic  matter,  even  if  the  loss  in  weight  is 
inconsiderable;  while  the  absence  of  a  charred  residue  indicates 
the  absence  of  organic  matter,  even  if  considerable  loss  in  weight 
occurs.  The  presence  of  nitrates  is  indicated  by  sparking  of  the 
carbon  on  ignition.  On  account  of  the  difference  in  practice,  the 
temperature  at  which  the  total  solids  were  dried  should  be  stated  in 
reporting  the  analysis. 

BACTERIOLOGICAL  EXAMINATION  OF  WATER. 

643.  Bacteria  Present  in  Water. — All  natural  waters  contain 
varying  numbers  of  bacteria  which  are  microscopic  single-celled 
organisms  generally  living  on  organic  matter.  The  bacteria  which 
feed  on  living  organic  matter,  and  are  therefore  parasitic,  con- 
stitute the  class  of  disease  germs.  This  class  is  far  less  numer- 
ous than  the  bacteria  which  live  on  dead  matter  and  frequently 
cannot  exist  for  any  length  of  time  out  of  their  living  host.  Only 
a  few  of  this  class  can  multiply  or  even  live  in  water,  the  most 


424  WATER  ANALYSIS. 

common  being  B. /typhi,  which  produces  typhoid  fever  when 
it  secures  a  foothold  in  the  human  system.  Dysentery  and  similar 
intestinal  diseases  are  produced  by  bacteria  which  are  carried 
by  water,  but  which  have  not  been  identified  and  studied.  Another 
bacterium,  B.  coli,  which  is  almost  always  present  in  the  intestines 
of  healthy  human  beings  and  other  warm-blooded  animals,  can 
also  live  in  water.  The  presence  of  this  bacterium  in  water  is 
therefore  taken  as  evidence  of  sewage  contamination. 

Such  water  is  always  regarded  as  unwholesome,  since  disease 
germs  from  the  human  system  may  at  any  time  be  present  in 
the  water.  B.  typhi  will  almost  certainly  be  present  at  times  in  such 
water  if  sewage  is  the  source  of  contamination,  because  cases 
of  typhoid  fever  may  always  be  found  in  any  large  community. 

644.  Test  for  B.  Coli. — Fortunately  a  very  simple  method  of 
testing  for  B.  coli  has  been  devised.     During  its  life  activity,  this 
bacterium   decomposes  carbohydrates  with  the  production   of  a 
gas  which  is  mainly  hydrogen,  with  25  to  40  per  cent  of  carbon 
dioxide.     Bile  has  the  property  of  inhibiting  the  growth  of  other 
gas-forming  bacteria.     The  sample  of  water  is  therefore  mixed 
with  bile  and  lactose  and  kept  at  a  temperature  of  37J°  C.  for  48 
hours.     If  gas  is  evolved  during  this  incubation  period,  the  water 
almost   certainly  contains   B.   coli.     An   idea   of  the  number  of 
these  organisms  present  can  be  obtained  by  making  three  tests, 
using  respectively  .1  c.c.,  1  c.c.,  and  10  c.c.  of  the  water.     The  test 
is  made  in  two  test-tubes  of  such  a  size  that  the  smaller  one  can 
be  inverted  within  the  larger  one. 

645.  Bacterial  Count. — In  addition  to  the  test  for  B.  coli,  the 
total  number  of  bacteria  in  a  water  is  determined.     An  excessive 
number  of  bacteria  in  a  water  indicates  contamination  with  sew- 
age or  other  decomposing  animal  or  plant  tissue,  and  renders  the 
water  unfit  for  consumption.     This  test  is  made  by  adding  a 
measured  amount  of  the  water  to  a  plate  containing  a  sterile 
nutrient  medium  which  is  composed  of  meat  extract  and  pep- 
tone to  nourish   the  growing  bacteria,  and   gelatin  or  agar-agar 
to  make  a  semi-solid  media  in  which  the  bacteria  will  remain 
fixed.     Each  bacterium  develops  a  colony  around  it,  which  ulti- 
mately becomes  visible  to  the  eye  under  only  very  slight  magni- 
fication.    By   counting  these   spots   or   colonies,  the  number   of 


BACTERIOLOGICAL  EXAMINATION  OF  WATER.  425 

bacteria  originally  present  in  the  water  can  be  ascertained.  This 
determination  is  only  approximate,  as  all  of  the  bacteria  do  not 
grow  under  the  conditions  of  incubation  sufficiently  to  be  counted. 
Ill  material  used  must  be  thoroughly  cleaned  and  sterilized  by 
heat,  so  as  to  kill  all  organisms  present  before  the  water  to  be  tested 
is  added. 

EXERCISE  72. 

Bacterial  Count. 

646.  Sterilization   of  Apparatus.— Clean   thoroughly   12   test-tubes   and 
insert  firmly  a  plug  of  cotton.     Clean  6  Petri  dis"hes  and  6  one-c.c  pipettes, 
also  2  small  glass-stoppered  bottles.     Place  the  pipettes  in  a  copper  tube 
having  a  close-fitting  cap,  or  in  a  large  test-tube  closed  with  a  plug  of  cot- 
ton.    Sterilize  this  apparatus  by  placing  it  in  an  air-oven  heated  to  150a 
for    1    hour.      Prepare  sterile  water  by  nearly  filling  a  250-c.c.  flask  with 
distilled  water   and   boiling   vigorously  for    15   to   20   minutes.     Insert   a 
plug  of  cotton  and  allow  to  cool. 

647.  Preparation  of  Nutrient  Gelatin. — Place  500  grams  of  lean  beef  in  a 
large  beaker  or  flask,  and  add  1000  c.c.  of  distilled  water.     The  meat  must 
be  as  free  as  possible  from  fat  and  chopped  fine  or  run  through  a  sausage 
grinder.     The   beaker   or  flask   is  weighed   and   distilled   water   added  to 
make  up  the  loss  by  evaporation  after  standing  24  hours.     Place  a  piece 
of  new  cotton  flannel,  with  the  wool  side  up,  in  a  large  funnel  and  cover 
with  a  layer  of  clean  cotton  wadding.     Filter  the  meat  infusion  through 
the  flannel,  squeezing  out  the  last  portions.     It  is  advisable  to  place  a 
second   smaller   funnel   containing   the   same   filtering   medium   below  the 
first  funnel,  so  that  the  filtered  solution  from  the  first  funnel  passes  through 
the  second,  thus  giving  a  very  clear  solution.     Allow  the  solution  to  flow 
into  the  inner  vessel  of  an  agate-ware  double  boiler  of  at  least  two  quarts 
capacity.     This  vessel  should  be  weighed  empty  as  well  as  after  receiving 
the   meat  infusion.     Add  an  amount  of  Witte's  peptone  equal   to  1  per 
cent  of  the  weight  of  the  infusion,  and  10  per  cent  of   the  best  quality  of 
sheet  gelatin   (gold  label).     Dissolve  the  peptone  and  gelatin  by  stirring 
with  a  thermometer  and  heating  the  solution,  not  allowing  the  tempera- 
ture to  risa  above  60°  C.     For  this  purpose  the  outer  vessel  only  of  the 
double  boiler  is  heated  with  the  Bunsen  burner,  the  inner  vessel  being 
surrounded  by  the  hot  water.     After  the  gelatin  is  dissolved,  the  water 
in  the  outer  vessel  is  brought  to  a  boil  and  kept  boiling  for  30  minutes,  the 
inner  vessel  being  covered. 

648.  Adjusting  the  Acidity. — After  the  boiling  has  continued  for  20  min- 
utes, withdraw  5  c.c.  of  the  solution  with  a  pipette  and  place  in  a  porcelain 
dish  or  casserole.     Add  45  c.c.  of  distilled  water  and   boil  for  1  minute 
over   the   Bunsen-burner   flame.     Add  1  c.c.  of   phenolphthalein   solution 


426  WATER  ANALYSIS. 

and  titrate  while  hot  (preferably  while  boiling)  with  N/20  caustic  soda. 
Add  the  soda  solution  until  within  a  drop  or  two  of  the  end-point.  Cool 
by  standing  the  dish  in  cold  water,  and  if  a  distinct  pink  color  does  not 
develop,  add  the  soda  solution  drop  by  drop  until  the  end-point  is  reached. 

So  much  of  the  acid  in  the  solution  must  be  now  neutralized  that  the 
amount  of  acid  remaining  in  1000  grams  will  neutralize  10  c.c.  of  normal 
caustic-soda  solution.  The  calculation  of  the  amount  of  soda  to  be  added 
is  made  as  follows:  If  the  5-c.c.  portion  titrated  required  4£  c.c.  of  the  N/20 
soda  solution,  and  the  weight  of  the  solution  is  950  grams,  the  entire  solution 
will  require  855  c.c.  of  the  N/20  soda,  or  42.7  c.c.  of  normal  soda.  As  the 
acid  equivalent  to  10  c.c.  of  normal  soda  solution  must  be  left  free,  32.7  c.c. 
of  normal  soda  solution  is  added. 

To  coagulate  finely  suspended  matter  so  that  the  solution  may  be 
filtered  clear,  the  white  of  an  egg  is  added  at  this  point.  The  solution  is 
cooled  to  60^-70°  by  immersing  the  containing  vessel  in  cold  water,  and  the 
white  of  an  egg  added  with  stirring;  it  is  then  heated  in  the  double  boiler 
and  finally  boiled  for  2  minutes  over  the  free  flame,  with  constant  stir- 
ring. Weigh  and  add  distilled  water  to  make  up  for  loss  by  evaporation. 
Take  out  5  c.c.  and  titrate  with  N/20  sodium  hydroxide  and  calculate  the 
acidity  as  before.  If  the  acidity  per  1000  grams  is  less  than  S  c.c.  or  more 
than  12  c.c.  of  normal  acid,  acid  or  alkali  should  be  added  to  bring  the 
acidity  to  the  standard  1  per  cent.  Take  out  another  5-c.c.  portion  and 
titrate  a  third  time  to  ascertain  if  the  adjustment  of  the  acidity  has  been 
correctly  carried  out. 

649.  Sterilization  of  the   Culture    Medium. — Filter    the    solution   again 
through  absorbent  cotton  and  cotton  flannel,  passing  the  filtrate  through  the 
filter  until  clear.     The  nutrient  gelatin  must  now  be  measured  off  in  10-c.c. 
portions  into  the  sterilized  test-tubes  closed  with  cotton  plugs.     For  this 
purpose  a  glass  tube  graduated  every  10  c.c.  is  convenient.     The  cotton 
plug  is  removed  from  a  sterilized  test-tube  and  held  between  the  first  and 
second  finger  and  again  inserted  into  the  test-tubes  as  soon  as  the  10-c.c. 
portion  of  gelatin  has  been  added.     The  gelatin  should  not  be  allowed 
to  touch  the  upper  portion  of  the  test-tube,  which  is  set  aside  in  an  upright 
position  to  cool.    When  all  of  the  gelatin  has  been  measured  out  into  test- 
tubes,  it  is  sterilized  by  heating  for  5  minutes  in   an  autoclave  at   120°, 
which  is  the  temperature  of  steam  at  15  pounds  pressure.    The  tubes  of 
gelatin  must  be  kept  in  an  ice-chest.      If  the  gelatin  has  not  been  com- 
pletely sterilized,  colonies  will  make  their  appearance  in  a  few  days. 

650.  Count  of  Bacteria. — To  make  the  count  of  bacteria  in  water,  the 
sample  must  be  taken  in  a  sterilized  glass-stoppered  bottle.     It  is  advisable 
before  sterilization  to  cover  the  stopper  with  tin-foil  to  prevent  entrance  of 
bacteria  with  the  dust  from  the  air;    1  c.c.  is  withdrawn  with  a  sterilized 
pipette  and  transferred  to  a  sterilized   test-tube,  and  9  c.c.  of  sterilized 
water  added.     This  gives  a  dilution  of  1  to  10.    By  diluting  this  solution 


BACTERIOLOGICAL  EXAMINATION  OF  WATER.  427 

in  the  same  manner,  a  dilution  of  1-100  is  obtained.  A  clean  pipette 
must  be  used  for  each  dilution.  To  one  of  these  dishes  1  c.c.  of  the 
sample  of  water  is  transferred;  to  the  second  dish  1  c.c.  of  the  1-10  dilu- 
tion, and  to  the  third  dish  1  c.c.  of  the  1-100  dilution.  Tubes  of  the 
nutrient  gelatin  are  melted  by  placing  in  warm  water,  the  plug  of  cotton 
removed,  the  open  end  of  the  test-tube  sterilized  by  passing  it  through  the 
flame  of  a  Bunsen  burner,  and  the  gelatin  poured  into  three  sterilized  Petri 
dishes.  The  covers  are  raised  only  when  pouring  in  water  or  gelatin,  so 
as  to  prevent  entrance  of  bacteria  from  atmospheric  dust.  The  water 
and  nutrient  gelatin  are  mixed  by  slightly  tilting  the  dish,  so  that  the  con- 
tents flow  from  one  side  to  the  other.  The  dish  is  then  placed  in  a  hori- 
zontal position  in  a  thermostat  or  refrigerator  kept  at  about  20°  C.  After 
48  hours  the  number  of  colonies  on  the  plate  are  counted.  If  the  number 
is  not  over  200,  the  total  number  may  be  counted.  If  the  number  is  large, 
some  counting  device  must  be  employed.  This  generally  consists  of  a 
plate  marked  off  in  sections  or  other  divisions,  so  that  the  total  number 
of  divisions  covered  by  the  plate  may  be  counted,  and  the  average  num- 
ber of  colonies  per  division  is  obtanied  by  counting  the  number  of  colonies 
in  several  divisions,  so  as  to  obtain  a  fair  average  of  the  number  per  divi- 
sion. Small  specks  of  dust  must  not  be  mistaken  for  colonies  which  are 
circular  in  shape.  The  count  must  generally  be  made  with  a  small  mag- 
nifying lens.  When  the  colonies  are  small  it  is  sometimes  advisable  to 
allow  the  sample  to  incubate  for  72  hours.  This  fact  should  be  stated, 
however,  in  reporting  the  analysis.  The  best  results  are  generally  obtained 
by  counting  the  colonies  on  plates  containing  about  200. 

651.  Preparation  of  Nutrient  Agar-agar. — When  this  medium  is  prepared 
the  meat  is  soaked  in  one-half  the  quantity  of  water,  that  is,  500  c.c.  Fifteen 
grams  of  thread  agar  are  dissolved  in  500  c.c.  of  water  by  boiling  for  one- 
half  hour.  The  loss  of  water  is  then  restored  and  the  infusion  allowed  to 
cool  to  about  60°  C.  The  remaining  operations  are  identical  with  those 
used  in  preparing  gelatin,  except  that  2  per  cent  of  Witte's  peptone  is 
dissolved  in  the  filtered  meat  infusion,  after  which  the  agar  is  added  to  the 
meat  infusion,  care  being  taken  to  keep  the  temperature  below  60°  C. 

The  samples  of  water  are  plated  as  described  for  gelatin.  It  is  neces- 
sary, however,  to  heat  the  agar  to  a  higher  temperature  in  order  to  melt 
it.  It  should  be  cooled  to  about  40°  before  pouring  on  the  plate.  The 
plates  are  incubated  at  body  temperature  (37£°-40°  C.)  for  48  hours. 
The  number  of  colonies  obtained  is  usually  higher  than  with  gelatin.  It 
is  advisable  to  use  porous  earthenware  covers  with  agar  to  prevent  the 
spreading  of  colonies  by  the  condensation  of  water. 


428  WATER  ANALYSIS. 

EXERCISE  73. 
Test  for  B.  Coli. 

Obtain  a  quantity  of  fresh  ox-bile.  Place  in  a  flask  and  sterilize  by 
heating  in  an  autoclave  at  15  pounds  pressure  for  20  minutes,  or  heat  in 
steam  for  one-half  to  three-quarters  of  an  hour.  Filter  through  cotton 
and  add  1  per  cent  of  lactose.  Add  a  sufficient  amount  of  the  bile 
to  a  large  test-tube  containing  an  inverted  smaller  test-tube,  so  that  the 
bile  will  fill  the  small  tube  and  act  as  a  seal  at  the  bottom.  Fill  other 
test-tubes  in  the  same  manner  until  all  the  bile  has  been  utilized,  and 
plug  with  cotton.  Sterilize  in  an  autoclave  at  15  pounds  pressure  for  5 
minutes.  The  air  in  the  smaller  tube  will  be  expelled  by  the  high  tem- 
perature, and  the  steam  produced,  so  that  on  cooling,  the  bile  will  entirely 
fill  the  smaller  tube.  Cool  and  store  in  a  cool  place. 

Take  three  of  these  tubes  and  add,  with  sterilized  pipettes,  0.1  c.c.,  1  c.c., 
and  10  c.c.  of  the  water  to  be  tested.  Incubate  at  37£°  C.  for  48  hours. 
The  presence  of  gas  in  the  small  tube  is  evidence  of  the  presence  of  B.  coli 
in  the  water. 

INTERPRETATION  OF  THE  RESULTS  OF    A  SANITARY 
WATER  ANALYSIS. 

No  definite  limits  can  be  set  for  the  amount  of  chlorides,  nitro- 
gen, or  other  elements  which  may  be  present  in  water  without 
rendering  it  unsafe  for  domestic  use.  None  of  these  substances 
are  themselves  poisonous  or  injurious  in  the  amounts  found  in 
water.  The  amount  present  can  merely  be  used  as  evidence  of  more 
or  less  recent  contamination  of  the  water  with  sewage.  A  knowl- 
edge of  the  condition  of  the  water  before  contamination  is  the  best 
possible  basis  for  a  judgment.  On  comparing  the  analysis  of  a 
water  which  is  suspected  to  have  produced  disease  with  the  analysis 
of  the  same  water  before  the  disease  appeared,  if  a  marked  increase 
in  the  amount  of  chlorides  or  nitrogen  is  shown,  contamination  of 
the  water  may  be  suspected,  and  its  source  should  be  looked  for  by 
examining  the  watercourse  from  which  the  supply  is  drawn. 

In  cities  where  the  water  is  analyzed  at  regular  intervals  and 
where  a  careful  record  of  the  cases  of  typhoid  and  other  diseases 
known  to  be  due  to  impure  water  is  kept  the  amount  of  nitrogen, 
chlorides,  etc.,  in  the  water  which  indicates  a  contaminated  water- 
supply  is  soon  learned. 


INTERPRETATION  OF  RESULTS.  429 

652.  Normal    Chlorine. — These   difficulties    in    interpretation 
arise  from  the  fact  that  the  substances  introduced  into  the  water 
with  sewage  also  find  their  way  into  the  water  from  sources  which 
cannot   produce  dangerous  contamination.     While,  for  instance, 
the  chlorides  present  in  water  may  be  due  to  sewage  contamina- 
tion, which  always  contains  large  amounts  of  chlorides,  their  pres- 
ence is  by  no  means  prima  facie  evidence  of  such  contamination. 
As  the  earth  contains  many  salt  deposits  with  which  the  water 
may  come  in  contact,  and  the  salt  from  the  ocean  undoubtedly 
diffuses   through   the    earth   for   considerable    distances    inland, 
natural  waters  may  contain  large  amounts  of  chlorides  derived 
from  these  sources.     The  amount  of  chlorides  derived  from  these 
sources  by  the  water  of  a  given  region  is  known  as  the  normal 
chlorine  for  the  region.     Maps  have  been  prepared  for  many  states 
giving  isochlors,  or  lines  passing  through  regions  of  equal  chlorine. 
When  the  amount  of  chlorine  found  is  in  excess  of  the  normal 
chlorine,  contamination  of  the  water  with  sewage  is  indicated. 

653.  Nitrogen. — As  has  been  already  stated,  the  ALBUMINOID 
AMMONIA  may  be  derived  from  vegetable  as  well  as  from  animal 
sources.     A  surface-water  which  has  passed  through  a  swampy 
region   almost   always   contains   a  large  amount  of   albuminoid 
ammonia.     The  presence  of  dissolved  vegetable  matter  is  almost 
always  indicated  by  the  high  color  of  the  water.      On  the  other 
hand,  the  presence  of  much  albuminoid  ammonia  in  a  light-colored 
water  is  almost  always  suspicious,  the  absence  of  color  making 
it  probable  that  the  organic  matter  is  partly  of  animal  origin.     If 
the  free  ammonia  and  nitrites  are  also  high,  the  evidence  of  pol- 
lution is  strong. 

654.  The  Characteristics  of  Polluted  Surface-waters  are  given 
by  Geo.  C.  Whipple  as  follows: 

"They  contain  an  excess  of  chlorine  above  the  normal  of  the 
region. 

"They  contain  considerable  organic  matter,  shown  by  the 
albuminoid  ammonia  and  loss  on  ignition ;  but  it  should  be  remem- 
bered that  the  dissolved  albuminoid  ammonia  may  be  explained 
by  a  high  color. 

"They  contain  free  ammonia  and  nitrites,  if  the  pollution  is 
recent  and  the  organic  matter  is  in  a  decomposing  state. 

"They  show  coincident  high  nitrates  and  high  hardness. 


430  WATER  ANALYSIS. 

11  They  are  characterized  usually  by  a  mouldy  or  musty  odor 
and,  if  much  polluted,  by  turbidity  and  sediment." 

655.  Ground- waters  are  very  different  from  surface-waters. 
As  a  rule  they  are  clear,  almost  colorless,  and  without  odor.  Hav- 
ing been  much  in  contact  with  the  soil,  they  are  rich  in  mineral 
matter.  The  fixed  solids  are  higher,  and  usually  the  hardness. 
The  amount  of  chlorine  and  nitrogen  may  be  high,  indicating  pollu- 
tion, but  if  the  nitrogen  is  nearly  all  present  as  nitrate,  the  water 
has  been  purified  by  filtration  through  the  soil  and  oxidation  of 
the  organic  matter  so  that  it  may  be  safely  used. 

The  determination  of  the  amount  of  TOTAL  SOLIDS  as  well  as 
HARDNESS  is  carried  out  not  so  much  to  determine  the  sanitary 
character  of  the  water  as  its  suitability  for  laundry  purposes, 
though  the  continued  drinking  of  hard  water  is  undoubtedly 
injurious  to  many  constitutions.  The  presence  of  considerable 
amounts  of  iron  renders  water  unfit  for  laundry  use,  the  presence 
of  more  than  0.5  part  per  million  being  objectionable.  If  the 
water  contains  much  organic  matter,  more  iron  may  be  present 
without  being  objectionable.  A  hardness  of  25  to  50  is  most  de- 
sirable for  ordinary  use.  A  water  of  hardness  of  50  is  noticeably 
hard,  while  if  above  100  it  is  classed  as  very  hard. 

The  COLOR  of  water  should  not  exceed  25.  The  amount  of 
color  which  this  figure  implies  may  be  judged  from  the  fact  that 
if  the  color  of  a  water  is  15  to  20  it  will  be  noticed  when  the 
water  is  allowed  to  flow  into  a  bath-tub,  while  if  the  color  is  30  it 
may  be  noticed  in  a  tumblerful. 

Waters  having  a  TURBIDITY  of  more  than  3  or  4  are  objection- 
able. 

ANALYSIS  OF  WATER  FOR  USE  IN  BOILERS. 

Aside  from  the  sanitary  analysis  of  water,  the  most  commonly 
conducted  analysis  is  that  carried  out  for  the  purpose  of  deter- 
mining the  adaptability  of  the  water  in  question  to  use  in  boilers. 
The  water  may  not  be  suitable  for  use  in  boilers  for  two  reasons. 

It  may,  in  the  first  place,  contain  substances  which  cause  the 
CORROSION  of  the  iron  of  which  the  boiler  is  made.  FREE  ACIDS, 

MAGNESIUM  CHLORIDE,  AMMONIUM  SALTS,  MUCH  DISSOLVED  OXY- 
GEN, HUMUS,  AND  FATTY  SUBSTANCES  are  the  commonly  occurring 
substances  which  have  been  found  to  cause  corrosion  in  boilers. 


BOILER-SCALE.  431 

A  water  may  also  be  objectionable  for  boiler  use  from  contain- 
ing SCALE-FORMING  INGREDIENTS,  among  which  the  most  impor- 
tant are  CALCIUM  SULPHATE  and  CARBONATE  and  MAGNESIUM 
CARBONATE.  Others  occurring  commonly,  though  in  smaller 
amounts,  are  SILICA  and  the  OXIDES  OF  IRON  AND  ALUMINIUM. 
Although  rarely  forming  parts  of  the  boiler-scale,  the  alkalies 
should  in  a  careful  analysis  be  determined,  as  the  distribution  of 
the  hydrochloric  and  sulphuric  acids  present  may  then  be  more 
correctly  determined. 

656.  Hardness. — The  fitness  of  a  water  for  boiler  use  may  be 
somewhat  roughly  determined  by  simply  determining  the  tem- 
porary and  permanent  hardness,  as  given  in  Chapter  XXI,  page 
269.     All  of  the  calcium  and  magnesium  carbonates  constituting 
temporary  hardness   are  precipitated  by  boiling  and  become  part 
of  the  boiler-scale.     The  calcium  and  magnesium  existing  as  per- 
manent hardness  may  be  present  as  chlorides  or  sulphates.     The 
determination  of  permanent  hardness  does  not  give  the  amount 
of  magnesium  chloride  present  which  causes  corrosion,  nor  the 
amount  of  calcium  sulphate  present  which  is  precipitated  at  the 
temperature  existing  in  the  boiler  and  constitutes  part  of  the 
boiler-scale. 

657.  For  a  Complete  Analysis  an  amount  of  water  should  be 
taken  which  will  give  from  J  to  1  gram  of  residue.     It  is  evapo- 
rated to  dryness  in  a  platinum  dish  and  the  TOTAL  SOLIDS  deter- 
mined as  already  directed.     The  ignition  of  the  residue  to  volatilize 
organic  matter  and  burn  the  carbon  must  be  conducted  at  dull 
redness  and  the  ignition  continued  only  for  a  few  minutes,  or  a  loss 
of  alkalies   may  occur.      After  weighing,  the  residue  is  digested 
with  water  and  a  few  cubic  centimeters  of  hydrochloric  acid. 
The    insoluble    residue    is    filtered    off,    washed,   ignited    in    the 
platinum  dish  and  weighed.     It  is  mainly  SILICA,  though  a  little 
iron,   aluminium,  and   calcium  sulphate  may  be   present.      The 
silica   may   be  volatilized  by  treatment  with   hydrofluoric  acid 
and  a  drop  or  two  of  sulphuric  acid,  the  residue  weighed  after 
ignition,  and  the  weight  deducted  from  that  of  the  impure  silica. 
The  .residue  in  the  dish  is  dissolved  by  digestion  with  a  little 
hydrochloric   acid  and  water  and  is   added  to   the   main    solu- 
tion. 


432  WATER  ANALYSIS. 

The  IRON  and  ALUMINIUM  may  be  precipitated  with  ammonia 
and  weighed  as  oxides  in  the  usual  manner. 

The  CALCIUM  is  precipitated  as  oxalate  from  the  nitrate  and 
weighed  as  oxide. 

The  MAGNESIUM  and  the  ALKALIES  in  the  nitrate  may  be  weighed 
together  as  sulphates,  the  solution  being  evaporated  to  dryness 
in  a  platinum  dish  and  the  ammonium  salts  volatilized  after  the 
addition  of  a  few  drops  of  sulphuric  acid.  A  small  amount  of 
silica  and  other  insoluble  matter  generally  remains  with  the  sul- 
phates of  magnesium  and  the  alkalies.  This  is  separated  by  treat- 
ing the  residue  in  the  dish  with  water  and  filtering.  The  filtrate 
is  again  evaporated  to  dryness  in  the  dish,  and  after  gentle  igni- 
tion is  weighed.  The  weighed  sulphates  are  dissolved  in  water, 
the  solution  diluted  to  100  c.c.  and  divided  into  two  portions,  in 
one  of  which  the  magnesium  is  determined  in  the  usual  manner 
as  pyrophosphate,  while  in  the  other  portion  the  potassium  is 
precipitated  as  platinochloride,  washed  according  to  the  Lindo- 
Gladding  method  and  weighed.  The  amount  of  magnesium 
present  as  sulphate  is  calculated  from  the  weight  of  the  pyro- 
phosphate, and  the  weight  of  potassium  as  sulphate  is  calcu- 
lated from  the  weight  of  the  potassium  platinochloride.  The 
difference  between  the  sum  of  these  two  weights  and  the  weight 
of  the  combined  magnesium  and  alkali  sulphates  gives  the  weight 
of  sodium  sulphate  present. 

The  amount  of  CHLORIDES  present  is  determined  volumetric- 
ally  according  to  the  method  given  under  the  sanitary  analysis 
of  water  on  p.  421.  The  amount  of  SULPHATES  present  is  also 
determined  in  a  separate  portion.  Unless  a  considerable  amount 
is  present,  500  c.c.  are  evaporated  down  to  a  bulk  of  about  100  c.c. 
and,  after  acidifying  with  a  drop  or  two  of  hydrochloric  acid,  the 
sulphuric  acid  is  precipitated  by  the  addition  of  barium  chloride 
solution.  After  digestion  on  the  water-bath  the  barium  sul- 
phate is  filtered  off,  washed  with  hot  water,  ignited,  and  weighed 
in  the  usual  manner. 

If  FREE  MINERAL  ACIDS  are  present,  the  amount  is  determined 
volume trically  by  titration  with  a  standard  alkali. 

658.  Method  of  Combining  Acids  and  Bases. — The  elements 
present  in  the  water  having  been  determined,  it  remains  to  com- 


CALCULATION  OF  RESULTS.  433 

bine  them  in  such  a  manner  that  the  properties  of  the  water 
when  used  in  a  boiler  may  be  easily  shown,  no  attempt  being  made 
to  combine  them  as  they  are  believed  to  exist  when  in  solution. 
The  following  method  is  generally  followed :  The  chlorine  is  com- 
bined with  the  bases  in  the  following  order:  sodium,  potassium, 
magnesium,  and  calcium.  The  sulphuric  acid  is  combined  with 
the  alkalies  if  the  amount  of  chlorine  was  insufficient,  then  with 
the  calcium  and,  if  any  remains,  with  the  magnesium.  The  cal- 
cium and  magnesium  remaining  are  calculated  as  carbonates. 

That  this  method  of  combination  does  not  exist  in  solution 
is  evident  from  the  consideration  that  the  strong  base  calcium 
would  undoubtedly  be  in  combination  with  some  of  the  hydro- 
chloric acid,  while  the  alkalies  would  also  be  combined  partly 
with  the  sulphuric  and  partly  with  the  hydrochloric  acid.  When, 
however,  the  temperature  was  reached  at  which  the  calcium 
sulphate  is  almost  absolutely  insoluble  in  water  this  salt  would 
be  precipitated,  even  if  the  sulphuric  acid  were  combined  with 
the  alkalies  and  the  calcium  with  the  hydrochloric  acid.  The 
ultimate  result  would  therefore  be  that  the  alkalies  would  be  found 
in  combination  with  the  chlorine,  and  the  calcium  with  the  sul- 
phuric acid.  Some  authorities  recommend  that  as  much  magne- 
sium chloride  as  can  be  formed  from  the  amounts  of  these  elements 
present  be  reported  in  order  to  bring  out  the  fact  that  the  presence 
of  the  two  elements  together  leads  to  the  formation  of  hydrochloric 
acid  and  magnesium  oxide.  This  method  of  reporting  the  analy- 
sis probably  overstates  the  danger,  as  some  of  the  magnesium  is 
undoubtedly  precipitated  as  carbonate  if  that  acid  is  present  in 
the  solution. 

659.  Calculation  of  Results. — The  simplest  way  in  which  to 
state  the  results  is  in  grams  per  liter  or  in  milligrams  per  liter, 
which  would  be  identical  with  parts  per  million.  The  results 
must  frequently  be  given  in  grains  per  United  States  gallon  (58,318 
grains)  or  per  imperial  gallon  (70,000  grains),  to  suit  the  conve- 
nience of  engineers  who  still  use  the  English  instead  of  the  metric 
system. 

The  method  of  calculation  may  be  more  clearly  understood 
from  the  following  example.  The  analysis  gave  the  following 
results : 


434  WATER  ANALYSIS. 

Si02 0.0063  gram  per  liter 

S03 0.0245  "  "  " 

Cl 0.0075  "  "  " 

K20 0.0021  "  "  " 

Na,0 0.0065  "  "  " 

MgO 0.0125  "  "  " 

CaO 0.0223  "  "  " 

Fe203+Al203 0.0040  "'  "  " 

The  sodium  is  first  calculated  to  sodium  chloride  as  follows: 

Na20:2NaCl::  0.0065  :x 
62.  :  117.9  ::0.0065:o; 
x  =  0.0123  gram  Nad. 

The  amount  of  chlorine  in  this  weight  of  sodium  chloride  is 
then  calculated: 

NaCl:   Cl    ::  0.0123  :x 

58.5: 35.45::  0.0123  :z 

x  =  0.0075  gram  Cl. 

As  this  is  the  amount  of  chlorine  found  by  analysis,  no  other 
chlorides  are  present. 

The  potassium  is  next  calculated  to  sulphate : 

K20:K2S04::  0.0021:  x 
94.3: 174.36:.  0.0021:* 
x  =  0.0039  gram  K2S04 
K20  =  0.0021      " 

S03  =  0.0018     " 

The  amount  of  potassium  oxide  being  subtracted,  it  is  found 
that  0.0018  gram  of  sulphur  trioxide  has  been  taken  to  combine 
with  the  potassium,  leaving  0.0227  gram  (0.0245-0.0018)  which  is 
combined  with  calcium  oxide  to  form  sulphate. 

S08:CaS04::  0.0227:  a; 
80:136.2  ::0.0227:z 
x =0.0386  gramCaSO, 
S0,  =  0.0227      " 


CaO  =0.0159 


CALCULATION  OF  RESULTS.  435 

0.0064  gram  (0.0223-0.0159)  of  calcium  oxide  remains  after 
all  of  the  sulphur  trioxide  has  been  combined  to  form  sulphate. 
This  calcium  oxide  will  be  present  as  carbonate. 

CaO-CaC03::0.0064:z 
56  :100      ::0. 0064:s 
x  =  0.01 14  gram  of  CaC03. 

The  chlorine  and  sulphur  trioxide  being  all  combined,  all  of 
the  magnesium  will  be  present  as  carbonate. 

MgO:MgC03::0.0125:z 
40.3  :  84.3:: 0.0125: x 
j  =  0.0261  gram  MgC03. 

The  result  of  the  analysis  is  therefore  reported  as  follows: 

NaCl=  0.0123  gram  per  liter 

K2S04  =  0.0039  "  "  " 

CaS04  =  0.0386  "  "  " 

CaC03  =  0.0114  "  il  li 

MgC33  =  0.0261  "  "  " 

Fe203+Al203= 0.0040  "  "  " 

Si02=0.0063  "  "  " 


LITERATURE. 

WATER  SUPPLY.    Mas^n  (John  Wiley  &  Sons) 

AIR,  WATER,  AND  FOOD.    Richards  &  Woodman  (John  Wiley  &  Sons). 

WATER  SUPPLY.     Nichols  (John  Wiley  &  Sons). 

SEWAGE.    Rideal  (John  Wiley  &  Sons) . 

EXAMINATION  OF  WATER.     Leffman  (Blakiston). 

WATER  SUPPLIES.     T.  C.  Thresh  (Rebman  Pub.  Co.,  London). 

SPECIAL  REPORT  ON  EXAMINATION  OF  WATER  SUPPLY,  1890,  by  the  Mass. 
State  Board  of  Health. 

SPECIAL  REPORT  ON  PURIFICATION  OF  SEWAGE  AND  WATER,  1890,  by  the 
Mass.  State  Board  of  Health. 

TYPHOID  FEVER,  ITS  CAUSATION,  TRANSMISSION,  AND  PREVENTION.  Geo. 
C.  Whipple  (John  Wiley  &  Sons). 

ELEMENTS  OF  WATER  BACTERIOLOGY,  WITH  SPECIAL  REFERENCE  TO  SANI- 
TARY WATER  ANALYSIS.  Prescott  and  Winslow  (John  Wiley  &  Sons), 

THE  BACTERIOLOGICAL  EXAMINATION  OF  WATER-SUPPLIES.  W.  G.  Savage 
(Blakiston  &  Sons,  Philadelphia). 


CHAPTER  XXX. 
ANALYSIS  OF  FATS  AND   OILS. 

660.  The    Quantitative  Analysis  of  a  fat  or  oil  is  conducted 
for  the  purpose  of  identifying  the  constituent  fats  or  oils  as  well 
as  ascertaining  the  proportion  in  which  they  are  present.     The 
difficulties  of  the   subject  arise  from  the  fact  that  not  only  are 
most  pure  fats  and  oils  mixtures  of  a  considerable  number  of  chemi- 
cal substances,  but  a  given  fat  or  oil,  being  produced  as  a  part  of 
an  animal  or  vegetable  structure,  is  by  no  means  uniform  in  com- 
position, but  varies  with  the  conditions  under  which  it  was  pro- 
duced.    Changes  in  climate,  season,  food,  etc.,  produce  consid- 
erable variations  in  the  fat  or  oil  produced.     The  number  of  oils 
and  fats  which  are  produced  on  a  commercial  scale  is  very  large, 
thus  adding  very  much  to  the  complexity  of  the  subject.     The 
mineral  oils,  being  obtained  by  distillation  of  crude  products  from 
various  parts  of  the  earth,  differ  still  more  widely  in  composition. 
The  strictly  chemical  examination  is  confined  almost  exclusively 
to  vegetable  fats  and  oils.     The  methods  employed  are  essen- 
tially inorganic,  and  therefore  come  within  the  scope  of  this  work. 
The  interpretation  of  the  results  should  be  undertaken  only  after 
the  consultation  of  a  more  extensive  work  on  the  subject,*  except 
in  the  case  of  pure  oils  and  comparatively  simple  mixtures. 

66 1.  Chemical   Composition. — All    animal   and   vegetable   oils 
and  fats    are  organic    salts  or  esters  in  which  a  common   base, 
GLYCERINE,  is  always  present.     Besides  glycerine,  most  fats  and 
oils  and  especially  the  waxes  contain  a  larger  or  smaller  quantity 
of  a  base  peculiar  to  themselves.     Of  these,  Benedikt  and  Ulzer 

*  A  very  excellent  work  for  this  purpose  is  that  by  Lewkowitsch  on  "Chemical 
Analysis  of  Oils,  Fats,  and  Waxes."  (Macmillan  &  Co.)  A  recent  German 
publication  is  also  excellent,  "Analyse  der  Fette  und  Waehsarten."  Benedikt 
und  Ulzer.  (Julius  Springer,  Berlin.) 

436 


FREE  ACID.  437 

mention  twelve.  Three  of  these  belonging  to  the  aromatic  series 
furnish  a  means  of  distinguishing  between  fats  and  oils  of  vege- 
table or  animal  origin.  CHOLESTERIN  and  ISOCHOLESTERIN  occur 
exclusively  in  substances  of  animal  origin,  while  vegetable  products 
contain  another  base  called  PHYTOSTERIN.  These  may  be  liber- 
ated from  the  fatty  acids  by  saponificaticn,  and  after  extraction 
with  ether,  may  be  crystallized  from  alcohol,  cholesterin  and 
isocholesterin  forming  irregular  plates  or  tables,  phytosterin, 
groups  of  needles.  The  first  melts  at  147°,  the  second  at  138°, 
and  the  third  at  132°-134°.  Failure  to  obtain  these  substances, 
however,  is  no  indication  that  the  oil  or  fat  was  not  derived  from 
its  proper  source,  since  they  often  exist  in  such  small  quantity  as 
to  be  isolated  with  difficulty.  The  organic  acids  present  in  com- 
bination with  the  glycerine  are  very  numerous  and  differ  greatly 
in  their  properties.  On  these  differences  are  founded  most  of  the 
methods  of  identifying  the  oils  and  fats. 

662.  Acidity. — In  many  oils  and  fats,  especially  those  of  vege- 
table origin,  part  of  the  acid  exists  uncombined  with  the  base. 
The  amount  of  this  free  acid  increases  with  the  age  of  the  oil, 
this  being  especially  true  of  palm-oil.     The  amount  of  the  free 
acid  in  most  fresh  animal  oils  is  generally  so  small  as  to  be  neg- 
ligible.    Old  and  rancid  fats  become  strongly  acid. 

The  method  of  determining  this  value,  as  well  as  preparing  the 
solutions  required,  is  given  in  the  following  paragraphs. 

663.  Standard  N/2   Hydrochloric   Acid    Solution.  —  Prepare  2 
liters  of  a  half -normal  solution  of  hydrochloric  acid  by  measuring 
out  about  40  c.c.  concentrated  acid  and  diluting  to  a  liter.     Repeat 
the  operation  and  pour  the  two  solutions  together  and  mix  thor- 
oughly.    Standardize  the  solution  by  one  of  the  methods  given 
in  Chapter  XXI,  page  258,  and  dilute  to  exact  strength. 

664.  Standard  N/2    Caustic  Potash    Solution.  —  Prepare  some 
pure  alcohol  by  adding  to  about  2  liters  of  the  ordinary  alcohol  a 
few  pieces  of  caustic  potash,  shaking  thoroughly,  and  after  allowing 
the  solution  to  stand  for  a  few  hours,  distilling  off  the  alcohol.     To 
1  liter  of  the  alcohol  add  30  grams  of  caustic  potash.     The  alcohol 
should  be  poured  into  a  liter  bottle,  the  potash  added,  and  the 
bottle  closed  with  a  rubber  stopper.    Shake  until  the  caustic  pot- 
ash is  dissolved.    The  potassium  carbonate  will  not  dissolve,  but 


438  ANALYSIS  OF  FATS  AND  OILS. 

will  adhere  quite  firmly  to  the  sides  and  bottom  of  the  bottle  if 
the  solution  is  allowed  to  stand  undisturbed  for  a  few  hours. 
Decant  or  siphon  off  the  clear  liquid  into  another  liter  bottle, 
exposing  the  solution  to  the  air  as  little  as  possible. 

The  solution  is  standardized  by  titrating  with  the  standard 
hydrochloric  acid,  using  phenolphthalein  as  the  indicator.  It 
should  be  protected  from  the  carbon  dioxide  of  the  air  both 
when  being  withdrawn  from  the  bottle  and  during  a  titration.  A 
simple  method  of  accomplishing  this  object  is  to  arrange  a  siphon 
for  withdrawing  the  solution  from  the  bottle,  as  shown  in  Fig.  50, 
p.  290.  The  siphon  should  be  passed  through  a  rubber  stopper 
with  two  holes,  a  straight  tube  containing  soda-lime  being  inserted 
in  the  second  hole  of  the  stopper  The  end  of  the  siphon  exposed 
to  the  air  should  be  kept  closed  with  a  piece  of  rubber  tubing 
having  one  end  closed  with  a  glass  plug.  After  being  filled,  the 
burette  should  be  immediately  closed  with  a  small  rubber  stopper 
through  which  passes  a  soda-lime  tube. 

665.  Determination  of  Free  Acid. — For  the  determination  of  the 
acid  value  of  oils,  calculate  the  volume  of  the  N/2  caustic  potash 
solution  to  measure  out  and  dilute  with  alcohol  to  250  c.c.  to  make 
a  fifth-normal  solution.  As  the  solution  is  not  stable  it  must  be 
used  very  soon  after  being  made.  All  solid  fats  must  be  melted  at 
as  low  a  temperature  as  possible,  and  after  standing  until  the 
impurities  have  settled,  the  clear  fat  is  carefully  decanted  through 
a  dry  filter-paper.  The  vessel  containing  the  oil  or  fat  is  weighed 
together  with  a  small  glass  rod  or  spoon.  From  1  to  2  grams  are 
transferred  to  an  Erlenmeyer  flask,  the  exact  amount  being  obtained 
by  difference.  50  c.c.  of  the  redistilled  alcohol  are  now  added  and 
the  flask  shaken  until  the  oil  is  dissolved.  If  the  fat  does  not 
readily  dissolve  in  alcohol,  50  c.c.  of  a  mixture  of  equal  parts  of 
alcohol  and  ether  may  be  used.  One  drop  of  phenolphthalein  is  now 
added.  The  solution  is  titrated  with  the  fifth-normal  caustic  potash 
solution,  which  should  be  added  drop  by  drop  with  constant  and 
vigorous  shaking.  The  first  appearance  of  pink  color  is  taken  as  the 
end-point.  It  may  fade  on  standing  a  few  minutes,  but  this  is  due 
to  saponification  of  the  neutral  glycerides  by  the  excess  of  alkali. 

When  oils  of  high  free  acid  value,  such  as  palm-oil  or  old  and 
rancid  *°ts  are  being  tested,  less  than  1  gram  may  be  taken  for  a 


KOTTSTORFER  VALUE.  439 

titration.  When  the  acid  value  is  small,  as  in  most  solid  fats,  as 
much  as  5  or  10  grams  may  be  taken  to  obtain  a  reliable  determina- 
tion. The  number  of  milligrams  of  KOH  per  gram  of  fat  or  oil  is 
calculated,  which  is  the  acid  value.  The  acid  value  is  also  fre- 
quently expressed  in  terms  of  the  amount  of  oleic  acid  corre- 
sponding to  the  amount  of  alkali  used.  The  molecular  weight  of 
oleic  acid  being  282, 1  c.c.  of  N/5  alkali  is  equal  to  0.0564  gram  of 
oleic  acid. 

666.  Kottstorfer  Value. — As  the  acids  present  in  fats  and  oils 
differ  in  molecular  weight  and  basicity,  varying  amounts  of  alkali 
will  be  required  to  neutralize  the  acid  in  a  definite  amount  of  the 
oil  or  fat.     Caustic  potash  is  the  alkali  invariably  used,  1  gram 
of  the  fat  or  oil  being  taken  for  the  determination.     The  so-called 
Kottstorfer  value  is  the  number  of  milligrams  of  KOH  required  to 
neutralize  the  acids  present  in  1  gram  of  the  fat  or  oil.     When  an 
oil  or  fat  is  treated  with  an  alcoholic  solution  of  caustic  potash 
the  union  between  the  glycerine  and  the  fatty  acids  is  broken, 
so  that  the  acid  is  left  in  combination  with  the  potassium.     This 
process  is  called  SAPONIFICATION.     An  excess  of  standard  caustic 
potash  dissolved  in  alcohol  is  added  to  a  weighed  amount  of  the 
fat  or  oil.      When  the  saponification  is  complete  the  excess  of 
alkali  is  titrated  with  standard  acid.    The  number  of  milligrams 
of  KOH  neutralized  by  one  gram  of  the  fat  or  oil  is  then  calcu- 
lated to  find  the  Kottstorfer  value. 

667.  Determination   of  Kottstorfer  Value. — To  determine  the 
Kottstorfer  or  saponification  value,  weigh  out  as  directed  for  the 
determination  of  free  acid  from  1  to  2  grams  of  the  fat  or  oil  into 
a  small  Erlenmeyer  flask  fitted  with  a  cork  to  a  return  condenser. 
Add  25  c.c.  of  the  alcoholic  caustic  potash  solution  with  a  pipette 
or  a  burette,  and  heat  on  the  water-bath  until  the  oil  or  fat  is 
dissolved,  which  will  require  from  ten  minutes  to  half  an  hour, 
depending  on  the  fat  or  oil  being  investigated.     Measure  out  25  c.c. 
of  the  caustic  potash  solution  into  a  similar  flask  connected  with 
a  return  condenser,  and  digest  on  the  water-bath  for  the  same  time. 
Cool  for  a  few  minutes  and  titrate  the  contents  of  each  flask  with 
the  N/5  hydrochloric  acid,  using  rather  more  phenolphthalein  than 
usual  as  the  indicator.     The  difference  between  the  acid  used  in 
the  two  titrations  gives  the  acid  necessary  to  neutralize  the  alkali 


440  ANALYSIS  OF  FATS  AND  OILS. 

used  in  the  saponification.     Calculate  the  number  of  milligrams  of 
KOH  necessary  to  saponify  1  gram  of  the  fat  or  oil. 

668.  Ether  Value. — The  ether  value  is  the  difference  between 
the  saponification  and  the  acid  value. 

669.  Reichert  and  Reichert-Meissl  Value. — Some  of  the  fatty 
acids  of  small  molecular  weight  are  quite  volatile,  and  also  soluble 
in  water.     It  has  been  found  that  by  liberating  the  acids  con- 
tained in  a  given  fat  or  oil,  adding  water,  and  distilling  off  this 
water  again,  a  distillate  is  obtained  which  contains  a  considerable 
percentage  of  the  volatile  acids.     It  is  found  in  practice  that  the 
total  amount  of  volatile  acid  present  is  not  separated  in  this 
manner  by  one  distillation.     The  amount  which  comes  over  depends 
upon  the  dilution  of  the  solution,  the  amount  of  the  distillate,  and  the 
rapidity  of  the  distillation.    The  amount  of  acid  in  the  distillate  is 
determined  by  titration    with  decinormal  caustic  potash.      The 
REICHERT  VALUE  indicates  the  number  of  cubic  centimeters  of  N/10 
potash  required  to  neutralize  the   acid  from   2.5  grams  of  fat 
contained  in  100  c.c.  of  distillate,  110  c.c.  of  which  has  passed 
over  in  one-half  hour.     The  REICHERT-MEISSL  value  is  obtained 
in  the  same  manner,  5  grams  of  fat  having  been  used  instead  of 
2.5  grams.    This  number  is  not  necessarily  twice  the  Reichert 
number.    This  determination  has  been  used  more  especially  to 
distinguish  butter  from  the  artificial  substitutes  which  have  been 
put  on  the  market. 

670.  Determination  of  Reichert-Meissl  Value. — For  the  deter- 
mination of  the  Reichert-Meissl  number  a  sample  of  butter-fat 
should  be  used  which  has  been  freed  from  casein,  water,  salt, 
etc.,  by  melting  and  filtering  as  already  directed.     A  quantity  of 
this  fat  as  nearly  as  possible  5  grams  is  weighed  out  into  a  250-c.c. 
Erlenmeyer  flask,  2  c.c.  water,  8  c.c.  alcohol,  and  a  stick  of  caustic 
soda  weighing  about  2  grams  are  added.     Attach  the  flask  to  an 
inverted  condenser  by  means  of  a  cork  stopper  and  heat  on  the 
water-bath  for  one  hour,  shaking  the  contents  of  the  flask  occa- 
sionally.   Remove  or  incline  the  condenser  and  distil  off  the  alco- 
hol by  immersing  the  flask  in  the  hot  water  and  heating  for  about 
three-quarters  of  an  hour.    After  the  alcohol  is  removed  add 
100  c.c.  recently  boiled  distilled  water  and  dissolve  the  soap  by 
warming  on  the  water-bath  and  shaking.     Cool  the  solution  to 


POLENSKE   VALUE.  441 

60°  or  70°  and  add  50  c.c.  of  a  solution  of  sulphuric  acid  containing 
25  c.c.  concentrated  acid  per  liter.  Again  attach  the  flask  to  the 
inverted  condenser  and  heat  on  the  water-bath  until  the  insoluble 
acids  collect  in  a  clear  oily  layer  on  top  of  the  liquid.  Allow  the 
contents  of  the  flask  to  cool  to  the  room  temperature.  Introduce 
a  few  pieces  of  pumice-stone  which  have  been  heated  to  redness 
and  dropped  into  distilled  water.  Attach  the  flask  to  an  inclined 
condenser  and  distil  off  110  c.c.  into  a  graduated  cylinder,  regulating 
the  heat  so  that  thirty  minutes  as  nearly  as  possible  shall  be 
required  for  the  distillation.  Mix  the  distillate  thoroughly  by 
pouring  back  and  forth  into  a  dry  beaker.  Filter  through  a  dry 
paper  into  a  100-c.c.  flask.  Pour  into  a  beaker,  add  a  few  drops 
of  phenolphthalein,  and  titrate  with  N/10  alcoholic  KOH  made  by 
diluting  the  half -normal  solution.  When  the  end-point  is  reached 
the  100-c.c.  flask  is  rinsed  and  the  titration  completed.  The  num- 
ber of  cubic  centimeters  of  N/10  alkali  used  multiplied  by  1.1  gives 
the  Reichert-Meissl  number. 

671.  The  Polenske  Value.— This  method  of  examining  butter- 
fat  has  come  into  use  for  the  detection  of  cocoanut-oil,  which  has 
been  largely  used  for  the  adulteration  of  butter.  The  Reichert- 
Meissl  value  has  been  generally  relied  on  to  detect  the  admixture 
of  foreign  fats  in  butter,  on  account  of  its  high  value  in  the  case 
of  butter-fat  and  the  low  value  given  by  other  fats,  except  cocoa- 
nut-oil,  which  gives  a  Reichert-Meissl  value  of  7  to  7.8,  so  that  a 
considerable  amount  of  this  oil  can  be  mixed  with  a  butter  having 
a  high  value,  30  for  instance,  producing  a  mixture  which  would 
have  a  Reichert-Meissl  value  above  the  legal  limit  of  20  to  24. 
Polenske  found  that  the  greater  part  of  the  volatile  fatty  acids 
of  butter  obtained  in  the  Reichert-Meissl  determination  are  solu- 
ble in  water,  while  the  reverse  is  true  of  cocoanut-oil.  The 
amount  of  the  soluble  and  insoluble  acids  is  obtained,  as  in  the 
Reichert-Meissl  determination,  by  titration  with  decinormal  caus- 
tic potash,  and  the  values  are  given  in  cubic  centimeters  of  the 
alkali.  For  butter-fat  the  insoluble  volatile  fatty  acids  required 
1.5  to  3.0  c.c.  of  the  decinormal  alkalies,  while  16.8  to  17.8  c.c. 
was  required  for  the  insoluble  volatile  fatty  acids  from  cocoanut- 
oil.  The  Polenske  value  varies  with  the  Reichert-Meissl  value, 
as  is  shown  in  the  table  on  page  459. 


442  ANALYSIS  OF  FATS  AND  OILS. 

672.  The  Determination  of  the  Polenske  Value  is  made  in  a 
manner  very  similar  to  the  Reichert-Meissl  determination.  It 
has  been  found  that  the  details  of  the  method  must  be  adhered 
to  very  strictly  in  order  to  obtain  concordant  results.  Five  grams 
of  the  butter-fat  are  placed  in  a  300-c.c.  flask  and  saponified  with 
caustic  soda  and  glycerine.  For  this  purpose  2  c.c.  of  a  caustic- 
soda  solution,  prepared  from  equal  parts  of  caustic  soda  and  water, 
and  20  grams  of  glycerine,  are  added,  and  the  flask  heated  over 
the  free  flame  until  saponification  is  complete.  The  solution  is 
allowed  to  cool  below  100°,  and  90  c.c.  of  water  added.  The  mass 
is  dissolved  by  warming  on  the  water-bath  to  50°  C.  The  solu- 
tion must  be  clear  and  colorless.  If  it  is  brown  it  has  been  over- 
heated during  the  saponification  and  must  be  rejected  and  the 
saponification  repeated.  50  c.c.  of  dilute  sulphuric-acid  solution 
containing  25  c.c.  concentrated  sulphuric  acid  per  liter  are  now 
added,  and  the  volatile  acids  distilled  off.  For  this  purpose  i 
gram  of  pumice  is  added.  This  pumice  must  be  broken  and  sifted 
so  as  to  include  only  pieces  of  0.5  to  1  mm.  diameter.  The  flask 
is  placed  on  copper  or  brass  gauze  of  0.5  mm.  mesh.  The  flask 
is  connected  to  an  upright  condenser  by  means  of  a  bulb  tube. 
The  bulb  is  immediately  above  the  stopper  of  the  flask,  and  the 
tube  passing  to  the  condenser  is  inclined  toward  the  bulb,  which 
must  be  70  mm.  from  the  condenser.  The  heat  is  regulated  so 
that  110  c.c.  are  distilled  off  in  19  to  20  minutes.  The  cooling 
water  must  flow  rapidly  enough  through  the  condenser  to  cool 
the  distillate  to  20°-23°  C.  as  it  drops  into  a  110-c.c.  flask,  which 
serves  as  a  receiver.  When  110  c.c.  have  distilled  over,  the  flask 
is  replaced  with  a  20-c.c.  measuring  cylinder. 

The  110-c.c.  flask  containing  the  distillate  is  immersed,  with- 
out shaking,  in  water  at  15°  C.  After  about  5  minutes  the 
neck  of  the  flask  is  tapped  lightly,  so  as  to  cause  the  drops  of  oil 
floating  on  the  surface  of  the  liquid  to  adhere  to  the  walls  of  the 
flask.  After  another  10  minutes  the  consistency  of  the  oily  drops 
is  noted.  If  the  butter  is  pure  and  has  a  high  Reichert-Meissl 
value,  the  drops  will  be  semi-solid  and  milky.  If  the  Reichert- 
Meissl  value  is  low,  or  10  per  cent  or  more  of  cocoanut-oil  is  pres- 
ent, the  insoluble  fatty  acids  do  not  solidify,  but  remain  clear 
and  oily.  The  contents  of  the  flask  are  mixed  by  inverting  it 


DETERMINATION  OF   THE  POLENSKE  VALVE.  443 

several  times  without  violent  shaking,  and  100  c.c.  filtered  through 
an  8-cm.  filter-paper  and  titrated  with  tenth-normal  caustic  potash. 
The  insoluble  acids  on  the  paper  are  washed  free  from  soluble  acids 
by  washing  with  three  15-c.c.  portions  of  water  which  have  been 
passed  through  the  condenser  tube  and  the  20-c.c.  cylinder  and 
the  110-c.c.  flask.  The  insoluble  acids  are  then  dissolved  in  neu- 
tral 90  per  cent  alcohol  by  passing  three  15-c.c.  portions  through 
the  condenser  tube,  the  20-c.c.  measuring  cylinder,  the  110-c.c. 
flask,  and  the  filter-paper.  Each  portion  is  allowed  to  drain 
completely  before  adding  a  fresh  portion.  The  combined  alco- 
holic solution  is  then  titrated  with  tenth-normal  alkali  to  obtain 
the  Polenske  value. 

673.  The  Hehner  Value  is  the  converse  of  the  Reichert  value, 
the  percentage  of  insoluble  acids  being  determined  by  saponifying 
the  fat,  adding  an  excess  of  mineral  acid  so  as  to  liberate  the  fatty 
acids,  then  filtering  off,  washing,  and  weighing  the  insoluble  portion. 

674.  The  Iodine  Value  of  oils  and  fats  is  obtained  by  allowing 
a  measured  amount  of  an  iodine  solution  to  act  on  the  oil  or  fat. 
It  has  been  found  that  a  considerable  amount  of  the  halogen  is 
absorbed  by  the  fatty  acids  present.     The  percentage  of  the  halo- 
gen absorbed  is  taken  as  the  iodine  value.     The  chemical  reactions 
which  take  place  are  not  fully  understood.     In  general  it  is  thought 
that  in  the  unsaturated  acids  the  double  bond  is  broken  and  the 
iodine  enters  at  this  point  into  the  molecule.     There  seems  to  be 
no  doubt,  however,  that  substitution  takes  place  to  a  limited 
extent,  an  atom  of  iodine  taking  the  place  of  an  atom  of  hydrogen, 
which  unites  with  another  atom  of  iodine,  forming  hydroiodic 
acid.     Solutions  of  bromine  have  been  used  in  the  same  manner, 
while  recently  solutions  of  iodine  monochloride  and  also  solutions 
of  iodine  monobromide  have  come  into  use.     The  results  are  almost 
universally  computed  as  percentage  of  iodine  absorbed.     Slightly 
different  values  are  obtained  by  the  use  of  these  different  solu- 
tions.   Strictly    comparable    results    can   be    obtained    only   by 
working  with  the  same  solutions  and  under  the  same  conditions, 
especially  as  to  the  time  during  which  the  halogen  is  allowed  to 
act  on  the  oil  or  fat. 

675.  Wijs*    Iodine    Monochloride    Solution. — For    the    deter- 
mination of  the  iodine,  the  Wijs  solution  of  iodine  is  the  most 


444  ANALYSIS  OF  FATS  AND  OILS. 

advantageous  for  use.  This  is  a  solution  of  iodine  monochloride 
in  glacial  acetic  acid.  The  acid  must  be  the  strongest  and  purest 
obtainable,  giving  no  reduction  with  sulphuric  acid  and  potas- 
sium dichromate.  The  percentage  of  acid  must  not  be  less 
than  9-9  and  can  be  most  readily  determined  by  taking  the  melt- 
ing-point which  varies  with  the  amount  of  water  present,  as  is 
evident  from  the  following  table: 

Per  cent  of  H.C2H302.       Melting-point. 

100  16.75 

99.5  15.65 

99  14.80 

97.09  11.95 

95.24  9.40 

The  test  is  made  by  half  filling  an  8-inch  test-tube  with  the 
acetic  acid.  This  test-tube  is  suspended  within  a  larger  test-tube 
by  means  of  a  cork,  through  which  the  smaller  tube  is  passed.  A 
delicate  and  accurate  thermometer  is  placed  in  the  acid,  which 
is  cooled  by  immersing  the  whole  in  ice-water  As  soon  as  the 
acetic  acid  begins  to  crystallize,  the  temperature  rapidly  rises  to 
the  true  melting-point  of  the  acid.  The  advantage  of  using  this 
solution  is  that  its  titre  remains  unchanged  for  months,  and  the 
time  necessary  for  a  determination  is  reduced  from  four  or  six 
hours  to  a  few  minutes. 

Thirteen  grams  of  pure  iodine  is  dissolved  in  a  liter  of  the 
acid.  The  exact  strength  is  then  determined  by  tit  rat  ion  against 
a  standard  thiosulphate  solution.  Chlorine  free  from  hydro- 
chloric acid  is  passed  into  the  solution  until  the  dark-brown  color 
changes  to  a  light-brown  or  yellow  color.  The  strength  should 
then  be  determined  again  by  titration  against  the  thiosulphate 
solution,  and  should  be  double  the  original  strength.  It  i|  advis- 
able, however,  to  have  a  slight  excess  of  iodine  present  rather 
than  of  chlorine. 

676.  Harms'  Monobromide  Solution.  —  The  Hanus  solution 
serves  the  same  purpose  as  the  Wijs  solution,  and  is  more  easily 
made.  Instead  of  passing  chlorine  into  the  acetic-acid  solution 
of  iodine,  liquid  bromine  is  added  until  the  titre  is  doubled.  About 
3.0  c.c.  will  be  found  sufficient. 


IODINE   VALVE.  445 

677.  Sodium-thiosulphate    Solution.  —  An   N/10    sodium-thio- 
sulphate  solution  is  made  up  and  standardized  by  means  of  re- 
sublimed  iodine.    This  solution  is  then  titrated  against  the  iodine 
solution.    For  this  purpose  20  c.c.  of  the  iodine  solution  are  meas- 
ured out  into  a  beaker,  100  c.c.  distilled  water  added,  and  1  gram 
potassium  iodide  dissolved  in  10  c.c.  water.     The  thiosulphate 
solution  is  then  added  until  a  faint  yellow  color  is  obtained,  the 
starch  solution  is  added,  and  the  titration  completed. 

678.  Determination  of  Iodine  Value. — To  determine  the  iodine 
value  of  an  oil,  from  0.15  to  0.2  gram  of  a  drying-oil,  0.3  to  0.5 
gram  of  a  non-drying  oil,  or  0.7  to  1  gram  of  a  solid  fat  is  weighed 
out  and  transferred  to  a  glass-stoppered  bottle.    The  most  con- 
venient method  of  weighing  out  oils  is  to  pour  a  suitable  amount 
into  a  small  beaker,  place  in  the  beaker  a  short  glass  tube,  and 
weigh  carefully.    By  means  of  the  glass  tube,  drop  some  of  the 
oil  into  the  bottle,  replace  the  tube  in  the  beaker,  and  weigh  again. 
The  oil  may  also  be  weighed  in  a  tared  homeopathic  shell  vial. 
The  vial  containing  the  oil  is  introduced  into  the  glass-stoppered 
bottle.    The  oil  or  fat  is  dissolved  in  10  c.c.  of  chloroform  and  25  to 
50  c.c.  of  the  iodine  solution  are  added.     The  iodine  must  be  pres- 
ent in  excess.     If  the  iodine  solution  is  very  much  decolorized  by 
the  oil,  more  must  be  added.    The  iodine  should  be  allowed  to  act 
for  fifteen  minutes  for  non-drying  oils,  thirty  minutes  for  semi- 
drying  oils,  and  one  hour  for  drying  oils.     A  blank  determination 
is  carried  on  at  the  same  time,  10  c.c.  of  the  chloroform  being 
treated  with  iodine  solution  for  the  same  length  of  time. 

After  the  iodine  has  acted  on  the  oil,  10  c.c.  of  a  10%  solution 
of  potassium  iodide  free  from  iodate  are  added  to  the  bottle,  and 
this  is  followed,  after  thorough  agitation,  by  about  100  c.c.  of 
distilled  water,  care  being  taken  that  all  of  the  iodine  solution  is 
washed  down  from  the  stopper,  neck,  and  walls  of  the  bottle. 
The  excess  of  iodine  is  then  titrated  with  thiosulphate.  The  num- 
ber of  cubic  centimeters  of  thiosulphate  used,  subtracted  from  the 
amount  required  for  the  blank,  represents  the  amount  of  iodine 
absorbed  by  the  oil.  The  equivalent  weight  of  iodine  thus  found 
is  divided  by  the  weight  of  oil  taken,  and  the  result  expressed  in 
per  cent  as  the  iodine  number. 


446  ANALYSIS  OF  FATS  AND  OILS. 

679.  Determination  of  Rosin  in  Shellac  by  A.  C.  Langmuir's 
Method.*  —  The  iodine  value  of  unbleached  shellac  varies  from  15  to 
18  per  cent,  while  that  of  rosin  varies  from  175  to  about  260.  It 
is  possible,  therefore,  to  ascertain  approximately  the  amount  of 
rosin  present  in  a  given  sample  of  shellac  by  a  determination  of 
the  iodine  number.  For  calculating  the  result,  18  is  used  as  the 
iodine  number  of  the  shellac,  and  228  as  that  of  the  rosin.  By 
using  these  figures  the  percentage  of  the  adulterant  which  is 
found  may  be  slightly  lower  than  that  actually  present.  This 
is  in  accord  with  the  accepted  commercial  principle  that  the 
inaccuracies  in  a  method  of  analysis  shall  favor  the  seller  rather 
than  the  buyer,  by  assigning  the  lowest  possible  value  to  the 
amount  of  impurities  present.  The  calculation  is  made  accord- 
ing to  the  formula 

A-M 


where    ?/  =  per  cent  of  rosin; 

M  =  iodine  number  of  shellac; 
N=      "          "         "  rosin; 
A=      "          "        "  mixture. 

Substituting  the  fixed  values  of  M  and  N,  the  formula  becomes 

A-18 


y-100 


210 


The  iodine  number  is  determined  by  means  of  the  Wijs  solu- 
tion. Exactly  0.2  gram  of  the  ground  sample  is  introduced  into 
a  dry  250-c.c.  glass-stoppered  bottle.  20  c.c.  of  glacial  acetic 
acid  is  introduced  and  warmed  until  the  shellac  is  dissolved,  with 
the  possible  exception  of  a  little  wax.  10  c.c.  of  chloroform  is 
then  added  and  the  solution  cooled  to  21°-24°  C.  This  tem- 
perature must  be  maintained  during  the  remainder  of  the  test. 
20  c.c.  of  the  Wijs  solution  is  introduced  by  means  of  a  pipette. 
The  bottle  is  allowed  to  stand  in  water  having  a  temperature  of 
22°  to  23°  for  exactly  1  hour. 

*Ber.,  31,  750.  J.  Soc.  Chem.  Ind.,  17,  609.  Jour.  Am.  Chem.  Soc.,  29, 
1221. 


ACETYL   VALUE.  447 

Pure  shellac  will  scarcely  alter  the  color  of  the  Wijs  solution. 
If  rosin  is  present,  a  reddish-brown  color  will  be  produced,  the 
depth  of  which  depends  on  the  amount  of  rosin  present.  After 
standing  1  hour,  10  c.c.  of  a  10  per  cent  potassium-iodide  solution 
is  added.  The  excess  of  iodine  is  immediately  determined  by 
means  of  a  tenth-normal  solution  of  sodium  thiosulphate.  When 
the  iodine  color  is  nearly  gone,  a  little  starch  is  added  and  the 
titration  completed.  Any  color  returning  after  a  half -minute 
is  disregarded. 

On  account  of  the  large  coefficient  of  expansion  of  the  Wijs 
solution,  a  blank  determination  should  be  made  with  that  of  the 
shellac.  The  determination  is  carried  out  in  exactly  the  same 
manner,  with  the  exception  of  the  addition  of  the  shellac. 

680.  Acetyl  Value. — This  permits  the  identification  of  castor- 
oil,  grapeseed-oil,  and  of  oxidized  fish  oils  in  mixtures.  Besides 
this,  many  fish  oils  show  a  comparatively  high  acetyl  value. 
The  test  is  founded  on  the  action  of  acetic  anhydride  upon  alco- 
holic hydroxyls  as  they  exist  in  the  oxyacids,  as  is  shown  in  the 
following  reaction: 

C17H32(OH)COOH  +  (C2H30)20 = C17H32(OC2H30)COOH + HC2H302. 

Upon  saponification  of  the  acetylated  fatty  acid  thus  produced, 
the  hydroxyl  is  again  replaced  and  the  acetyl  group  liberated  as 
an  acetate.  The  amount  of  acetic  acid  liberated  may  be  deter- 
mined, giving  the  acetyl  value.  Benedikt  first  proposed  this 
method,  operating  on  the  fatty  acids,  but  the  process  was  modified 
by  Lewkowitsch,  who  works  on  the  oils  or  fats  directly,  giving 
more  exactly  the  true  content  of  hydroxy  acids. 

68 1.  Determination  of  Acetyl  Value. — The  method  adopted  by 
the  Association  of  Official  Agricultural  Chemists  is  carried  out  as 
follows:  Boil  a  portion  of  the  oil  or  fat  with  an  equal  volume  of 
acetic  anhydride  for  two  hours  and  pour  the  mixture  into  a  large 
beaker  containing  500  c.c.  of  water,  and  boil  for  half  an  hour.  To 
prevent  bumping,  a  slow  current  of  carbon  dioxide  is  passed  into 
the  liquid  through  a  finely  drawn-out  tube,  reaching  nearly  to  the 
bottom.  Allow  the  mixture  to  separate  into  two  layers,  siphon 
off  the  water,  and  boil  the  oily  layer  with  fresh  water  until  it  is 
no  longer  acid  to  litmus-paper.  The  acetylated  fat  is  then  sepa- 
rated from  the  water,  filtered,  and  dried  in  a  drying-oven. 


448  ANALYSIS  OF  FATS  AND  OILS. 

Weigh  from  2  to  4  grams  of  the  acet-ylated  fats  into  a  flask  and 
saponify  with  alcoholic  potash,  as  in  the  determination  of  the 
saponification  number.  If  the  distillation  process  is  to  be  adopted 
it  is  not  necessary  to  work  with  a  standardized  alcoholic-potash 
solution.  In  case  the  filtration  method  is  used,  which  will  be 
found  much  shorter,  it  is  necessary  that  the  alcoholic  potash  be 
measured  exactly.  In  either  case  evaporate  the  alcohol  after 
saponification  and  dissolve  the  soap  in  water.  Now  two  pro- 
cedures are  possible,  either  distillation  or  filtration. 

(a)  DISTILLATION  PROCESS. — Acidify  with  dilute  sulphuric  acid 
(1-10)  and  distil  the  liquid,  as  in  the  Reichert  test.     As  several 
hundred  cubic  centimeters  must  be  distilled,  either  a  current  of 
steam  is  run  through  or  portions  of  water  are  added  from  time 
to  time.     From  500  to  700  c.c.  of  distillate  will  be  found  suffi- 
cient.    Filter  the  distillates  to  remove  any  insoluble  acids  carried 
over  by  the  steam,  and  titrate  the  filtrate  with  decinormal  potas- 
sium hydroxide,  using  phenolphthalein  as  the  indicator.     Multiply 
the  number  of  cubic  centimeters  of  alkali  employed  by  5.61,  and 
divide  by  the  weight  of  substance  taken.     This  gives  the  acetyl 
value,  which  is  the  number  of  milligrams  of  KOH  required  to 
neutralize  the  acetic  acid  liberated  from  one  gram  of  the  acetyl- 
ated  fatty  acid. 

(b)  FILTRATION  PROCESS. — Add  to  the  soap  solution  a  quantity 
of   the   standard    sulphuric   acid   exactly   corresponding   to   the 
amount  of  alcoholic  potash  added,  warm  gently,  and  the  free  fatty 
acids  will  collect  on  top.     Filter  off  the  liberated  fatty  acids,  wash 
with  boiling  water  until  the  washings  are  no  longer  acid,  and 
titrate  the  filtrate  with  decinormal  potassium  hydroxide,  using 
phenolphthalein  as  the  indicator.    Calculate  the  acetyl  value  as 
before. 

PHYSICAL   TESTS. 

Besides  the  constants  obtained  by  strictly  chemical  quantita- 
tive means,  there  are  a  few  physical  tests  that  are  extremely  im- 
portant and  widely  used,  which  it  seems  advisable  should  be 
mentioned  under  the  head  of  oil-analysis. 

682.  Specific  Gravity.  —This  may  be  very  accurately  obtained 
by  means  of  a  Westphal  balance  as  described  on  p.  275. 


PHYSICAL  TESTS.  449 

If  the  oil  is  viscous,  however,  a  pycnometer  must  be  used  as 
described  on  p.  274. 

683.  Index  of  Refraction. — This  is   found   by   means   of  an 
instrument  called  a  refractometer,  of  which  there  are  several  on 
the  market.     The  Zeiss  butyro-refractometer,  one  of  the  simplest 
of  these,  is  also  one  of  the  most  efficient.     The  oil  or  fat  is  intro- 
duced as  a  thin  film  between  two  halves  of  a  Nicol's  prism,  which 
are  enclosed  in  a  hollow  metal  jacket  through  which  water  is 
allowed  to  flow,  maintaining  a  constant  temperature.     The  degree 
of  diffraction  is  indicated  on  an  arbitrary  scale  by  a  shadow  which 
is  observed  through  a  telescope  attached  to  the  prism,  and  this 
reading,  by   comparison  with   a  table,  made  especially   for  the 
instrument,  is  converted  into  the  refractive  index. 

The  Abbe  refractometer  is  better  suited  for  general  use,  being 
constructed  with  prisms  and  water-jackets  like  the  Zeiss  butyro- 
refractometer,  but  instead  of  an  arbitrary  scale  fixed  in  the  tele- 
scope, the  border  of  a  shadow,  visible  through  the  latter,  is 
brought  to  the  intersection  of  cross-hairs  by  moving  an  arm  at  the 
side  of  the  instrument,  and  the  refractive  index  may  then  be  read 
directly  on  a  scale  along  which  the  movable  arm  slides. 

684.  Maumene  Number  and  Specific  Temperature  Reaction. — 
It  is  found  that  by  mixing  various  oils  with  concentrated  sul- 
phuric acid  a  reaction  ensues,  causing  a  rise  of  temperature  in  the 
mixture,  which  for  the  same  oil  under  the  same  conditions  and 
with  the  same  concentration  of  acid  is  a  constant.      In  order  to 
bring   more   uniformity   into   the   results   obtained   by   different 
operators  working  under  different  conditions  and  using  different 
strengths  of  acid,  the  old  Maumene  test  has  been  modified  with 
considerable  success.     Maumene  mixed  50  grams  of  oil  with  10  c.c. 
of  concentrated  acid  in  a  small  beaker  sunk  in  a  larger  vessel 
filled  with  cotton  or  some  other  insulating  material.     Before  mix- 
ing, both  oil  and  acid  were   at  the  same   temperature,  as  close 
to  20°  as  possible.     This  temperature  was  noted,  and,  after  com- 
bining, the  highest  point  registered  by  the  thermometer  used  for 
stirring  the  mass  was  observed,  which,  minus  the  initial  tempera- 
ture,   gave    the   Maumene   number.     The   errors   introduced   by 
different  degrees  of  insulation  and  strength  of  acid  have  been 
eliminated  by  conducting  another  test  exactly  as  the  one  just 


450  ANALYSIS  OF  FATS  AND  OILS. 

described,  using  the  same  apparatus  and  the  same  acid,  but  sub- 
stituting 50  c.c.  of  pure  water  for  the  oil.  Then,  by  the  following 
formula : 

100A 

&••    B 

in  which  S  =  specific  temperature  number, 
^4.=  Maumene  number, 

B--=  rise  of  temperature  obtained  with  pure  water  under 
the  same  conditions  as  those  used  in  the  Maumene 
test, 

results  are  obtained  which  compare  admirably,  provided  the  acid 
used  is  fairly  concentrated — between  95  and  99  per  cent.* 

685.  The  determination  is  carried  out  as  follows:  f  A  beaker,  5 
inches  by  H  inches,  is  placed  inside  another,  6  inches  by  3  inches, 
and  a  wet  mixture  of  asbestos  and  plaster  of  Paris  tightly  packed 
around  the  inner  one.  This,  when  dried,  makes  a  hard,  solid 
packing  which  radiates  heat  very  slowly. 

Remove  the  inner  beaker,  weigh  into  it  50  grams  of  fat  or  oil, 
and  note  the  temperature  carefully.  Then  from  a  pipette  which  will 
deliver  it  in  approximately  one  minute  add  10  c.c.  of  96-98% 
sulphuric  acid,  which  is  at  the  same  temperature  as  the  oil.  While 
the  acid  is  being  introduced,  stir  the  oil  and  acid  with  an  accurate 
thermometer.  Then  hold  the  thermometer-bulb  carefully  in  the 
centre  of  the  mixture,  and  when  the  mercury  reaches  the  highest 
point  note  the  reading.  It  is  easy  to  determine  this  point,  as  the 
column  of  mercury  remains  stationary  for  some  time.  It  is  neces- 
sary to  take  care  not  to  read  the  temperature  too  soon,  as  some 
oils  take  considerable  time  to  reach  their  maximum  point.  The 
difference  between  the  initial  reading  and  the  final  reading,  ex- 
pressed in  degrees  centigrade,  gives  the  Maumene  number. 

In  order  to  get  the  specific  temperature  number  the  operation 
is  repeated  with  the  same  apparatus  and  the  same  acid,  using 
50  c.c.  of  pure  water  in  place  of  the  oil.  It  is  advisable  in  this 
case  to  cover  the  beaker  with  a  piece  of  cardboard  or  heavy  paper 

*  References:  Journal  Soc.  Chem.  Ind.,  1191,  10,  233;  Jour.  Amer.  Chem. 
Soc.,  1902,  24,  266. 

t  Mien,  Com.  Org.  Anal.,  3d  Ed.,  Vol.  II,  Pt.  1,  p.  76. 


PHYSICAL  TESTS.  451 

twice  the  diameter  of  the  latter,  the  thermometer  being  thrust 
through  a  small  hole  in  the  centre.  This  interferes  with  the 
evaporation  of  the  water  and  the  consequent  fall  of  the  tempera- 
ture of  the  mixture,  but  if  employed  should  be  used  with  the  oils 
also. 

686.  Melting-point   of  Fatty  Acids. — This  test  is  much  used  in 
corroborating  results  obtained  by  other  methods  when  the  ques- 
tion of  identity  or  purity  of  an  oil  or  fat  arises.     The  sample  is 
saponified  by  boiling  with  an  excess  of  alcoholic  potash,  and  the 
fatty  acids  liberated,  as  in  the  Hehner  method,  with  dilute  sul- 
phuric acid  and  washed  with  warm  water  until  the  washings  show 
a  neutral  reaction.     The  melted  fatty  acids  are  then  drawn  into  a 
very  thin-walled  capillary  tube  1  or  2  inches  long  according  to  the 
length  of  the  bulb  of  the  thermometer  used.     The  capillary  is 
sealed  about  one-fourth  inch  above  the  filled  portion  by  holding 
in  the  outer  mantle  of  a  Bunsen  flame  and  drawing  out  when  the 
glass  softens.     The  tube  thus  prepared,  after  cooling  in  a  refrig- 
erator, is  fastened  to  the  bulb  of  the  thermometer  by  an  elastic 
band  and  immersed  in  a  glycerine  bath,  which,  to  secure  a  slower 
and  more  even  rise,  may  in  turn  be  surrounded  by  a  second  bath 
of  the  same  material  which  is  directly  heated  by  a  Bunsen  flame. 
By  regulating  the  flame  the  rise  of  temperature  may  be  readily 
controlled.     A  slow  rotary  motion  is  maintained  by  the  ther- 
mometer with  its  capillary  until  the  fatty  acids  become  trans-- 
parent, when  the    temperature  is  noted.      It  is  almost  always 
necessary  to  conduct  three  tests;  the  first  to  get  the  approximate 
temperature,  after  which  the  heating  may  be  done  very  slowly  as 
the  melting-point  approaches,  and  the  average  of  the  two  last 
determinations  taken  as  the  true  point. 

687.  Detection  of  Phytosterol  and  Cholesterol. — Fats  and  oils 
of  vegetable  origin  may  be  detected  in  those  derived  from  animals 
by  the  presence  of  phytosterol  and,  vice  versa,  animal  products 
in  vegetable  oils  and  fats  by  the  presence  of  cholesterol.     Boil 
50  grams  of  the  fat  or  oil  in  a  flask  having  a  reflux  condenser,  with 
75  c.c.  of  95%  alcohol,  for  five  minutes,  and  separate  the  alcoholic 
solution.     Repeat  with  another  portion  of  alcohol  and  separate. 
Mix  the  alcoholic  solution  with  15  c.c.  of  30%  sodium  hydroxide, 


452  ANALYSIS  OF  FATS  AND  OILS. 

and  boil  in  a  flask  with  an  air  condenser  until  one-fourth  of  the 
alcohol  is  evaporated.  Evaporate  nearly  to  dryness  in  a  porce- 
lain dish  and  extract  the  residue  with  successive  portions  of  ether. 
Evaporate  the  ethereal  solution  to  dryness,  take  up  with  a  little 
ether,  filter,  again  evaporate,  dissolve  in  hot  95%  alcohol,  and 
allow  to  crystallize.  Examine  a  few  of  the  crystals  under  a  micro- 
scope. Phytosterol  thus  prepared  appears  as  tufts  of  needles, 
cholesterol  as  thin  rhombic  tables.  The  melting-point  of  phy- 
tosterol  is  130°-137.5°,  that  of  cholesterol  146°. 

The  melting-point  is  better  determined,  however,  on  the  acetates 
of  these  alcohols.  The  crystals  obtained  as  described  are  heated 
in  a  dish,  with  2-3  c.c.  of  acetic  anhydride,  over  a  small  flame  until 
it  boils,  the  dish  being  covered  with  a  watch-glass.  The  watch- 
glass  is  then  removed  and  the  excess  of  acetic  anhydride  evapo- 
rated off  on  the  water-bath.  The  contents  of  the  dish  are  next 
heated  with  the  smallest  quantity  of  absolute  alcohol,  and  in 
order  to  prevent  immediate  solidification  or  crystallization,  a  few 
c.c.  of  alcohol  are  added,  and  the  mass  allowed  to  crystallize  by 
spontaneous  evaporation  of  the  alcohol,  of  which  one-half  or  a 
third  is  allowed  to  volatilize.  The  crystals  are  filtered  off  through 
a  small  filter  and  washed  with  a  little  95%  alcohol.  They  are 
then  dissolved  in  5-10  c.c.  of  hot  absolute  alcohol  and  again  allowed 
to  crystallize.  These  crystals  are  filtered  off,  pressed  dry,  and 
their  melting-point  determined.  Cholesteryl  acetate  melts  at 
114.3°  to  114.8°  (corr.),  and  phytosteryl  acetate  at  125.6°  to  137° 
(corr.).  In  doubtful  cases  the  crystals  should  be  redissolved  in 
hot  absolute  alcohol  and  recrystallized  at  least  three  times  before 
obtaining  the  melting-point. 

688.  Flash  Point. — This  is  the  temperature  at  which  the  vapors 
given  off  by  the  oil  may  be  ignited,  producing  a  flash  when  brought 
in  contact  with  a  small  flame.  This  test  is  made  in  order  to  deter- 
mine the  temperature  at  which  the  use  of  the  oil  involves  the 
danger  of  fires  or  explosions.  The  flash  point  varies  with  the 
form  of  apparatus  and  the  method  of  making  the  test,  so  that 
the  conditions  must  be  rigidly  fixed  in  order  to  obtain  constant 
results.  Various  forms  of  apparatus  have  been  devised,  all  of 
which  embody  a  few  essential  features,  such  as  a  cup  to  hold  the 
oil,  some  device  for  gradually  raising  the  temperature  of  the  oil, 


PHYSICAL   TESTS.  453 

a  thermometer  to  indicate  the  temperature,  and  a  small  flame 
to  be  used  for  igniting  the  vapor.  A  cover  containing  two  open- 
ings, one  for  the  thermometer  and  another  at  which  to  apply 
the  test-flame,  is  sometimes  provided.  Such  an  instrument  is 
called  a  closed  tester,  while  an  instrument  without  a  cover  is 
called  an  open  tester.  The  open  testers  give  higher  readings 
than  the  closed  testers.  More  accurate  determinations  can  be 
made  with  the  closed  testers. 

689.  Oil  Testers. — A  simple  tester  may  be  made  from  ordi- 
nary laboratory  apparatus.     A  shallow  porcelain  crucible  of  about 
100  c.c.  capacity  will  serve  as  the  cup  to  contain  the  oil.     It  may 
be  heated  on  a  sand-bath,  or  it  may  be  placed  in  a  circular  opening 
in  a  piece  of  asbestos  board  and  heated  over  a  Bunsen-burner 
flame.    A  thermometer  is  suspended  in  the  oil,  which  is  then  slowly 
heated  while  a  small  test-flame  is  passed  over  the  surface  of  the 
oil.    The  test-flame  may  be  produced  by  drawing  out  a  piece 
of  glass  tubirg  to  a  fine  tube  and  attaching  it  to  the  gas-supply 
by  means  of  rubber  tubing.     The  temperature  is  noted  at  the 
moment  a  flash  is  produced.     After  the  oil  has  cooled  some- 
what, the  test  is  repeated,  the  oil  being  heated  more  slowly  as 
the  temperature  approaches  the  point  at  which  the  oil  flashes. 
By  placing  a  metallic   or  glass  cover  containing  two  openings 
over  the  crucible,  it  can  be  used  as  a  closed  tester. 

Tagliabue's  tester  is  constructed  entirely  of  copper  and  con- 
tains two  cups.  The  large  one  serves  as  a  receptacle  for  a  non- 
volatile oil  in  which  the  smaller  cup  containing  the  oil  to  be 
tested  is  immersed.  The  heat  is  applied  to  the  larger  cup.  By 
this  device  the  temperature  of  the  oil  in  the  inner  cup  is  raised 
more  uniformly.  In  a  similar  form  of  tester  known  as  the  "  Say- 
bolt,"  the  test-flame  is  replaced  by  a  spark  of  electricity  produced 
by  an  induction-coil. 

690.  Viscosity  is  that  property  of  a  liquid  which  determines 
the  relative  rate  of  flow  under  fixed  conditions.     It  is  usually 
determined  by  noting  the  time  required  by  a  definite  volume  of 
oil  at  a  definite  temperature  to  flow  through  a  small  aperture. 
Water  is  usually  taken  as  the  standard.     The  simplest  form  of 
apparatus  is  that  used  in  the  Pennsylvania  and  other  railroad 
laboratories.     A  100-c.c.  pipette,  having  a  bulb  1.5  to  1.75  inches 


454  ANALYSIS  OF  FATS  AND  OILS. 

in  diameter  and  about  4J  inches  long,  is  graduated  so  that  the 
100-c.c.  mark  is  just  at  the  bottom  of  the  bulb.  The  opening  of 
the  pipette  is  then  made  of  such  a  size  that  100  c.c.  of  water 
at  100°  F.  will  run  out  of  the  pipette  in  34  seconds.  After  dry- 
ing the  pipette,  the  oil  to  be  tested  is  warmed  to  100°  and  100  c.c. 
is  drawn  up  into  the  pipette.  The  time  required  for  the  oil  to 
flow  out  is  noted.  This  gives  the  viscosity  of  the  oil.  It  is  also 
becoming  customary  to  take  the  ratio  between  the  number  of 
seconds  required  by  the  water  and  the  oil  as  the  viscosity. 

A  considerable  number  of  instruments  have  been  devised  for 
this  determination,  Engler's,  Tagliabue's,  Redwood's,  and  Gibbs' 
being  standard  instruments.  All  of  these  instruments  contain 
a  cup  having  a  small  opening  at  the  bottom,  through  which  the 
oil  is  allowed  to  flow.  A  jacket  is  generally  provided  which  can 
be  filled  with  water  or  oil  and  heated,  thus  maintaining  the  oil 
at  a  constant  temperature.  The  oil  is  delivered  into  a  flask  of 
known  capacity. 

691.  Oil  Analysis. — In   the  examination   of   an   unknown   oil 
or  fat,  the  first  test  to  be  made  is  the  determination  as  to  whether 
the  oil  is  of  mineral  or  organic  origin,  or  a  mixture  of  these  two 
classes  of  oils.     For  this  purpose  advantage  is  taken  of  the  fact 
that  organic  fats  and  oils  can  be  saponified,  while  mineral  oils 
and  waxes  cannot  be  converted  into  a  soap,  but  can  be  extracted 
with  ether,  while  the  soap  produced  from  the  organic  fats  and 
oils  is  insoluble  in  ether.     On  evaporating  off  the  ether,  the  mineral 
oil  can  be  recovered  in  pure  condition.     On  acidifying  the  water 
solution  of  the  soap,  the  fatty  acids  of  the  vegetable  or  animal 
oils  separate  out  as  an  oily  layer.     As  the  fatty  acids  represent 
about  95%  of  most  animal  or  vegetable  oils,  the  weight  of  these 
acids  can  be  used  to  calculate  the  amount  of  these  constituents. 
A  determination  of  the  chemical  and  physical  constants  of  the 
fatty  acids  can  be  made  for  the  identification  of  the  oils  present. 
If  mineral  oils  are  absent,  these  determinations  are  made  on  the 
original  oil. 

692.  The  Saponification  is  carried  out  in  the  same  manner  as 
the  determination  of  the  Kottstorfer  value  (page  439).     Stand- 
ard solutions  of  the  caustic  potash  and  acid  are  not  necessary. 


PHYSICAL  TESTS.  455 

60  grams  of  caustic  potash  are  dissolved  in  one  liter  of  95%  alco- 
hol. 5  to  10  grams  of  the  fat  or  oil  to  be  tested  are  weighed  out 
into  an  Erlenmeyer  flask  and  50  to  75  c.c.  of  the  alcoholic  caustic 
potash  added.  The  flask  is  heated  on  the  water-bath  with  a 
return  condenser  until  the  oil  is  saponified.  75  c.c.  of  water  is 
then  added  and  the  heating  continued  without  the  condenser 
until  the  alcohol  has  been  expelled.  The  soap  solution  is  trans- 
ferred to  a  separatory  funnel  and  75  c.c.  of  ether  added.  The 
funnel  is  corked  and  the  contents  thoroughly  shaken  and  then 
allowed  to  stand  quietly  until  the  upper  ether  solution  of  the 
unsaponified  or  mineral  fat  is  completely  separated  from  the 
lower  water  solution  of  the  soap  formed  from  the  animal  or  vege- 
table fat.  The  lower  layer  is  drawn  off  into  a  beaker,  the  stop- 
cock being  closed  when  the  last  drop  of  the  water  solution  has 
passed  out.  A  little  ether  may  be  allowed  to  pass  out  with  the 
water  solution,  but  none  of  the  latter  must  re  allowed  to  remain 
with  the  ether.  The  ethereal  solution  is  drawn  off  into  a  small 
weighed  flask  and  the  water  solution  returned  to  the  funnel  and 
the  extraction  with  ether  repeated.  Three  or  four  extractions 
with  ether  are  generally  sufficient  to  remove  all  the  unsaponified 
fat  or  oil.  The  last  portions  of  ether  should  leave  no  residue 
when  allowed  to  evaporate  on  a  watch-crystal.  The  flask  contain- 
ing the  ethereal  solution  is  connected  with  an  inclined  condenser 
and  the  ether  distilled  off  over  a  water-bath,  or,  still  better,  over 
an  electric-light  bulb.  As  the  ether  vapor  is  both  inflammable 
and  explosive  when  mixed  with  air,  a  flame  should  not  be  brought 
near  vessels  containing  ether.  The  residue  remaining  after  dis- 
tilling off  the  ether  is  dried  on  the  water-bath,  or  at  a  lower  tem- 
perature if  volatile  oils  are  present  and  weighed.  This  gives  the 
weight  of  mineral  oil  present. 

693.  Determination  of  the  Fatty  Acids. — The  aqueous  solution 
of  the  soap  is  acidified  with  hydrochloric  acid  and  heated  on  the 
water-bath  until  the  fatty  acids  form  a  clear  layer  on  the  water 
solution.  The  oily  layer  is  filtered  off  on  a  wet  filter-paper  which 
has  been  dried  for  one  hour  in  the  steam-oven  together  with  a 
small  beaker  and  weighed.  The  oil  is  washed  with  hot  water 
without  allowing  the  funnel  to  become  more  than  half  empty. 
When  the  chlorides  have  been  completely  washed  out,  the  paper 


456  ANALYSIS  OF  FATS  AND  OILS. 

containing  the  fatty  acids  is  transferred  to  the  beaker  which  was 
weighed  with  the  paper,  and  the  whole  placed  in  the  steam-oven 
to  dry.  After  one  hour  it  is  weighed  and  again  dried  and  weighed 
until  the  weight  is  constant.  In  order  to  determine  what  organic 
oils  are  present  in  the  sample  being  analyzed,  a  determination 
of  the  various  constants  of  the  fatty  acids  is  made. 

694.  Identification  of  Oils. — For  the  identification  of  an  un- 
known organic  oil,  the  most  useful  constant  is  the  iodine  value, 
on  account  of  the  great  difference  in  this  value  for  various  oils. 
When  two  or  more  oils  have  the  same  iodine  value  as  given  in  the 
table,  page  457,  other  constants  must  be  determined.  An  iodine 
value  of  103,  for  example,  might  indicate  cottonseed,  mustard, 
rape,  or  sesame  oil.  A  saponification  value  of  193  would  show 
that  the  oil  must  be  cottonseed  or  sesame  oil,  while  an  acetyl  value 
of  16.6  would  indicate  that  the  oil  must  be  cottonseed.  This 
conclusion  could  be  confirmed  by  qualitative  color  tests,  of  which 
some  very  excellent  ones  are  known. 


CONSTANTS. 


457 


IODINE  VALUE  OF  OILS. 


By  Hiibl's 
Solution. 

By  Wijs's 
Solution. 

ByHanus 
Solution 

Authority. 

Butter  

35.3 

36.2 

35.3 

Tolman  and  Munson 

a 

34.8 

35.9 

35.4 

u          n        tt 

u 

30  7 

30  6 

Hanus 

It 

19  5  to  38  9 

Cocoanut  

8  93 

9  05 

8  6C 

Tolman  and  Munson 

« 

9  03 

9  0? 

Hanus 

(i 

8  to  9  5 

Corn  

119  0 

123  2 

120  2 

Tolman  and  Munson 

(i 

119  0 

122  2 

119  6 

(t          tt        tt 

ii 

123  3 

129  2 

126  0 

(i          tt        tt 

ii 

124.8 

128.4 

Wijs 

u 

111  2  to  122  9 

Cottonseed-oil.  .  .  . 
ii 

ii 
u 

Konut  

103.8 
106.2 
104.8 
100.  9  to  116.  9 
6.09 

105.3 
107.3 
106.2 

6  43 

105.2 
107.8 
106.7 

6  40 

Tolman  and  Munson 
«          tt        ti 

tt          tt        tt 
tt          tt        u 

Lard-oil. 

73  7 

74  5 

73  9 

(t          tt        tt 

i  < 

69  3 

70  5 

69  8 

tt          tt        it 

ii 

56  4 

56  9 

TTami5? 

ii 

50  4  to  77  28 

Linseed-oil  

(Decreases  to  22 
169.8 

with  age") 
186  5 

184  5 

Tolman  and  Munson 

ii 

179  5 

188  7 

183  7 

(t          tt        tt 

ii 

174  8 

177  3 

m* 

Hunt 

<i 

180.9 

182  1 

Wijs 

iC 

170  2 

mn 

II 

171  to  188 

Magnolia-oil.  . 

81  7 

79  4 

78  Q 

u 

76  1 

75  6 

74  0 

tt            tt          tt 

Mustard-oil.  .    . 

110  4 

118  5 

115^ 

tt            tt          tt 

(i 

113  0 

118  2 

11fi  8 

tt            tt          tt 

a 

98  4 

104  3 

103  S 

tt            tt          tt 

tt 

103  5 

112  5 

m9 

tf            tt          tt 

(( 

106  4 

117  3 

114  8 

tt            tt          tt 

a 

92.1  to  106  57 

Oleo-oil 

42  6 

43  ^ 

43  3 

tt            tt          tt 

Oleomargarine  .  .  . 
u 

53.6 

52.8 

53.5 
53.7 

52.3 
52.2 

tt            tt          tt 

t            «           t 

458 


ANALYSIS  OF  FATS  AND  OILS. 


IODINE  VALUE  OF  OILS  *— Continued. 


By  Hiibl's 
Solution. 

By  Wijs's 
Solution 

By  Hanus' 
Solution. 

Authority. 

Oleomargarine  

52.5 

52  9 

52  0 

Tolman  and  Munson 

66.3 

66  0 

64  8 

ii          u        u 

Olive-oil  

79.2 

79  9 

80  6 

it          ti        n 

80  5 

80  9 

81  6 

it          n        it 

it 

84  8 

86  7 

86  5 

ti          tt        tt 

ti 

89  8 

91  4 

90  0 

ii          tt        tt 

"  av.  36  samples  . 

84.1 
77.28  to  91  5 

85.3 

84.7 

Peanut-  or  Arachis-oil 
a        it            ii 

96.3 
94.5 

88.3 

99.0 
95.2 

97.4 
94.1 

88.4 

Tolman  and  Munson 
it          u        it 

Hanus 

ti        ti            ii 

87.2 

87.2 

Wijs 

ti        u            it 
ii        it            n 

Poppv.  . 

91.8 
85.  6  to  105 
133.4 

93.4 
135  2 

91.6 
132  9 

Hanus 
it          tt        tt 

a 

134.9 

139  1 

138  4 

u          tt        tt 

tt 

122.4 

122  6 

Hanus 

it 

134.6 

135  2 

it 

u 

119  6 

119  6 

Wijs 

it 

132.6  to  143  3 

Rape 

101  3 

105  7 

105  2 

Tolman  and  Munson 

tt 

100  2 

104  1 

102  8 

it          it        it 

u 

99  3 

98  8 

Hanus 

tt 

102.9 

103.3 

Wijs 

ii 

103.0 

102.1 

101.9 

Hunt 

tt 

97  to  106 

Sesame  

106.4 

107.0 

106  5 

Tolman  and  Munson 

it 

110.3 

111.7 

Wijs 

ii 

107.1 

107.5 

Hanus 

it 

102.7  to  112 

Sunflower  

106.4 

109  2 

107.7 

Tolman  and  MunsoD 

117.8 

119.0 

Wijs 

it 

119.7  to  135 

*  For  other  iodine  values  and  other  constants,  see  Van  Nostrand's  Chemical 
Annual. 


CONSTANTS. 


459 


REICHERT-MEISSL-VALUE. 

c.c.  N/10  KOH. 


c.c.  N/10  KOH. 


Butter 19.8-33.5 

Legal  minimum  in  Great 
Britain,    Germany,    and 

France 24.0 

Legal  minimum  in  Sweden.  .  .  23.0 

"  Italy 20.0 

Rabbit-fat..  5.6 


Cocoanut-oil  * 7.0-7.8 

Margarine 2.6 

Oleomargarine 0.8.-0.9 

Palm-oil 5.0 

Wool-fat , 8.0 

Beef-marrow 2.0 

Turkey-fat 5.6 


POLENSKE  VALUE  OF  BUTTER-FAT. 

VALUES   FOUND   BY   E.    POLENSKE.  VALUES  FOUND  BY  M.  FRITZSCHE 

Reichert-  Normal  Maximum  ON  DUTCH  BUTTER. 


Meissl  Value. 
c.c  .C/loKOH. 

20-21 

Pol.  Value. 
C.c.  N/10  KOH. 

1.3-1.7 

Limit, 
c.c.  >  /1  0  KOH. 

2.1 

V^il 

Reichert- 
Meissl  Value, 
c.c.  N/10  KOH. 

1      .LHJ   J.  \_/-LJ.     13  \J  ±  J 

Normal 
Pol.  Value. 
c.c.  N/1U  KOH. 

..EJXV. 

Maximum 
Limit, 
c.c.  N/10  KOH. 

21-22 

1.4-1.8 

2.2 

30-31 

2.1-3.2 

3.2 

22-23 

1.5-1.9 

2.3 

31-32 

1.9-3.2 

3.2 

24-45 

1.7-1.8 

2.3 

32-33 

2.1-3.4 

3.4 

25-26 

1.8-1.9 

2.4 

33-34 

2.5-2.9 

3.4 

26-27 

1.9-2.0 

2.5 

27-28 

2.0-2.2 

2.7 

28-29 

2.2-2.5 

3.0 

29-30 

2.5-3.0 

3.5 

KOTTSTORFER  SAPONIFICATION  VALUE. 

Mg.  KOH  per  Gram.  Mg.  KOH  per  Gram. 


Butter 216-233 

Oleomargarine 192-200 

Cocoanut-oil 246-270 

Lard 193-200 

Olive-oil 187-195 

Niger-oil 189-191 

Linseed-oil 190-195 

Cottonseed-oil 191-196 

Castor 176-186 

Cod-liver  oil .  .  171-189 


Sperm 123-147 

Rape 167-1 7f 

Cocoa-butter 192-202 

Sesame 187-193 

Wool-fat 98-127 

Almond 187.9-195.5 

Peanut 185.6-197 

Palm 202 

Beeswax 90-100 

Mustard-oil.  .  .   170-180 


HEHNER  VALUE. 

AVERAGE    PERCENTAGE    OF    INSOLUBLE    ACIDS. 


Butter 87.5 

Cocoanut-oil 86.43 

Palm-nut  oil 91.1-95.6 

Lard 96.15 

Oleomargarine 95 . 56 

Olive 95.43 

Rape 95.10 


Cod-liver  oil 95.46-97.05 

Almond-oil 96. 6 

Cocoa-butter 94.59 

Tallow 95.50 

Cottonseed 95.75 

Peanut-oil 95 .00 

Sesame. .  .95.48 


*  De  Negri  and  Fabris  have  found  as  low  a  number  as  3. 


460 


ANALYSIS  OF  FATS  AND  OILS. 


MAUMENE   NUMBER   OF   OILS. 


Authority. 

Linseed  , 

103 

Maumene 

"      

,  104-124 

Baynes 

tt 

104-111 

Allen 

"      

122-126 

De  Nigri  and  Fabris 

Niger  

82 

Baynes 

tt 

81 

Allen 

Cottonseed  (raw)  

84 

Baynes 

«            « 

61 

Dobb 

tt            it 

70 

Archbutt 

tt            (t 

67-69 

De  Nigri  and  Fabris 

Cottonseed  (refined)  

77 

Baynes 

K                               ft 

75-76 

Archbutt 

it                               tt 

74-75 

Allen 

Sesame  

68 

Maumene 

tt 

65 

Archbutt 

u 

63-64 

De  Nigri  and  Fabris 

Colza  

57-58 

Maumene 

u 

54-56 

Dobb 

tt 

55-64 

Archbutt 

tt 

51-60 

Allen 

tt 

49-51 

De  Nigri  and  Fabris 

Almond  

52-54 

Maumene 

"       

51-53 

De  Nigri  and  Fabris 

Peanut  

67 

Maumene 

u 

47-60 

Archbutt 

"      

45.5-51 

De  Nigri  and  Fabris 

Olive  

42 

Maumene 

tt 

40 

Baynes 

tt 

39-43 

Dobb 

tt 

41-45 

Archbutt 

it 

41-43 

Allen 

tt 

32-37 

De  Nigri  and  Fabris 

Castor  

47 

Maumene 

"    

46 

Archbutt 

M 

46-47 

De  Nigri  and  Fabris 

36 

Olsen  and  Benoit 

CONSTANTS. 


461 


SPECIFIC   TEMPERATURE   NUMBER   OF   OILS.* 

Authority. 

Linseed 320-349  Thomson  and  Ballantyne 

"      313  Jenkins 

Cottonseed 163-170  Thomson  and  Ballantyne 

"          174.3  Tolman  and  Munson 

"           butter-oil 172.9                "         "         " 

"           summer  white....  191.1                 "         "         " 

"          cooking-oil 192 . 4                "        "         " 

blown 164  Jenkins 

Colza 125-144  Thomson  and  Ballantyne 

"    130  Jenkins 

Rape 135.6-152.5  Tolman  and  Munson 

Peanut 105-137  Thomson  and  Ballantyna 

"      129.1-135.3  Tolman  and  Munson 

Olive 89-94  Thomson  and  Ballantyne 

"    94  Jenkins 

"    94.5-109.7  Tolman  and  Munson 

Boiled  linseed 248  Jenkins 

Castor 89  Thomson  and  Ballantyne 

Sperm 100                      "          "             " 

Gocoanut 44  Tolman  and  Munson 

Lard-oil 96.0  Olsen  and  Benoit 

Mustard  (black) 189.4  Tolman  and  Munson 

(brown) 165.4                "        <<        " 

"        (black) 169.3                "        ?,        " 

(yellow) 130.9                "        "        " 

Almond 117.6                "        "        " 

Sunflower 166.7                "        "         " 

Maize 190.2                "        "<        " 

ACETYL  VALUE  OF  OILS. 

Linseed 8.5     Whale-oil  (old) 23.1 

Sesame 11.5  "         (recent) 13.2 

Cottonseed 16.6  (various  sam- 

Colza 6.3  pies) 11.6-17.2 

Almond 5.8      Grape-seed 144 . 5 

Peanut 3.4      Cod-liver  oil 41 .1-50.6 

Olive 10.6     Seal-oil 33.0-33.9 

Castor 153 .4-156 

*  This  modification  of  Maumene's  test  being  comparatively  new,  there  are 
less  data  of  results  to  choose  from  than  is  the  case  with  the  older  methods. 


CHAPTER  XXXI. 
GAS  ANALYSIS. 

695.  General  Methods  of  Gas  Analysis. — Two  general  methods 
are  in  use  for  the  analysis  of  gases.     The  gas  to  be  determined 
may  be  absorbed  in  an  appropriate  liquid  and  the  amount  deter- 
mined by  VOLUMETRIC  or  GRAVIMETRIC  METHODS.     The  determina- 
tion of  carbon  dioxide  illustrates  these  methods.      This  gas  may 
be  absorbed  in  caustic  potash  solution  in  a  Liebig  bulb  which  is 
weighed  before  and  after  absorption.     It  may  also  be  absorbed 
in  a  measured  volume  of  standard  barium  hydroxide   solution 
and    the    excess  of   alkali   titrated   with  standard   acid.      Such 
methods  should  always  be  used  when  the  gas  to  be  determined 
is  mixed  with  a  large  amount  of  other  gases,  as  is  the  case  with 
carbon  dioxide  in  the  air. 

If  the  gas  to  be  determined  constitutes  a  relatively  large  pro- 
portion of  a  gaseous  mixture,  its  amount  may  be  ascertained  by 
taking  a  MEASURED  VOLUME  of  the  gaseous  mixture  and  measuring 
the  residue  after  the  constituent  to  be  determined  has  been 
absorbed.  Such  determinations  may  be  made  very  rapidly  and 
with  a  high  degree  of  accuracy,  the  methods  having  been  well 
developed  and  excellent  apparatus  devised  by  Bunsen,  Hempel, 
and  others. 

ABSORBENTS   FOR  OXYGEN. 

Two  reagents  are  commonly  in  use  for  absorbing  oxygen  in 
gas  analysis — an  ALKALINE  SOLUTION  OF  PYROGALLOL  and  YELLOW 

PHOSPHORUS  IN  STICKS. 

696.  The  Pyrogallol,  being  in  solution,  may  be  used  in  appa- 
ratus of  a  great  variety  of  form.     The  solution  must  not  be  used 
at  a  temperature  less  than  15°,  since  at  lower  temperatures  the 

462 


OXYGEN.  463 

absorption  of  oxygen  is  very  slow.  The  absorption  is  not  com- 
plete unless  a  large  excess  of  the  reagent  is  present.  When  the 
solution  has  absorbed  a  few  per  cent  of  the  amount  of  oxygen 
which  it  will  take  up,  it  leaves,  during  subsequent  use,  a  larger  and 
larger  residue  of  unabsorbed  oxygen  in  the  gas  which  is  in  contact 
with  it.  A  solution  which  has  been  exposed  to  the  air  for  even 
a  short  time  must  therefore  not  be  used.  A  record  should  be 
kept  of  the  amount  of  oxygen  absorbed  by  a  given  solution,  so 
that  it  may  be  discarded  when  no  longer  efficient.  The  solution 
is  made  according  to  Hempel  *  by  dissolving  5  grams  of  pyrogallol 
in  15  c.c.  of  water  and  adding  120  grams  of  caustic  potash  dis- 
solved in  80  c.c.  of  water.  The  caustic  potash  purified  by  alcohol 
should  not  be  used.  1  c.c.  of  this  solution  will  absorb  completely 
2  c.c.  of  oxygen. 

697.  Yellow  Phosphorus  has  the  advantage  that  its  absorbing 
power   is   practically   unlimited.      It   should   be   protected  from 
the  light,  which  converts  it  into  the  red  inactive  modification. 
It  is  fully  as  efficient  an  absorbent  of  oxygen  as  the  solution  of 
pyrogallol.     It  is  kept  in  a  gas  pipette  under  water,  which,  besides 
protecting  it  from  the  oxygen  of  the  air,  serves  to  dissolve  the  ox- 
ides of  phosphorus  produced  during  the  absorption  of  the  oxygen. 
As  these  oxides  are  solid  and  have  practically  no  vapor  tension 
it  is  not  necessary  to  wait  for  their  absorption  before  reading  the 
volume  of  the  residual  gas.     The  temperature  of  the  phosphorus 
must  not  be  allowed  to  fall  below  15°  or  the  absorption  will  be 
very  slow,  20°  being  a  more  favorable  temperature. 

698.  Pure  Oxygen  at  atmospheric  pressure  is  not  absorbed  by 
phosphorus.     If  the  oxygen  is  diluted  by  admixture  with  another 
gas  or  the  pressure  reduced  by  means  of  an  air-pump,  action 
begins  when  the  pressure  of  the  oxygen  is  75%  of  atmospheric 
pressure.    The  action  is  then  violent,  being  accompanied  by  the 
evolution  of  light  and  heat,  so  that  the  phosphorus  is  melted. 
A  gas  containing  more  than  50%  of  oxygen  should  not  be  brought 
into  contact  with  the  phosphorus.     If  a  gas  containing  more  than 
this  amount  of  oxygen  is  to  be  analyzed,  it  may  be  diluted  with 
nitrogen  which  is  prepared  from  air  by  absorption  of  the  oxygen 

*Gas  Analysis,  1902,  p.  149. 


404  GAS  ANALYSIS. 

by  phosphorus.  The  absorption  of  oxygen  is  entirely  or  partly 
prevented,  according  to  Schonbein,  by  the  presence  of  ethylene 
and  other  hydrocarbons,  ethereal  oils,  alcohol,  and  traces  of 
ammonia. 

699.  The  Phosphorus  is  Formed  into  Sticks  by  melting  a  suffi- 
cient amount  in  a  test-tube  under  a  layer  of  water.     The  test- 
tube  is  most  conveniently  heated  by  placing  it  in  warm  water. 
Glass  tubes  having  an  internal  diameter  of  2  to  3  mm.  are  selected 
and  inserted  into  the  test-tube  with  the  larger  end  down.     It 
will  be  found  that  the  diameter  of  most  glass  tubing  varies  so 
that  tubes  having  a  slight  difference  in  diameter  at  the  two  ends 
may  easily  be  found.     On  placing  the  finger  over  the  upper  end 
of  the  glass  tube  and  taking  it  out  it  will  remain  filled  with  phos- 
phorus, which  may  be  solidified  by  holding  the  tube  in  cold  water. 
If,  on  cooling,  the  stick  of  phosphorus  does  not  drop  out  on  tap- 
ping the  tube,  it  may  be  shoved  out  with  a  stiff  wire.    The  operation 
is  more  easily  performed  if  a  short  piece  of  rubber  tubing  with  a 
pinch-cock  is  placed  on  the  upper  end  of  each  glass  tube.     While 
one  tube  is  cooling  in  the  water  others  may  be  filled  with  phosphorus. 

700.  Chromous  Chloride. — Oxygen  may  also  be  quantitatively 
absorbed  by  a  solution  of  chromous  chloride,  which  does  not  absorb 
hydrogen  sulphide  nor  carbon  dioxide. 

701.  For    the    Absorption    of    Carbon   Dioxide    a   solution   of 
caustic  potash  is  used.     It  is  made  by  dissolving  1  part  of  com- 
mercial caustic  potash  in  2  parts  of  water.     1  c.c.  of  this  solution 
will  absorb  quantitatively  40  c.c.  of  carbon  dioxide. 


ABSORBENTS  FOR  CARBON  MONOXIDE. 

This  gas  is  absorbed  by  CUPROUS  CHLORIDP:  dissolved  in  HYDRO- 
CHLORIC ACID  or  in  AMMONIA.  As  solutions  of  cuprous  chloride 
rapidly  absorb  oxygen  from  the  air,  the  cuprous  chloride  being 
converted  into  cupric  chloride,  the  solution  must  be  protected 
from  the  atmosphere.  Hydrochloric  acid  solutions  may  be  kept 
reduced  by  the  presence  of  metallic  copper,  the  most  convenient 
form  being  wire  gauze.  Any  cupric  chloride  formed  in  such  a 
solution  is  slowly  reduced  to  cuprous  chloride  by  the  metallic 


CARBON  MONOXIDE.  465 

copper,  which  dissolves,  the  brown  solution  becoming  colorless. 
If  the  hydrogen  in  a  mixture  of  gases  is  to  be  absorbed  by 
palladium  after  the  absorption  of  the  carbon  monoxide,  the 
ammoniacal  solution  must  be  used. 

702.  The  Absorbing   Power  of  neither  solution  is  large,  and 
the  carbon  monoxide  is  held  in  such  loose  combination  that  after 
a  moderate  amount  has  been  absorbed  the  solution  gives  up  a 
small  part  of  the  carbon  monoxide  again  to  a  gas  in  which  it  is 
absent.     As  acetylene  and  ethylene  are  also  absorbed  by  cuprous 
chloride  solutions,  these  gases  must  be  removed  before  determin- 
ing carbon  monoxide.     This  is  accomplished  by  the  absorbents 
used  to  determine  the  " heavy  hydrocarbons"  or  "illuminants." 

703.  The  Hydrochloric  Acid  Solution  of  Cuprous  Chloride  may 
be  made,  according  to  Winkler,*  by  adding  a  mixture  of  86  grams 
of  copper  oxide  and  17  grams  of  finely  divided  metallic  copper  to 
1036  grams  of  hydrochloric  acid  (sp.  gr.  1.124),  the  mixture  being 
slowly  introduced  and  the  acid  frequently  stirred.     The  copper 
powder  is  best  prepared  by  reducing  copper  oxide  in  a  stream  of 
hydrogen.     The  solution  should  be  placed  in  a  flask  which  it  very 
nearly  fills.     A  spiral  of  copper  wire  is  introduced  into  the  flask, 
which  is  closed  with  a  rubber  stopper  and  allowed  to  stand  in 
the  dark  until  the  color  has  disappeared.     1  c.c.  of  this  solution 
will  absorb  4  c.c.  of  carbon  monoxide. 

The  solution  may  also  be  made  by  dissolving  15  grams  of 
commercial  cuprous  chloride  in  120  c.c.  of  concentrated  hydro- 
chloric acid.  Twenty  grams  of  copper  drillings  are  added,  and  the 
solution  is  allowed  to  stand  twenty-four  hours.  It  is  diluted  with 
water  to  200  c.c.  and  decanted  from  the  metallic  copper  for  use. 

704.  The  Ammoniacal  Solution  of    Cuprous    Chloride  may  be 
prepared  by  placing  15  grams  of  commercial  cuprous  chloride  in 
a  250-c.c.  flask  and  adding  200  c.c.  of  7%  ammonia.     After  in- 
serting the  stopper  the  solution  is  shaken  until  the  cuprous  chlo- 
ride is  dissolved. 

705.  The    Cuprous    Chloride  may  be    prepared  by  dissolving 
30  grams  of  copper  sulphate  and  15  grams  of  sodium  chloride  in 
100  c.c.  of  water.     Sulphur  dioxide  is  passed  into  this  solution 

*  Hempel,  Gas  Analysis,  1902,  p.  202. 


466  GAS  ANALYSIS. 

until  precipitation  is  complete.  The  solution  is  decanted  and  the 
cuprous  chloride  is  washed  several  times  by  decantation  and  dis- 
solved in  ammonia  or  hydrochloric  acid  as  desired. 


ABSORBENTS  FOR  THE  "  ILLUMINANTS." 

Ethylene  and  its  homologues,  as  well  as  benzene  and  its  homo- 
logues, which  taken  together  are  known  as  heavy  hydrocarbons 
or  illuminants  in  the  analysis  of  illuminating-gas,  are  absorbed  by 

FUMING   SULPHURIC   ACID   Or   BROMINE-WATER. 

706.  The  Fuming  Sulphuric  Acid  gives  off  vapors  of   sulphur 
dioxide  and  trioxide,  so  that  after  absorption  of  the  hydrocar- 
bons the  gas  remaining  must  be  passed  over  caustic  potash  to  free 
it  from  the  acid  vapors.     The  fuming  sulphuric  acid  must  con- 
tain a  large  amount  of  sulphur  trioxide  and  should  on  cooling 
deposit  crystals.     The  commercial  acid  is  seldom  strong  enough. 
It  may  be  concentrated  by  distilling  the  sulphur  trioxide  from 
one  portion  into  a  second  portion.     The  distillation  should  be  con- 
ducted in  a  tubulated  retort,  using  as  a  receiver  a  flask  the  neck 
of  which  fits  quite  snugly  over  the  stem  of  the  retort.     The  flask 
may  be  placed  in  a  large  funnel  to  the  stem  of  which  is  attached 
a  rubber  tube  leading  to  the  sink.     A  stream  of  water  is  allowed 
tc  flow  over  the  flask.     If  crystals  form  in  the  receiver,  they  may 
be  dissolved  at  the  end  of  the  distillation  by  gently  warming  the 
flask.     The  distillate  should  be  exposed  to  the  air  as  little  as  pos- 
sible.    One  c.c.  will  absorb  8  c.c.  of  the  illuminants. 

707.  If  Bromine  is  used  as  the  absorbent,  a  few  cubic  centi- 
meters of  liquid  bromine  are  placed  in  the  absorption  pipette  and 
distilled  water  added.     On  shaking,  the  water  becomes  saturated 
with  bromine.     When  the  gas  containing  heavy  hydrocarbons  is 
introduced  the  bromine  fumes  above  the  water  disappear.      On 
shaking  the  liquid  more  bromine  vapors  appear,  only  to  be  again 
absorbed  until  all  the  heavy  hydrocarbon  vapors  have  been  re- 
moved, when  the  bromine  fumes  persist  and  may  be  removed  by 
passing  the  gas  over  caustic  potash.     According  to  Cl.  Winkler,* 

*  Zeit.  f.  anal.  Chem.,  28,  269. 


HYDROGEN.  467 

the  absorption  of  ethylene  by  means  of  bromine  is  not  as  complete 
as  by  fuming  sulphuric  acid. 

708.  Absolute  Alcohol. — According  to  Hempel  and  Dennis,* 
benzene  and  its  homologues  may  be  absorbed  by  absolute  alcohol, 
1  c.c.  being  sufficient  for  100  c.c.  of  illuminating-gas.  The  alcohol 
is  placed  in  a  pipette  over  mercury.  The  alcohol  vapor  is  after- 
wards removed  by  washing  the  gas  with  1  c.c.  of  water.  As  these 
vapors  are  soluble  in  caustic  potash,  they  should  be  absorbed 
before  passing  the  gas  over  caustic  potash,  otherwise  the  result 
for  carbon  dioxide  will  be  high  and  for  heavy  hydrocarbons  low. 


DETERMINATION  OF  HYDROGEN. 

No  liquid  absorbent  for  hydrogen  is  known.  It  is  determined 
by  adding  a  sufficient  amount  of  oxygen  to  form  water  with  the 
hydrogen  present  and  igniting  the  mixture.  The  volume  of  the 
mixed  gases  being  measured  before  and  after  combustion,  the 
diminution  in  volume  is  noted.  If  the  gas  was  saturated  with 
water  vapor  before  combustion,  two-thirds  of  this  diminution 
will  represent  the  volume  of  hydrogen  present,  since  two  volumes 
of  hydrogen  unite  with  one  volume  of  oxygen,  and  the  water 
formed  occupies  no  volume  except  that  necessary  to  saturate  the 
gases  present  with  water  vapor. 

709.  Explosion  of  Hydrogen  and  Oxygen. — Various  methods 
are  in  use  for  causing  the  union  of  the  hydrogen  with  the  oxygen. 
The  method  longest  in  use  consists  in  producing  an  explosion  in 
the  gas  mixture  by  means  of  an  electric  spark.  The  electric  cur- 
rent is  led  in  through  platinum  wires  fused  into  the  glass  of  which 
the  confining  vessel  is  made. 

The  proportion  of  explosive  gases  present  to  non-explosive  must 
for  various  reasons  be  carefully  regulated.  By  explosive  gases  is 
meant  the  combined  volume  of  oxygen  and  hydrogen  in  the 
proportion  to  form  water.  Any  hydrogen  or  oxygen  present  in 
excess  constitutes  a  non-explosive  gas,  as  well  as  nitrogen,  carbon 
dioxide,  etc.  If  all  or  nearly  all  of  the  gas  present  were  explosive, 

*  Journ.  fur  Gasbel.,  1891,  p.  414. 


468  GAS  ANALYSIS. 

the  action  would  be  so  violent  as  to  shatter  the  confining  vessel. 
If  not  violent  enough  to  shatter  the  vessel,  another  difficulty 
would  be  encountered  in  that  if  any  nitrogen  were  present  some 
of  it  would  be  oxidized,  giving  a  greater  diminution  in  volume  than 
corresponds  to  the  amount  of  hydrogen  present.  On  the  other 
hand  if  the  explosion  were  too  weak,  the  combustion  of  the  hydro- 
gen would  be  incomplete.  According  to  Bunsen  there  should  be 
present  for  100  volumes  of  non-explosive  gases  at  atmospheric 
pressure  not  less  than  26,  nor  more  than  60  volumes  of  explosive 
gases,  while  the  most  favorable  proportion  is  26  to  40  parts  of 
explosive  gas  to  100  parts  of  non-explosive. 

If  an  insufficient  amount  of  the  explosive  gas  is  present,  oxy- 
hydrogen  gas  produced  by  the  electrolysis  of  water  may  be  added. 
As  this  gas  is  entirely  converted  into  water  on  explosion,  the 
exact  amount  added  need  not  be  noted.  If  the  proportion  of 
explosive  gas  is  too  great,  air  or  pure  oxygen  may  be  added. 
Whenever  a  gas  poor  in  hydrogen  is  to  be  analyzed,  the  required 
amount  of  oxygen  should  be  added  in  the  form  of  the  pure  gas. 
This  may  be  produced  according  to  Bunsen  by  blowing  a  small 
retort  from  glass  tubing  and  half  filling  it  with  pure  powdered 
potassium  chlorate.  The  oxygen  is  evolved  by  heating  the  bulb 
with  the  Bunsen  burner,  the  first  portions  being  discarded  because 
contaminated  with  air.  If  the  gas  to  be  analyzed  contains  a 
large  percentage  of  hydrogen,  air  should  be  used  to  furnish  the 
oxygen. 

710.  Slow  Combustion  of  Hydrogen.  —  Because  of  the  large 
amount  of  inert  gas  which  must  be  present  to  diminish  the  violence 
of  the  explosion,  only  small  amounts  of  the  gas  to  be  analyzed 
can  be  operated  upon  when  it  is  exploded.  This  difficulty 
is  obviated  by  the  method  proposed  by  Dennis  and  Hopkins,*  by 
which  the  combustion  is  effected  by  means  of  a  red-hot  platinum 
wire.  Pure  hydrogen  may  be  burned  by  this  method.  It  is 
introduced  into  the  pipette  containing  the  red-hot  platinum  wire, 
and  a  mixture  of  equal  parts  of  air  and  oxygen  containing  more 
oxygen  than  sufficient  to  combine  with  the  hydrogen  is  intro- 
duced slowly  into  the  pipette  containing  the  hydrogen.  The 

*  Jour.  Am.  Chem.  Soc.,  21,  398. 


HYDROGEN.  469 

hydrogen  is  burned  almost  as  rapidly  as  the  necessary  oxygen 
is  introduced. 

711.  Combustion    of   Hydrogen    by    Palladium    Sponge. — The 
mixture  of  hydrogen  and  oxygen  may  also  be  burned  by  passing 
the  gas  over  palladium  sponge  or  palladium  asbestos,*  which  is 
prepared  as  follows:    One  gram  of  palladium  is  converted  into 
chloride,  and  the  solution  treated  with  a  few  cubic  centimeters  of 
a  saturated  solution  of  sodium  formate,  and  enough  caustic  soda 
to  make  the  solution  strongly  alkaline.     One  gram  of  fine  silky 
asbestos  is  immersed  in  the  solution,  which  should  be  entirely 
taken  up  by  the  asbestos.     The  material  is  dried  by  gently  heat- 
ing on  the  water-bath.     The  palladium  is  deposited  on  the  asbes- 
tos, to  which  it  adheres  firmly.     The  asbestos  is  placed  in  a  funnel 
and  thoroughly  washed  with  hot  water  and  again  dried.     This 
asbestos  may  be  twisted  into  threads  by  moistening  with  water 
and  inserted  into  capillary  tubes  and  again  dried  on  the  water- 
bath  before  being  used. 

712.  Theory  of  Combustion  of  Hydrogen  by  Palladium  Sponge. — 
The  palladium  is  able  to  absorb  large  quantities  of  hydrogen,  es- 
pecially at  high  temperatures.     When  air  is  led  over  palladium 
saturated  with  hydrogen,  water  is  formed,  and  the  combustion  is 
so  energetic  that  the  palladium  becomes  red-hot,  and  in  this  con- 
dition it  is  superficially  oxidized.     If  hydrogen  is  led  over  this 
palladium,  even  at  the  ordinary  temperature,  it  is  burned  at  the 
expense  of  the  oxygen  of  the  palladous  oxide.     If  the  hydrogen 
has  previously  been  mixed  with  pure  oxygen  or  air,  the  palladium 
which  has  been  heated  by  the  combustion  of  some  of  the  hydrogen 
with  the  oxygen  of  the  palladous  oxide  is  able  to  absorb  more 
hydrogen,   which   then   can   combine   with   the   gaseous   oxygen 
present.    When  all  of  the  hydrogen  present  has  been  burned  a 
film  of  palladous  oxide  will  be  formed  on  the  hot  palladium  by 
the  excess  of  oxygen  present,  when  the  palladium  will  be  in  con- 
dition for  the  next  combustion,  and  the  oxidation  will  begin  at 
the  ordinary  temperature.     If  the  palladium  has  not  been  rendered 
active  in  this  manner,  it  must  be  heated  to  100°,  or  even  to  redness, 
before  action  begins,  the  hydrogen  being  absorbed  more  readily 

*  Cl.  Winkler,  Technische  Gas-Analyse,  p.  86. 


470  GAS  ANALYSIS. 

at  high  temperatures,  and  the  film  of  oxide  being  always  formed 
when  the  palladium  is  heated  to  redness  in  the  presence  of  oxygen. 

713.  Temperature. — As  methane  is  also  oxidized  when  a  mix- 
ture of  oxygen  and  hydrogen  containing  this  gas  is  passed  over 
palladium  heated  to  200°  or  higher,  it  is  advisable  to  heat  the 
palladium  only  to  100°  when  methane  is  present,  in  order  to  effect 
the  combustion  of  the  hydrogen  alone.     The  tube  containing  the 
palladium  is  placed  in  boiling  water  and  the  mixture  of  gases  is  led 
through  it  very  slowly,  so  that  the  temperature  shall  not  rise  to  200°. 

714.  Determination    of   Methane   by   Combustion. — As  no  ab- 
sorbent for  methane  is  known,  this  gas  must  also  be  determined 
by  combustion  with  oxygen.     The  explosion  by  means  of  the 
electric  spark  and  the  combustion  with  the  red-hot  platinum  wire 
may  be  used  exactly  as  given  for  hydrogen.     The  same  precautions 
must  be  taken  in  regard  to  the  proportion  of  explosive  to  non- 
explosive  gas.     It  must  be  remembered  that  one  volume  of  methane 
requires  for  its  combustion  two  volumes  of  oxygen.     The  volume 
of  methane  will  be  equal  to  one-half  of  the  contraction  noted 
after   the  explosion,  provided  all  other   combustible    gases    are 
absent.     The  volume  of  methane  will  also  be  equal  to  the  volume 
of  carbon  dioxide  present  after  the  explosion,  provided  all  carbon 
dioxide  and  all  gases  which  produce  carbon  dioxide  on  combustion 
are  removed  before  the  explosion. 

715.  A  Mixture    of   Hydrogen   and   Methane  may  readily  be 
analyzed  by  combustion.     The  contraction  produced  is  noted,  and 
then  the  amount  of  carbon  dioxide  present  is  determined.     The 
volume  of  methane  will  be  exactly  equal  to  the  volume  of  carbon 
dioxide  produced,  as  1  molecule  of  each  of  these  gases  contains 
1  atom  of  carbon.     Twice  the  volume  of  methane,  or  of  the  carbon 
dioxide  found,  is  the  contraction  due  to  the  combustion  of  the 
methane.     On  subtracting  this  volume  from  the  total  contraction, 
that  due  to  the  hydrogen  present  is  obtained,  and  two-thirds  of 
this  volume  will  give  the  amount  of  hydrogen  present.     By  sub- 
tracting the  combined  volumes  of  methane  and  hydrogen  from 
the  original  volume  of  the  gas  analyzed,  the  amount  of  NITROGEN 
or  other  gas  present  may  be  ascertained. 

716.  Determination   of   Nitrogen. — As  no  convenient  method 
of  absorbing  nitrogen  is  known,  this  gas  is  generally  determined  by 
difference,  all  other  gases  having  been  absorbed  or  determined. 


APPARATUS  FOR  THE  ANALYSIS.  471 


APPARATUS  FOR  THE  ANALYSIS. 

The  apparatus  used  in  gas  analysis,  of  which  a  great  variety 
has  been  devised,  must  provide  for  two  essentially  different  opera- 
tions, namely,  the  measuring  of  the  gas  and  the  absorption  of 
individual  constituents.  The  measuring  of  the  gas  volumes  must 
be  accompanied  by  observations  of  the  temperature  and  pressure 
to  which  the  gas  is  subjected. 

717.  Regulation  of  Temperature. — Almost  invariably  the  gas  is 
allowed  to  assume  the  temperature  of  the  atmosphere  of  the  work- 
room and  the  temperature  is  noted  from  a  thermometer  which 
is  suspended  near  the  measuring-tube.    All  sources  of  artificial 
heat  are  removed  from  the  immediate  proximity  of  the  gas  or 
from  the  room  entirely.     The  heat  emanating  from  the  body  and 
especially  the  hands  of  the  operator  is  sufficient  to  appreciably 
change  the  volume  of  the  gas  unless  precautions  are  taken  to 
prevent  it.      For  this  reason  the  glass  measuring-tubes  are  fre- 
quently water-jacketed,  the  temperature  of  water  changing  much 
more  slowly  than  that  of  gases. 

718.  Regulation    of   Pressure. — The    temperature   of   the   gas 
being  fixed  as  that  of  the  atmosphere,  the  volume  varies  inversely 
with  the  pressure.     In  some  forms  of  apparatus  the  pressure  is 
kept  constant,  that  of  the  atmosphere  being  generally  used,  the 
volume  only  being  read,  while  in  other  forms  both  volume  and 
pressure  are  carefully  noted,  the  latter  as  well  as  the  former  being 
allowed   to   vary.     Both   the   temperature   and  pressure   of   the 
atmosphere  being  quite  constant  for  short  intervals  of  time,  gas 
analyses  are  frequently  carried  out  in  which  the  volume  only  of 
the  gas  is  noted,  the  results  being  given  in  percentage  by  volume. 
In  carrying  out  an  analysis  in  which  the  weight  of  a  gaseous  con- 
situent  must  be  obtained,  both  temperature  and  pressure  must 
be  noted  as  well  as  the  volume. 

719.  Confining  Liquids. — Both  WATER  and  MERCURY  are  used 
as  the  confining  liquids  in  gas  apparatus.     The  tension  of  mercury 
vapor  at  ordinary  temperatures  being  very  small  and  all  gases 
being  almost  absolutely  insoluble  in  this  metal,  it  is  almost  an  ideal 
confining  liquid,  and  for  some  operations  is  indispensable.     Because 


472  GAS  ANALYSIS. 

of  the  slight  solubility  of  most  gases  in  water,  the  results  obtained 
when  this  liquid  is  used  are  slightly  inaccurate.  The  errors  are 
considerably  reduced,  however,  by  using  water  which  has  been 
saturated  with  the  gas  to  be  analyzed.  This  is  accomplished  by 
shaking  the  water  with  a  portion  of  the  gas  or  by  passing  a  stream 
of  it  through  the  water. 


HEMPEL'S  GAS  APPARATUS. 

Very  convenient  forms  of  apparatus  have  been  perfected  by 
Hempel.  Water  is  used  as  the  confining  liquid  and  the  measure- 
ments of  volume  are  taken  at  atmospheric  temperature  and 
pressure. 

720.  The   Simple  Gas  Burette  consists  of  two  glass  tubes  set 
in  iron  or  loaded  wooden  feet,  as  shown  in  Fig.  66.    The  tubes  are 
bent  at  right  angles  at  the  bottom  and  drawn  out  so  as  to  afford 
convenient  attachments  for  the  rubber  tube  about  120  cm.  long 
which  joins  them.     The  measuring-tube  A  ends  in  a  capillary  tube, 
c,  over  which  a  short  piece  of  rubber  tubing  may  be  slipped.     A 
Mohr  pinch-cock,  d,  placed  on  this  tube,  makes  a  convenient  and 
perfectly  tight  stop-cock.     The  tube  is  graduated  in  cubic  centi- 
meters, and  is  capable  of  holding  100  c.c.  of  gas,  the  lower  mark 
being  made  a  little  above  the  foot.     The  numbers  run  both  up 
and  down.     The  second  tube  B  is  called  the  level  tube,  and  serves 
to  hold  the  confining  liquid. 

721.  The  Modified  Winkler  Gas  Burette  is  similar  in  construc- 
tion, except  that  a  glass  stop-cock  is  placed  at  both  ends  of  the 
measuring- tube.     The  upper  stop-cock  a  is  the  ordinary  two-way 
stop-cock,  while  the  one  at  the  bottom,  6,  is  a  three-way  cock,  the 
third  opening  being  through  the  key  and  the  tube  c.     A  rubber 
tube  may  be  attached  to  this  tube  and  gas  admitted  to  the  meas- 
uring-tube without  coming  in  contact  with  the  water  in  the  level 
tube.     This  burette  is  used  when  the  gas  to  be  analyzed  contains 
a  constituent  easily  soluble  in  water.     The  tube  is  dried  out  by 
rinsing  it  with  a  little  absolute  alcohol  and  ether,  the  vapor  of 
which  is  displaced  by  a  stream  of  air.     The  three-way  stop-cock 
is  turned  so  that  its  horizontal  opening  communicates  with   the 


HEMPEL'S  APPARATUS. 


473 


inside  of  the  burette,  and  by  means  of  a  tube  attached  to  this 
opening  the  gas  to  be  analyzed  is  passed  through  the  burette 
until  the  air  is  entirely  expelled.  The  three-way  stop-cock  is 
closed  and  then  the  upper  stop-cock.  The  reagent  for  absorbing 
the  soluble  gas  is  then  introduced  into  the  burette  through  a 


FIG.  66. 


FIG.  67 


funnel  attached  to  the  horizontal  opening  c  in  the  three-way 
stop-cock.  After  the  absorption  of  this  constituent  the  remainder 
of  the  analysis  is  conducted  in  the  usual  manner.  Whenever 
possible  the  burette  with  the  Mohr  pinch-cock  should  be  used  in 
place  of  the  Winkler  burette,  as  glass  stop-cocks  are  not  as  reliable 
as  rubber  tubing  and  a  pinch-cock. 


474 


GAS  ANALYSIS. 


722.  The  Absorption  Pipette  in  its  simplest  form  is  shown  in 
Fig.  68.  It  consists  of  two  bulbs,  a  and  b,  having  capacities  of 
100  c.c.  and  150  c.c.  respectively.  A  capillary  tube,  c,  bent  into 
a  U-shape  is  sealed  to  the  bulb  6,  and  the  whole  is  fastened  to 
the  iron  stand.  This  pipette  is  used  for  liquids  which  attack  rub- 
ber and  which  do  not  need  protection  from  the  air,  such  as  caustic 
potash  and  bromine-water.  The  bulb  b  is  filled  completely, 
while  a  is  left  empty,  so  that  when  the  gas  is  introduced  through 
the  tube  c  into  the  bulb  b  the  reagent  expelled  may  pass  into 
the  bulb  a. 


FIG.  68. 


FIG.  69. 


A  modified  form  of  this  pipette  which  is  suitable  for  solid 
reagents  is  shown  in  Fig.  69.  After  the  solid  has  been  intro- 
duced through  i  the  opening  is  closed  with  a  solid-rubber  stopper, 
or,  still  better,  with  a  closed  glass  tube  of  suitable  size,  over  which 
a  short  piece  of  rubber  tubing  has  been  slipped.  Very  little  rub- 
ber is  then  exposed  to  the  action  of  the  reagent.  Whichever 
stopper  is  used,  it  should  be  firmly  wired  on.  This  pipette  is 
suitable  for  the  sticks  of  phosphorus  used  for  the  absorption  of 
oxygen. 


HEM  PEL'S  APPARATUS. 


475 


723.  The  Double  Pipette  is  used  for  reagents,  such  as  cuprous 
chloride,  which  must  be  protected  from  the  air.  The  absorbing 
reagent  is  placed  in  the  bulb  a,  and  when  this  bulb  is  filled  with 
gas  the  liquid  is  forced  into  the  bulb  b.  The  bulb  c  is  filled  with 
water,  which  serves  to  retain  the  atmosphere  of  nitrogen,  carbon 
dioxide,  or  other  indifferent  gas  which  is  kept  in  the  bulb  b  over 


m 


FIG.  70. 

the  reagent.  To  fill  this  pipette  a  stream  of  carbon  dioxide  or 
nitrogen  is  passed  into  the  empty  pipette  at  m.  The  stream  of 
nitrogen  may  be  produced  by  placing  in  a  small  flask  a  mixture  of 
equal  parts  of  ammonium  chloride  and  sodium  nitrite.  After  the 
flask  has  been  attached  to  the  pipette  by  means  of  a  rubber  stopper 
and  a  delivery-tube  the  nitrogen  is  evolved  by  gently  heating  the 
flask  with  the  Bunsen  burner.  When  the  air  has  been  displaced 
the  delivery-tube  is  disconnected  and  the  bulb  c  filled  with  water. 
A  glass  tube,  about  a  meter  long,  is  connected  at  I  by  means  of  a 
short  piece  of  rubber  tubing,  and  the  cuprous-chloride  solution 
transferred  to  the  bulb  a  by  pouring  it  into  a  funnel  attached  to 
the  upper  end  of  the  long  glass  tube. 

If  FUMING  SULPHURIC  ACID  is  placed  in  a  double  pipette,  the 
liquid  seal  in  c  and  d  should  be  concentrated  sulphuric  acid.  Any 
water  in  the  bulbs  may  be  rinsed  out  with  concentrated  sulphuric 


476 


GAS  ANALYSIS. 


acid.  On  account  of  the  action  of  the  fuming  sulphuric  acid  on 
rubber  the  pipette  must  be  filled  by  inverting  it  so  that  the  end 
of  the  capillary  tube  may  be  dipped  into  the  acid.  Suction  is 
applied  by  the  lungs  or  a  Bunsen  filter-pump  at  m  until  the  bulb 
a  is  full.  The  pipette  is  then  inverted  and  concentrated  sulphuric 
acid  poured  into  the  bulb  d  to  prevent  the  entrance  of  moisture 
from  the  atmosphere. 

724.  The  Double  Pipette  for    Solid  and    Liquid  Reagents  has 
an  opening  in  the  bulb  a  for  the  insertion  of  solids.     This  bulb  is 
used  for  the  acid  cuprous  chloride  solution.     The  bulb  a  is  filled  with 
glass  tubes  containing  coils  of  copper  wire.     After  pouring  in  the 
cuprous  chloride  solution  the  opening  is  closed  with  a  rubber  stopper 
or  glass  stopper  as  directed  for  the  single  pipette  for  solid  reagents. 
This  bulb  need  not  be  filled  with  nitrogen,  as  any  cupric  chloride 
formed  by  the  oxygen  present  is  soon  reduced  by  the  metallic 
copper,  the  air  in  it  being  soon  deprived  of  its  oxygen. 

725.  The  Explosion  Pipette  consists  of  a  thick-walled  glass  bulb 
closed  with  a  glass  stop-cock  at  d.    The  level  bulb  b  is  connected 


FIG.  71. 

with  the  bulb  a  by  means  of  a  piece  of  thick-walled  rubber  tubing. 
Mercury  is  used  as  the  confining  liquid  in  the  pipette,  since  under  the 
high  pressure  developed  by  the  explosion  a  considerable  amount 


HEM  PEL'S  APPARATUS. 


477 


of  carbon  dioxide  would  dissolve  in  water.  Platinum  wires  are 
sealed  into  the  glass  at  c.  The  spark  which  ignites  the  gases  is 
produced  between  these  wires.  The  current  from  several  Bunsen 
dichromate  cells  or  other  convenient  battery  is  passed  through  a 
Ruhmkorff  induction-coil.  A  coil  about  15  cm.  long  is  of  con-, 
venient  size.  The  secondary  current  produced  by  the  induction- 
coil  should  produce  a  bright  spark  between  the  platinum  wires.  A 
box  made  of  strong  wire  netting  is  very  convenient  for  placing 
over  the  pipette  for  protection  in  case  it  should  be  shattered  by 
the  explosion.  If  this  is  not  at  hand,  it  should  be  placed  in  a  hood 
or  covered  with  a  towel  during  the  explosion. 

726.  The  Pipette  Arranged  for  Combustion  with  a  Hot  Platinum 
Wire  is  shown  in  Fig.  72.    A  stout  iron  wire  inclosed  in  a  glass 


FIG.  72. 

tube  is  pushed  through  the  rubber  stopper  and  the  lower  end 
of  the  glass  tube  closed  with  a  piece  of  rubber  tubing  which  is 
wired  around  the  iron  wire.  A  second  stout  iron  wire  is  pushed 
through  the  rubber  stopper  and  the  upper  end  connected  with  the 
first  iron  wire  by  means  of  a  coil  of  platinum  wire  J  mm.  in  diameter. 
The  coil  should  be  about  2  mm.  in  diameter  and  contain  from 
20  to  30  turns.  After  filling  the  bulb  with  mercury  or  water  the  air 
is  sucked  out  of  the  glass  tube  containing  the  iron  wire  by  closing 
the  capillary  tube  with  a  rubber  tube  and  pinch-cock  and  applying 
suction  with  a  pump  to  the  tube  of  the  level  bulb.  The  gas  to  be 


478  GAS  ANALYSIS. 

burned  is  transferred  to  the  pipette,  and  a  measured  volume  of 
oxygen  is  slowly  passed  into  the  pipette  after  heating  the  plati- 
num wire  to  redness  with  a  current  of  electricity. 

727.  The  Method  of  Manipulation  of  the  Hempel  apparatus 
will  be  more  fully  understood  from  the  following  description  of 
the  analysis  of  illuminating-gas. 

EXERCISE  7^. 
Analysis  of  Illuminating-gas. 

72-8.  Collecting  the  Sample.— 200  c.c.  of  water  are  placed  in  a  convenient- 
sized  flask,  and  the  gas  to  be  analyzed  allowed  to  bubble  through  for  a  few 
minutes,  the  flask  being  occasionally  shaken.  The  level  tube  of  the  gas 
burette  is  filled  with  the  water  saturated  with  the  gas,  and  the  air  displaced 
from  the  measuring-tube  by  opening  the  pinch-cock  and  raising  the  level 
tube.  The  measuring-tube  is  connected  with  the  gas-supply  by  means  of  a 
rubber  tube,  from  which  the  air  is  expelled  by  allowing  the  gas  to  flow  a  few 
minutes.  If  the  gas  has  not  been  burned  for  some  time,  it  should  be  allowed 
to  flow*  for  a  few  minutes  in  order  to  sweep  out  any  air  which  has  accumu- 
lated in  the  supply-pipes,  which  are  seldom  gas-tight.  More  than  100  c.c. 
are  drawn  into  the  measuring-tube  by  lowering  the  level  tube.  The  pinch- 
cock  of  the  measuring-tube  is  closed,  and  the  tube  connecting  with  the  gas- 
supply  is  removed.  Exactly  100  c.c.  of  the  gas  is  obtained  by  holding  the 
level  tube  in  the  hand,  so  that  the  bottom  of  the  meniscus  of  the  water 
is  exactly  opposite  the  100-c.c.  mark  on  the  burette.  The  pinch-cock  is 
now  opened  for  a  moment,  thus  allowing  the  excess  of  gas  to  escape.  If 
the  gas  does  not  measure  exactly  100  c.c.  when  the  level  tube  is  held  so 
that  the  water  is  on  the  same  level  in  the  two  tubes,  the  operation  is  repeated. 
After  a  little  practice  the  exact  amount  of  gas  is  readily  obtained.  The  gas 
measuring-tube  should  be  handled  as  little  as  possible.  By  taking  hold  of 
the  foot  heating  the  glass,  and  consequently  the  inclosed  gas,  is  avoided. 

729.  Carbon  Dioxide. — The  burette  is  now  connected  to  the  caustic  potash 
bulb,  as  shown  in  Fig.  73.  The  capillary  tube  F  is  inserted  into  the  rubber 
tube  d  to  the  pinch-cock.  The  rubber  tube  m  is  then  pinched  with  the 
fingers  to  expel  the  air,  and  the  other  limb  of  the  capillary  tube  inserted. 
After  noting  the  height  of  the  caustic  potash  solution  in  the  long  arm  of  the 
capillary  U-tube  of  the  pipette,  the  gas  is  passed  into  the  pipette  by  opening 
the  pinch-cock  and  raising  the  level  tube  B.  The  pinch-cock  is  closed,  and 
three  minutes  allowed  for  the  absorption  of  the  carbon  dioxide,  the  pipette 
being  shaken  occasionally.  The  gas  is  then  passed  back  into  the  burette 
by  opening  the  pinch-cock  and  lowering  the  level  tube,  the  caustic  potash 
solution  being  brought  to  the  same  point  in  the  capillary  tube  to  which  it 
came  before  passing  in  the  gas.  The  pinch-cock  is  closed,  and  after  three 


ILL  UMINA  TING-GAS. 


479 


minutes  the  volume  of  the  gas  is  read  by  bringing  the  water  in  the  two  tubes 
to  the  same  level  by  raising  or  lowering  the  level  tube.  The  three-minute 
intervals  are  most  readily  gauged  by  means  of  a  small  three-minute  sand-glass. 


•FIG.  73. 

730.  Uluminants. — While  waiting  three   minutes  for  the  water  to  run 
down  in  the  burette  and  the  gas  to  reach  the  atmospheric  temperature,  the 
caustic  potash  pipette  should  be  replaced  by  the  pipette  containing  the 
fuming  sulphuric  acid.     After  reading  and  recording  the  volume,  the  gas  is 
passed  over  the  fuming  sulphuric  acid  and  shaken  for  three  minutes.     The 
gas  is  then  passed  back  into  the  measuring-burette,  and  once  more  passed 
into  the  caustic  potash  bulb,  then  back  into  the  burette  and  measured 
after  three  minutes.     If  the  white  fumes  of  the  oxides  of  phosphorus  do 
not  appear  oxygen  is  absent  or  the  phosphorus  is  too  cold. 

73 1 .  The  Oxygen   is  absorbed   by  passing  the  gas  into  the  phosphorus 
pipette.     After  three  minutes  it  is  passed  back  into  the  burette  and  meas- 
ured. 

732.  The  Carbon  Monoxide  is  absorbed  by  passing  the  gas  into  the  pipette 
containing  the  cuprous  chloride  solution.     If  this  solution  is  fresh,  the  carbon 


480  GAS  ANALYSIS. 

monoxide  will  be  completely  absorbed  after  shaking  for  three  minutes,  and 
the  residue  may  be  passed  into  the  burette  and  measured.  If  a  fresh  solu- 
tion of  cuprous  chloride,  and  also  one  which  has  been  used  considerably, 
are  at  hand,  the  bulk  of  the  carbon  monoxide  may  be  absorbed  by  shaking 
for  two  minutes  with  the  old  solution,  and  the  last  traces  absorbed  by 
shaking  the  gas  with  the  fresh  solution  for  three  minutes.  A  careful  record 
should  be  kept  on  a  piece  of  paper  pasted  on  these  pipettes  of  the  amount 
ot  carbon  monoxide  absorbed.  If  a  hydrochloric  acid  solution  of  cuprous 
chloride  is  used,  the  gas  should  be  passed  into  the  caustic  potash  bulb  before 
measuring  in  order  to  remove  the  hydrochloric  acid  gas. 

733.  Explosion  of  Hydrogen  and  Methane- — The  gas  now  remaining  con- 
sists of  methane,  hydrogen,  and  nitrogen.     These  constituents  may  be  deter- 
mined by  explosion,  or  by  combustion  with  the  red-hot  platinum  wire.     If 
the  explosion  pipette  is  to  be  used,  the  gas  is  passed  back  into  the  cuprous 
chloride  pipette,  which  is  closed  wi'  h  a  rubber  tube  and  a  pinch-cock.     The 
water  in  the  burette  is  replaced  by  distilled  water  which  has  been  saturated 
with  air.     From  12  to  15  c.c.  of  the  gas  residue  is  then  drawn  into  the  burette 
and  carefully  measured.     Air  is  then  drawn  into  the  burette  until  the  total 
volume  of  the  gas  is  very  nearly  100  c.c.     The  volume  is  again  carefully 
read  and  the  gas  is  passed  into  the  explosion  pipette,  the  large  bulb  of 
which,  including  the  capillary  tube,  has  been  filled  with  mercury.     When 
the  gas  has  been  passed  in,  enough  water  to  nearly  fill  the  capillary  tube 
should  also  be  forced  over  and  the  glass  stop-cock  of  the  pipette  closed. 
A  pinch-cock  is  placed  on  the  rubber  tube,  which  is  securely  wired  on  the 
capillary  tube  of  the  pipette.     The  end  of  th's  rubber  tube  is  also  closed 
with  a  piece  of  glass  rod.     After  placing  the  pipette  under  the  hood  or 
other  safe  place,  the  secondary  coil  of  the  Ruhmkorf  coil  is  connected  with 
the  platinum  wires  of  the  explosion  pipette,  arid  the  primary  coil  is  connected 
with  several  Bunsen  or  other  cells  connected  in  series.     The  explosion 
should  be  vigorous  enough  to  present  a  single  flash  of  light,  the  course  of 
which  cannot  be  followed  by  the  eye  across  the  bulb.     The  glass  stop-cock 
of  the  pipette  is  opened,  and  the  gas  passed  into  the  burette,  and  after 
three  minutes  the  volume  is  read.     The  burette  is  then  connected  with 
the  caustic  potash  pipette,  the  carbon  dioxide  absorbed,  and  the  volume 
of  the  gas  again  determined  with  the  burette. 

If  the  explosion  was  not  satisfactory  the  operation  should  be  repeated, 
using  a  little  more  of  the  gas  residue.  As  this  operation  is  attended  with 
considerable  liability  of  error,  it  is  well  in  any  case  to  repeat  the  explosion. 

734.  Combustion  of  Hydrogen  and  Methane  by  Red-hot  Platinum  Wire. — 
If  the  pipette  fitted  with  the  platinum  wire  for  burning  the  gas  is  at  hand, 
it  should  be  used  rather  than  the  explosion  pipette.     The  entire  gas  residue 
may  then  be  used.     After  being  carefully  measured,  it  is  transferred  to  the 
pipette  containing  the  platinum  wire  heated  to  redness.     10%  more  oxygen 
than  the  volume  of  the  gas  residue  is  carefully  measured  in  the  burette, 
and  then  slowly  passed  into  the  pipette,  the  wire  having  been  heated  to 
redness  by  a  suitable  current  of  electricity.     The  oxygen  used  must  be  freed 
from  carbon  dioxide  by  pa^in<r  ihr  uo-h  istrong  caustic  potash  solution  or 


ORSAT  APPARATUS.  481 

by  being  shaken  in  the  caustic  potash  pipette.  The  presence  of  nitrogen 
is  not  objectionable.  The  amount  of  contraction  as  well  as  the  volume  oi 
carbon  dioxide  formed  must  be  noted  as  when  the  explosion  pipette  is  used. 
735.  Calculation  of  the  Analysis. — The  following  analysis  indicates  the 
method  of  recording  and  calculating  the  results: 

Volume  of  gas  taken 100.0  c.c. 

After  absorption  by  caustic  potash 97 . 4  c.c. 

Per  cent  of  carbon  dioxide fc  26 

After  absorption  by  fuming  sulphuric  acid 84 . 2  c.c. 

Per  cent  of  illuminants 13 .2 

After  absorption  by  phosphorus 83 . 9  c.c. 

Per  cent  of  oxygen .3 

After  absorption  by  cuprous  chloride 56.6  c.c. 

Per  cent  of  carbon  monoxide 27.3 

Taken  for  explosion 14       c.c. 

After  addition  of  air 98 . 5  c.c. 

Volume  after  explosion 75 . 9  c.c. 

Contraction 22 . 6  c.c. 

Volume  after  absorption  by  caustic  potash 69.9  c.c. 

Volume  of  carbon  dioxide 6.0  c.c. 

Volume  of  methane  in  14  c.c.  residue 6.0  c.c. 

The  methane  in  56.6  c.c.  is  found  by  the  propor- 
tion 14  :  56.6  :  :  6  :  x,  z  =  24.3  c.c. 

Per  cent  of  methane 24.3 

The  contraction  due  to  methane,  being  twice  its 

volume,  will  be 12  c.c. 

Contraction  due  to  hydrogen (22.6  — 12)  — 10.6  c.c. 

The  volume  of  hydrogen,  being  two-thirds  of  this 

contraction,  will  be 7.1  c.c. 

The  volume  of  hydrogen  in  56.6  c.c.  will  be  found 

by  the  proportion  14  :  56.6  :  :  7.1  :  x,  z  =  28.7  c.c. 

Per  cent  of  hydrogen 28.7 

Per  cent  of  nitrogen 3.6 

The  combined  volume  of  methane  and  hydrogen  being  53.0  c.c.,  the 
remainder,  or  3.6  c.c.,  is  considered  to  be  nitrogen. 

736.  The  Orsat  Apparatus. — A  very  convenient  and  compact 
apparatus  for  gas  analysis  was  devised  by  Orsat,  and  with  a  num- 
ber of  modifications  is  very  largely  in  use  in  the  form  shown  in 
Fig.  74.  The  measuring-burette  a  has  a  capacity  of  100  c.c., 
although  the  graduations  do  not  always  extend  over  the  enlarged 
portion  at  the  top,  the  absorbable  constituents  in  many  cases 
amounting  to  less  than  50%  of  the  gas  analyzed.  This  burette  is 


482 


GAS  ANALYSIS. 


water-jacketed,  so  that  the  gas  may  quickly  come  to  a  constant 
temperature.  The  absorption-bulbs,  b,  c,  and  d,  contain  glass 
tubes  so  as  to  give  a  larger  absorbing  surface  to  the  reagent. 
Each  of  these  bulbs  is  connected  with  a  bulb  of  the  same  capacity, 
which  is  only  partially  seen  at  the  rear  in  Fig.  74.  The  reagent 
forced  out  of  the  bulb  in  front  by  the  gas  passes  into  the  bulb  in 
the  rear.  The  bulb  6  is  filled  with  caustic  potash  for  absorbing 


FIG.  74. 

carbon  dioxide,  c  is  filled  with  an  alkaline  solution  of  pyrogallol 
for  the  absorption  of  oxygen,  while  d  is  filled  with  cuprous  chloride 
solution  for  the  absorption  of  carbon  monoxide.  The  palladium 
tube  /,  with  the  alcohol-lamp  g  and  the  bulb  h,  to  be  used  for  the 
absorption  of  hydrogen,  are  not  always  attached,  since  the  appa- 
ratus is  very  largely  used  for  the  analysis  of  flue  gases  in  which 
hydrogen  is  absent.  The  large  U-tube  at  the  side  is  filled  with 


ORSAT  APPARATUS.  483 

cotton,  which  removes  dust  particles  from  the  gas  as  it  enters  the 
apparatus. 

The  bottle  m,  connected  by  means  of  a  rubber  tube  with  the 
measuring-tube,  serves  as  a  level  tube,  by  which  the  gas  may  be 
forced  into  or  out  of  the  measuring-tube  and  absorption-bulbs. 
Water  is  used  as  the  confining  liquid.  The  whole  apparatus  is 
enclosed  in  a  wooden  case,  the  front  and  back  of  which  may  be 
readily  removed. 

737.  The  Manipulation  of  this  apparatus  is  very  similar  to 
that  of  the  Hempel  apparatus.  The  stop-cocks  and  rubber  joints 
must  first  be  tested  for  leaks  by  bringing  the  solution  in  the  ab- 
sorption-bulbs to  the  zero-marks  on  the  capillary  stems  and  clos- 
ing the  stop-cocks.  If  after  a  few  minutes  the  liquid  has  fallen 
in  any  of  the  bulbs,  the  stop-cock  or  rubber  connection  leaks,  and 
the  key  of  the  former  must  be  removed,  cleaned,  and  lubricated 
with  vaseline  and  the  rubber  joints  made  tight  by  wiring  the  rub- 
ber on  the  glass  tubes.  The  stop-cock  k  and  the  rubber  connec- 
tions with  the  measuring-tube  are  tested  by  bringing  the  water  in 
the  measuring-tube  to  the  zero-mark  on  the  upper  capillary  by 
holding  the  level  bottle  at  the  same  height.  On  closing  the  stop- 
cock k  and  lowering  the  bottle,  no  air  should  enter  the  apparatus 
as  indicated  by  the  height  of  the  water-column  when  the  bottle 
is  again  raised. 

All  of  the  joints  having  been  made  tight  and  the  solutions  in 
the  absorption-bulbs  being  at  the  marks  on  the  capillary  stems, 
the  measuring-tube  is  filled  with  the  gas  to  be  analyzed.  In  order 
to  displace  the  air  from  the  connecting-tubes  it  is  necessary  to 
fill  the  measuring-tube  several  times  and  then  expel  the  gas. 
When  a  fair  sample  is  finally  obtained  the  volume  is  made 
exactly  100  c.c.  by  holding  the  bottle  so  that  the  level  of  the 
water  is  the  same  as  that  in  the  measuring-tube.  The  stop- 
cock k  is  then  opened  and  immediately  closed.  The  gas  is  then 
forced  into  the  absorption-bulbs  in  turn  and  drawn  back  into 
the  measuring-tube  in  order  to  note  the  diminution  of  volume. 
The  absorption  of  oxygen  and  carbon  monoxide  being  some- 
what slower  than  that  of  carbon  dioxide,  it  is  well  to  pass  the 
gas  back  into  the  same  bulb  after  measurement  to  ascertain  by  a 
second  measurement  if  absorption  is  complete.  As  the  reagents 


484  GAS  ANALYSIS. 

are  not  well  protected  from  the  air,  they  must  frequently  be 
renewed  or  tested  for  their  efficiency. 

738.  In  the  Analysis  of  Flue  Gases  for  judging  the  efficiency 
of  the  firing,  the  most  difficult  part  of  the  operation  is  taking 
the  sample  of  the  gas.  When  the  fires  are  hand  stoked  the  com- 
position of  the  flue  gas  will  vary  with  the  time  which  has  elapsed 
since  fresh  coal  was  put  on  the  grate,  as  the  following  analyses  * 
show: 

One  Minute      Twelve  Minutes 
After  Stoking.  Later. 

Carbon  dioxide 13.5  4.0 

"       monoxide 0.0  0.0 

Oxygen 5.5  16.5 

Nitrogen 81.0  79.5 

739-  Taking  an  Average  Sample. — A  very  common  practice 
consists  in  collecting  the  sample  in  a  large  bottle  or  gasometer  from 
which  the  water  is  allowed  to  flow  slowly,  so  that  six,  twelve,  or 
twenty-four  hours  are  required  to  fill  the  gas  receiver.  This  aver- 
age sample  is  then  analyzed.  When  this  method  is  used  every 
precaution  must  be  taken  to  make  the  connections  absolutely  tight 
and  to  shellac  all  rubber  connections.  The  method  is  subject 
to  the  errors  due  to  the  solubility  of  the  gases  in  the  water  used. 

The  tube  extending  into  the  flue  may  be  of  iron  if  the  temper- 
ature of  the  gases  is  low.  Should  the  iron  become  heated  to  even 
very  dull  redness  alternate  oxidation  and  reduction  may  take 
place,  which  very  materially  changes  the  composition  of  the  gases. 
A  glass  or  porcelain  tube  must  then  be  used.  Several  openings 
should  be  made  in  the  tube  so  that  the  gas  in  the  entire  cross- 
section  of  the  flue  may  be  sampled.  The  entrance  to  the  flue 
should  be  made  as  near  the  fire  as  practicable,  as  the  diffusion  of 
both  air  and  the  products  of  combustion  through  the  usual  leaks 
in  the  flues  is  considerable. 

740.  Interpretation  of  Results. — The  method  of  taking  an 
average  sample  of  the  flue  gases  as  already  given  is  much  more 
suitable  for  mechanically  stoked  fires  than  those  hand  fed.  For 
the  latter  it  is  advisable  to  take  numerous  samples  at  various 

*  Lunge,  Chem.-tech.  Untersuchungsmethoden,  Vol.  I,  p.  183. 


PROBLEMS.  485 

intervals  of  time  after  stoking.  The  ideal  flue  gas  should  contain 
only  carbon  dioxide  and  nitrogen.  The  percentage  of  the  former 
will  not  be  quite  equal  to  that  of  the  oxygen  in  the  air,  because 
some  of  the  oxygen  combines  with  the  hydrogen  of  the  coal. 
The  remainder  of  the  oxygen,  however,  will  give  an  equal  volume 
of  carbon  dioxide,  since  a  molecule  of  the  latter  contains  a  molecule 
of  oxygen.  Generally,  however,  the  flue  gases  from  even  well- 
managed  fires  contain  several  per  cent  of  unused  oxygen.  If 
oxygen  is  entirely  absent  and  carbon  monoxide  is  present,  the 
draught  of  air  is  insufficient.  Carbon  monoxide  may  also  occur 
in  the  presence  of  oxygen  if  clinkers  are  allowed  to  remain  on  the 
grate. 


Problem  16. — A  coal  contains  3%  hydrogen,  82%  carbon,  and  15% 
ash.  What  would  be  the  composition  by  volume  of  the  flue-gas  (a)  if  per- 
fect combustion  took  place,  all  the  oxygen  of  the  air  being  used  up  and  no 
combustible  left  in  the  ash?  (6)  If  only  half  of  the  oxygen  of  the  air  were 
used? 

Problem  17. — A  flue-gas  produced  by  the  same  coal  contains  3.3% 
carbon  dioxide  and  16.7%  oxygen.  Calculate  the  excess  of  air  passing 
through  the  grate. 


CHAPTER  XXXII. 
STOICHIOMETRY. 

741.  Atomic-weight  Tables.  —  In  the  calculation    of    the  re- 
sults of   many  chemical   analyses  the  atomic  weight  of   one  or 
more  of  the  elements  is  used.    The  percentage  found  will  vary 
somewhat  according  to  the  atomic  weights  used.     Several  tables 
of  atomic  weights  compiled  by  various  authorities  are  in  use. 
Besides  differing  in  the  values  assigned  to  elements  whose  atomic 
weights  are  in  dispute,  the  tables  differ  as  to  the  element  whose 
atomic  weight  is  taken  as  the  basis  for  computing  the  remaining 
values.    Until  recently  hydrogen  has  been  taken  as  the  basis,  the 
value  1.000  having  been  assigned  to  it.    Tables  computed  on  the 
basis  of  16.000  as  the  value  for  oxygen  are  now  more  commonly 
used.     On  this  basis  hydrogen  has  the  value  1.008.     It  is  advis- 
able to  select  a  set  of  atomic  weights  and  in  all  calculations  to  use 
these  values.    The  values  used  in  this  book  are  those  given  in 
Table  I,  p.  509,  which  is  the  table  published  by  the  International 
Atomic  Weights  Committee  in  1908.     The  figures  given  by  this 
Committee  are  based  on  the  value  for  oxygen  of  16.000,  the  values 
based  on  hydrogen  as  unity  being  no  longer  given. 

742.  Limit  of  Accuracy  in  Atomic  Weights. — It  will  be  observed 
that  the  number  of  figures  given  after  the  decimal  point  varies 
from  one  to  three.     This  is  due  to  the  fact  that  the  values  have 
been  determined  with  a  varying  degree  of  accuracy.     In  each  case 
the  last  figure  given  is  somewhat  doubtful.     The  atomic  weight  of 
silver,  for  instance,  is  certainly  between  the  values  107.9  and 
108.0,  the  value  107.93  being  the  average  of  several  determinations, 
in  which  the  variations  were  in  the  second  place  of  decimals.     To 
write  the  atomic  weight  of  silver  with  three  figures  after  the  deci- 
mal point,  as  107.930,  would  be  introducing  an  absolutely  mean- 

486 


FACTORS.  487 

ingless  figure.  As  most  quantitative  work  is  far  less  accurate 
than  atomic-weight  determinations,  even  the  last  figure  given  may 
frequently  be  dropped.  Generally  the  figure  107.9  for  silver  will 
be  near  enough  to  its  true  value,  the  figure  dropped  representing 
only  -yinnr  of  its  atomic  weight,  which  is  smaller  than  the  ordi- 
nary analytical  errors.  If  in  the  same  calculation  the  value  of 
another  element  enters  whose  atomic  weight  is  known  with  still 
less  certainty,  even  one  more  figure  may  be  dropped.  If  the  atomic 
weight  of  iron,  for  instance,  which  is  uncertain  within  1  part 
in  560,  enters  into  the  calculation,  the  atomic  weight  108  may  be 
used  for  silver,  the  error  being  7  parts  in  10,793,  or  about  1 
pail  in  1540.  Numbers  obtained  by  multiplying  or  dividing  such 
numbers  need  not  be  given  to  more  than  four  significant  figures. 

743.  "  Factors."  —  If  iron  has  been  precipitated  as  ferric  hy- 
droxide and  weighed  as  ferric  oxide  the  amount  of  iron  in  the 
precipitate  is  found  by  calculation,  using  the  atomic  weights  of 
iron  and  oxygen.  From  the  formula  of  ferric  oxide,  Fe203,  we 
know  that  2  atoms  of  iron,  or  111.8  parts,  are  combined  with  3 
atoms  of  oxygen,  or  48  parts.  In  159.8  parts  of  the  precipitate 
there  are  present  111.8  parts  of  iron.  If  the  weight  of  the  pre- 
cipitate is  represented  by  p,  the  amount  of  iron,  x,  is  found  by  the 
following  proportion: 

Fe203  :  2Fe    :  :  p  :  x 
159.8  :  111.8  :  :  p  :  x 

111.8 
~  159.8  P' 

111  8 
The  fraction  1     'g  ,  which  is  equal  to  .6996,  represents  the  percent- 


age  of  iron  in  ferric  oxide  and  enters  into  every  calculation  of  the 
amount  of  iron  as  found  by  weighing  ferric  oxide.  For  each 
precipitate  one  or  more  factors  are  calculated  in  this  manner,  thus 
obviating  the  use  of  the  proportion  and  simplifying  the  calcula- 
tion. The  factors  for  the  commonly  used  precipitates  are  given 
in  Table  II,  p.  510. 

744.  Logarithms.  —  To  obtain  the  weight  of  a  given  element  in 
a  precipitate  the  weight  of  the  precipitate  is  multiplied  by  the 
proper  factor.  The  labor  of  multiplication  may  be  greatly 


488  STOICHIOMETRY. 

lessened  by  using  logarithms.  For  this  reason  the  logarithms  of 
the  factors  are  also  given  in  Table  II.  In  order  to  multiply  two 
numbers  together  their  logarithms  are  added.  The  sum  is  the 
logarithm  of  the  product  of  the  two  numbers.  Two  numbers 
may  be  divided  by  subtracting  the  logarithm  of  the  divisor  from 
the  logarithm  of  the  dividend,  the  remainder  being  the  logarithm 
of  the  quotient.  The  abbreviation  log.  is  frequently  used  for 
logarithm.  The  logarithms  of  the  numbers  from  1  to  9999  may  be 
obtained  from  Table  III,  page  514.  The  logarithm  of  a  number  is 
composed  of  two  parts,  a  mantissa  and  a  characteristic.  The 
former  is  a  positive  decimal  fraction  and  is  the  same  for  any 
number  composed  of  the  same  figures  in  the  same  order  irrespec- 
tive of  the  position  of  the  decimal  point.  The  characteristic  is  a 
positive  or  negative  integral  number  and  indicates  the  position  of 
the  decimal  point  of  the  number.  In  the  first  column  at  the  left 
of  Table  III  are  found  the  natural  numbers  beginning  w'th  10. 
The  mantissas  of  the  logarithms  of  any  of  these  numbers  are 
found  in  the  next  column  headed  0.  In  the  following  columns 
are  found  the  mantissas  of  the  numbers  formed  by  adding  to 
those  in  the  first  column  the  significant  figure  which  is  found  at 
the  head  of  one  of  the  columns.  For  instance,  the  mantissa  of  21 
is  found  in  the  column  headed  0  opposite  21  and  is  .3222.  In  the 
next  column  opposite  21  is  found  .3243,  which  is  the  mantissa  of 
211.  Following  this  number  is  .3263,  which  is  the  mantissa  of  212. 
If  the  mantissa  of  a  number  of  four  places  is  desired  the  columns 
of  differences  must  be  utilized.  The  mantissa  of  211  being  .3243 
and  that  of  212  being  .3263,  the  mantissa  of  a  four-place  number 
between  2110  and  2120  will  have  a  value  between  .3243  and  .3263. 
The  difference  between  these  values  is  .0020,  while  the  difference 
between  2110  and  2120  is  10.  For  each  unit  added  to  2110,  .0002 
must  be  added  to  the  mantissa  of  2110,  or  .3243.  The  mantissa 
of  2115  is  .3253,  10  having  been  added.  This  number  may  be 
found  in  the  column  of  differences  headed  5  opposite  the  natural 
number  21.  As  ordinary  quantitative  results  are  not  accurate 
beyond  four  significant  figures,  there  is  no  necessity  of  obtaining 
the  mantissa  to  the  fifth  significant  place. 

745.   The    Characteristic  is    the    number    placed    before    the 
mantissa-  to  designate  the  position  of  the  decimal  point  in  the 


LOGARITHMS.  489 

natural  numbers.  The  characteristic  of  the  log.  of  a  number 
with  one  significant  figure  to  the  left  of  the  decimal  point  is  0, 
if  there  are  two  figures  it  is  1,  with  three  figures  it  is  2,  etc.  The 
characteristic  of -the  log.  of  a  decimal  is  a  negative  number  and  is 
equal  to  the  number  of  places  by  which  its  first  significant  figure 
is  removed  from  the  place  of  units.  In  order  to  make  this  char- 
acteristic positive  it  is  sometimes  added  to  10,  which  is  placed 
with  a  minus  sign  after  the  mantissa.  The  logarithms  may  then 
be  added,  the  proper  number  of  tens  being  subtracted  from  the 
result.  According  to  this  rule  the  complete  logs,  of  the  numbers 
containing  the  figures  2115  are  given  in  the  following  table: 

Numbers.  Logarithms. 

2115 3.3253 

211.5 2.3253 

21.15 1.3253 

2.115 0.3253 

.2115 1.3253     or     9.3253-10 

.02115 3/3253     or     8.3253-10 

.002115 3.3253     or     7.3253-10 

As  the  position  of  the  decimal  point  may  generally  be  ascer- 
tained by  inspection  of  the  numbers  to  be  multiplied  or  divided, 
the  characteristic  is  frequently  omitted  from  the  logarithms. 
Illustration. — If  a  precipitate  of  ferric  oxide  weighing  .3252 
gram  is  obtained  from  .8276  gram  of  material,  the  percentage 
of  iron  is  calculated  by  logarithms  as  follows: 

log.  of  .3252 1.5122  or     9.5122-10 

log.  of  factor  of  Fe  in  Fe203  1 . 8449  or     9 . 8449-10 


by  addition      log.  of  weight  of  iron 1 . 3571  or  19 . 3571-20 

log.  of  .8276 1.9178  or     9.9178-10 


by  subtraction  and  addition  of  2  (log.  of  100) 

log.  of  percentage  of  iron  .  .  1 . 4393  or     1 . 4393 
percentage  of  iron 27 . 50 

The  number  corresponding  to  a  given  logarithm  is  obtained 
from  the  table  of  antilogarithms  in  exactly  the  same  manner  as 
the  logarithm  of  a  number  is  obtained  from  the  table  of  logarithms, 
the  first  column  of  figures  and  the  figures  at  the  head  of  the  other 


490  STOICHIOMETRY. 

columns  being  the  logarithms,  while  the  numbers  corresponding 
are  found  in  the  body  of  the  table. 

746.  Factors  from  a  Single  Precipitate.  —  A  given  precipitate 
may  be  used  for  the  calculation  of  a  large  number  of  different 
substances,  as  is  shown  by  the  following  indicated  calculations 
based  on  determinations  in  which  the  weighed  substance  is 
barium  sulphate.  The  weight  of  barium  sulphate  is  indicated 
by  p. 

Substance  weighed  BaS04. 
Desired,  the  amount  of  barium. 

BaS04  :  Ba  :  :  p  :  x 
Mol.  wts.      233        137 

137  137 

fact°r=233- 


Desired,  the  amount  of  barium  oxide. 

BaS04  :  BaO  :  :  p  :  x 
Mol.  wts.        233      153 

153  153 

factor=233* 


Desired,  the  amount  of  sulphur  trioxide. 

BaS04  :  S03  :  :  p  :  x 
Mol.  wts.         233    80 

80  ,  80 

X=233P>  factor=233' 

Desired,  the  amount  of  sulphuric  acid. 

BaS04  :  H2S04  :  :  p  :  x 
Mol.  wts.         233    98 

98  ,  98 

factor=233' 


Desired,  the  amount  of  sodium  sulphate. 
BaS04  :  Na2S04  :  :  p  :  x 
Mol.  wts.         233    142 

142  ,  142 

a.=_p;  factor  =233. 


INDIRECT  GRAVIMETRIC  ANALYSES.  491 

Desired,  the  amount  of  crystallized  sodium  sulphate. 

BaSO,  :  Na2S04.10H20  :  :  p  :  x. 
Mol.  wts.         233    322 

322  322 


Desired,  the  amount  of  sulphur. 
BaS04  :  S  :  :  p 
Mol.  wts.         233    32 


32  32 

factor=233' 


747.  The  Calculation  of  the  Theoretical  Percentage  of  an  Ele- 
ment in  a  Compound  is  made  in  exactly  the  same  manner  as 
the  calculation  of  a  "  factor"  where  the  element  or  radicle 
sought  is  part  of  the  substance  weighed.  The  theoretical  per- 
centage is  the  ratio  of  the  atomic  weight  of  the  element  sought  to 
the  molecular  weight  of  the  compound.  For  example,  the  theo- 
retical percentage  of  silver  in  silver  nitrate  is  obtained  from  the 

rati°  WWch  6qUalS  169^94  °r  6 


The  theoretical  percentage  of  S04  in  crystallized  magnesium 

SO 
sulphate  is  obtained  from  the  ratio  i\/r  QO  7TT  (V  wn^c^  e(luals 

Hm or  38-96%- 

748.  Indirect  Gravimetric  Analyses  are  more  difficult  to  cal- 
culate. General  rules  or  factors  cannot  readily  be  given.  The 
general  method  may  be  seen  from  the  calculation  of  a  determina- 
tion of  potassium  and  sodium  in  a  mixture  of  the  chlorides  of 
these  metals.  This  determination  is  readily  carried  out  by  weigh- 
ing the  sodium  and  potassium  chlorides  together  and  then  deter- 
mining the  amount  of  chlorine  present.  As  the  percentage  of 
chlorine  in  potassium  chloride  is  47.53,  and  in  sodium  chloride  is 
60.6,  the  percentage  of  chlorine  in  a  mixture  of  these  two  salts  will 
be  between  these  numbers.  For  example,  a  sample  of  the  mixed 
chlorides  which  weighed  1.5802  grams  gave  .9023  gram  of  chlo- 
rine, which  is  equal  to  57.1  per  cent.  If  onlv  potassium  chloride 


492  STOICHIOMETRY. 

had  been  present  .7511  gram  of  chlorine  would  have  been  ob- 
tained. The  difference  between  .9023  and  .7511  is  the  amount  of 
chlorine  due  to  the  presence  of  sodium  chloride.  This  difference 
(.1512)  divided  by  the  difference  in  the  percentage  of  chlorine  in 
the  two  salts  (.1307)  gives  the  weight  of  sodium  chloride  present 

=  1.1568).    The  weight  of   the   mixed   chlorides   less  the 


\.1307 

weight  of  sodium  chloride  gives  the  weight  of  potassium  chloride 
present  (1.5802 -1.1568  =  .4234).  The  problem  may  be  solved 
algebraically  as  follows: 

Let  x  =  the  weight  of  sodium  chloride ; 

and  y=ii       "      "  potassium  chloride ; 

then  x+  y  =  1.5802  the  weight  of  the  mixed  chlorides; 

and       .606:r +  A753y  =  .9023  the  weight  of  chlorine. 

Solving  these  simultaneous  equations  we  find  that 

.9023 -(1.5802X. 4753) 
.606 -.4753 

On  carrying  out  the  indicated  operations  we  obtain  a  value  for  x, 
or  the  amount  of  sodium  chloride  present,  which  is  identical  with 
that  already  obtained. 

The  disadvantage  of  using  indirect  methods  of  this  kind  is  found 
in  the  fact  that  the  difference  upon  which  the  weight  of  one  of  the 
constituents  is  calculated  is  very  small.  In  the  illustration  given 

it  must  be  multiplied  by  the  factor  7.652  (  1007)'      ^e  exPer^~ 

mental  errors  of  the  analysis  are  increased  in  this  ratio  in  the  per- 
centage found.  The  method  is,  therefore,  not  applicable  to  the 
determination  of  small  amounts  of  one  constituent  in  the  presence 
of  large  amounts  of  the  other. 

749.  Factor  Weights. — When  many  determinations  of  a  given 
constituent  must  be  made  it  is  found  convenient  to  so  choose  the 
amount  of  substance  to  be  weighed  out  that  the  weight  of  the 
precipitate  obtained  shall  have  a  simple  ratio  to  the  percent- 
age to  be  calculated.  For  example,  if  1.373  grams  of  a  substance 
containing  .375  per  cent  of  sulphur  is  weighed  out,  the  sulphur 
converted  into  sulphuric  acid  and  weighed  as  barium  sulphate, 


FACTOR  WEIGHTS.  493 

.0375  gram  or  the  latter  will  be  obtained.  The  number  of  milli- 
grams of  barium  sulphate  obtained  is  evidently  equal  to  the  hun- 
dredths  of  per  cent  of  sulphur  present.  The  general  method  of 
finding  the  amount  to  be  weighed  out  in  a  given  case  may  be 
obtained  from  the  following  general  case: 

Let  w  =the  amount  of  substance  weighed  out; 
and        i>=the  weight  of  precipitate  obtained; 
and       p  =the  per  cent  of  the  given  constituent  in  the  precipitate  v. 
The  percentage  is  obtained  by  the  following  indicated  calcula- 
tions: 

pXv 

w 

If  the  value  of  w  bears  a  simple  ratio  to  p,  the  percentage  will  bear  a 
simple  ratio  to  v.  In  the  illustration  already  given  p,  or  the  per- 
centage of  sulphur  in  barium  sulphate,  is  13.73.  w  was  made  one- 
tenth  of  this  number,  or  1.373.  The  percentage  is  therefore  ten 
times  v  or  the  weight  of  the  barium  sulphate  obtained,  as  may  be 
seen  by  substituting  the  values  of  v  and  w  in  the  formula  giving 

vX  13.73 

1.373    ' 

If  w  is  made  13.73  grams,  the  weight  of  v  in  grams  will  be  equal  to 
the  percentage  of  sulphur  in  the  substance  analyzed.  In  general, 
then,  the  amount  of  substance  taken  should  bear  a  simple  ratio  to 
the  percentage  in  the  precipitate  to  be  weighed  of  the  constituent 
to  be  determined.  This  ratio  will  be  inversely  equal  to  the  ratio 
of  the  percentage  to  the  weight  of  the  precipitate.  For  example, 
the  percentage  of  chlorine  in  silver  chloride  being  24.72,  the  per- 
centage of  chlorine  in  a  given  substance  will  be  ten  times  the 
weight  of  silver  chloride  obtained  if  2.472  grams  of  the  substance 
to  be  analyzed  are  weighed  out. 

CALCULATION  OF  THE  FORMULAE  OF  SALTS  AND 
ISOMORPHOUS  MIXTURES. 

The  results  of  many  analyses  may  be  expressed  by  a  chemical 
formula.  The  composition  of  all  pure  chemical  compounds  as  well 
as  a  large  number  of  minerals  may  be  expressed  in  this  manner. 
All  chemical  formulas  have  been  derived  from  quantitative  anal- 


494  STOICHIOMETRY. 

yses  and  express  primarily  the  proportions  by  weight  in  which 
the  elements  are  present. 

750.  Salts. — Instead  of  using  the  percentage  notation,  the  system 
of  equivalent  or  atomic  weights  has  been  devised  so  that  a  single 
number  or  its  simple  multiple  expresses  the  proportion  in  which  a 
given  element  is  present  in  its  innumerable  compounds.  Sodium 
chloride,  for  example,  contains  39.40  per  cent  of  sodium,  and 
60.60  per  cent  of  chlorine  as  found  by  analysis,  but  this  fact  is 
generally  stated  by  giving  the  formula  NaCl,  which  states  that 
23.05  parts  of  sodium  are  combined  with  35.45  parts  of  chlorine. 
The  ratio  of  the  percentages  39.40  and  60.60  must  be  the  same  as 
the  ratio  of  the  numbers  23.05  and  35.45,  and  what  is  not  quite  so 
self-evident,  the  ratio  of  23.05  to  39.40  must  be  the  same  as  the 
ratio  of  35.45  to  60.60.  This  ratio  is  1.709;  that  is,  39.40  parts  of 
sodium  represent  1.709  atoms  of  this  element  and  60.60  parts  of 
chlorine  represent  1.709  atoms  of  chlorine.  In  this  way  it  is 
found  from  the  percentage  composition  and  atomic  weights  that 
in  sodium  chloride  an  equal  number  of  sodium  and  chlorine  atoms 
are  present.  If  the  percentages  of  copper  and  chlorine  found  in 
crystallized  cupric  chloride  are  divided  by  the  atomic  weights  of 
these  elements  the  numbers 

537  30  41  58 

-^V=.5864,  ^-^  =  1.1728 
o3.o  35.45 

are  obtained.  The  number  of  chlorine  atoms  is  evidently  twice 
the  number  of  copper  atoms.  The  formula  then  must  be  CuCl2. 
The  number  of  acid  radicles,  molecules  of  water,  etc.,  may  be 
found  in  the  same  manner,  as  may  be  seen  from  the  calculation  of 
the  formula  of  potash  alum  from  its  analysis. 

Per  Cent  Found.         Atomic  Weights          Atomic  Ratios. 

K 8.17          39.14          3JnS  =   -2087 

Al 5.78          27.1  |^|  -  .2132 

S04 40.26          96.06          §g^  = 

45.74          18.02          TO|==: 

Total..      .   99.95 


CALCULATION  OF   THE  FORMULA  OF  SALTS.  495 

The  atomic  ratios  of  potassium  and  aluminium  being  nearly  equal, 
one  atom  of  each  of  these  elements  will  be  present  with  two  sul- 
phuric acid  radicles  and  twelve  molecules  of  water,  since  their 
atomic  ratios  are  respectively  twice  and  twelve  times  as  great  as 
that  of  potassium,  giving  the  formula  KA1(S04)2.12H20. 

751.  Many  Minerals  are  found  in  the  form  of  sharply  defined 
homogeneous  crystals,  which  contain  varying  amounts  of  several 
elements.  These  minerals  contain  several  salts  which  crystallize 
in  the  same  system,  and  are  therefore  called  isomorphous.  Dolo- 
mite is  an  example  of  such  a  mineral,  in  which  varying  amounts 
of  magnesium  and  calcium  carbonates  are  present.  Small  amounts 
of  iron,  aluminium,  and  silica  are  also  present.  The  silicic  acid  is 
able  to  replace  a  portion  of  the  carbonic  acid  forming  calcium  or 
magnesium  silicates,  while  the  iron  and  aluminium  are  able  to 
replace  a  portion  of  the  calcium  and  magnesium  forming  carbon- 
ates or  silicates.  A  general  formula,  M"R03,  will  express  the 
composition  of  the  mineral.  M"  represents  the  bivalent  bases, 
calcium,  magnesium,  ferrous  iron,  or  even  two  valences  of  an 
aluminium  atom.  R  represents  the  carbon  or  silicon. 

The  method  of  obtaining  the  formula  from  an  analysis  is  shown 
by  the  following  analysis  of  dolomite: 

46  80 


Per  cent  of  C02  .......  46.80 

Si02  .......  62  =    .010 


"       *     " 


Total  acid  equivalents,  R"02  =  1.074 
"     "    "  FeO  .......  21          71^9=   -003 


"      "     " 


"     " 


CaO  ......  32.35 


MgO  ......   19.83  .491 

Total  .....   99.81 

Total  basic  equivalents,  M"0  =  1.071 

Ratio  of  acids  to  bases       '        =1.003 


496  STOICHIOMETRY. 

The  formula  of  the  dolomite  will  therefore  be  MgC03,CaC03,  small 
amounts  of  the  base  being  replaced  by  FeO,  and  a  little  of  the  acid 
by  Si02. 

For  each  atom  of  bivalent  metal  there  is  present  one  acid  radi- 
cle, the  number  1.003  being  so  close  to  1.000  that  the  difference 
may  be  ascribed  to  the  unavoidable  experimental  errors  of  the 
analysis.  Ratios  as  high  as  1.02  or  even  1.03  may  be  produced 
by  errors  which  occur  in  fairly  good  analytical  work,  although  the 
analytical  worker  should  not  be  satisfied  unless  the  ratio  is  usu- 
ally less  than  1.01. 

BALANCING   OF   EQUATIONS. 

The  calculation  of  nearly  all  volumetric  determinations  is 
based  on  the  chemical  equation  which  represents  the  reaction 
taking  place.  While  it  is  true  that  the  chemical  equation  is  the 
expression  of  a  quantitative  analysis  and  should  not  be  written 
unless  the  quantitative  analysis  has  first  been  made,  this  view  of  the 
matter  is  taken  only  by  the  research  chemist.  In  ordinary  analyt- 
ical work  the  correctness  of  the  equation  is  assumed,  and  the 
quantitative  result  is  calculated  from  it.  The  equation  must 
therefore  be  written  before  the  calculation  is  made. 

752.    The  Simple  Synthetical  Equation,  such  as 

2H2  +  02=2H20 
or 

CH4+202=C02+2H20, 

is  required  mainly  for  gas  analysis. 

753.  The  Analytical  Equations,  such  as 

CuC204=Cu+2C02 
or 

2H20=2H2+02, 

are  most  commonly  required  to  express  electrical  decompositions. 
When  the  elements  or  compounds  produced  by  the  reaction  are 
known,  the  equations  are  balanced  by  counting  the  number  of 
atoms  of  each  element  on  each  side  of  the  equation.  For  exam- 
ple, in  the  second  equation  given,  the  molecule  of  methane  requires 
for  its  oxidation  two  atoms  of  oxygen  for  the  carbon  atom  and 
two  atoms  for  the  four  hydrogen  atoms.  The  first  member  will 


BALANCING  OF  EQUATIONS.  497 

therefore  contain  C  .  .  .  1,  H  .  .  .  4,  and  0  ...  4,  while 
the  second  member  will  contain  C  .  .  .  1,  H  .  .  .  4,  and 
0  ...  4.  These  numbers  being  identical  for  each  element,  the 
equation  is  balanced. 

754.  Metathetical  Equations  are  far  more  commonly  met  with, 
as  they  represent  the  two  types  of  chemical  reaction  which  are 
most  commonly  carried  out;  namely  (a)  those  involving  the  pro- 
duction  of  a  precipitate  when  two  substances  in  solution  are 
brought  together,  such  as 

AgN03+ Nad  = AgCl+ NaN03 
or 

BaCl2+H2S04=BaS04+2HCl; 

(6)  those  involving  the  production  of  a  substance  which  is  hi  the 
gaseous  form  under  the  conditions  of  the  experiment,  as 

CaC03+ 2HC1  =CaCl2+ C02+  H20 
or 

2NH4Cl+Ca(OH)2  =2NH3+CaCl2+2H20. 

Equations  such  as  these  may  be  balanced  by  adding  or  sub- 
tracting molecules  of  one  or  the  other  substance  until  the  number 
of  atoms  of  each  element  is  identical  in  each  member  of  the  equa- 
tion. It  is  far  better,  however,  to  take  into  consideration  the  fact 
that  some  of  the  elements  are  combined  into  groups  or  radicles 
which  do  not  separate  into  their  constituent  atoms.  N03  is  such  a 
radicle,  and  in  the  first  equation  given  the  number  of  N03  radicles 
on  each  side  of  the  equation  must  be  identical.  S04  acts  as  a 
radicle  in  the  same  manner.  In  the  third  equation  the  carbon 
remains  combined  with  two  atoms  of  oxygen  as  C02,  and  two 
molecules  of  hydrochloric  acid  must  be  introduced,  because  one 
atom  of  calcium  requires  two  atoms  of  chlorine  to  form  calcium 
chloride,  while  the  two  atoms  of  hydrogen  unite  with  the  remain- 
ing atoms  of  oxygen  in  the  calcium  carbonate.  In  a  similar  man- 
ner the  ammonia  radicle  NH3  is  considered  as  a  unit  in  the  fourth 
equation. 

755.  Oxidation  and  Reduction  Equations  are  more  difficult  to 
balance  since  two  distinct  reactions  generally  take  place.    The 


STOICHIOMETRY. 

valence  of  two  substances  changes,  involving  the  transference  of 
oxygen  or  its  equivalent,  and  in  the  second  place  an  acid  or  an 
alkali  reacts  with  some  of  the  elements  present  to  form  salts.  In 
balancing  these  equations  the  oxidizing  and  reducing  substances 
must  first  be  introduced  in  equivalent  amounts.  Then  the  amount 
of  acid  or  base  necessary  to  combine  with  the  bases  or  acid  liber- 
ated by  the  first  reaction  must  be  introduced. 

In  order  to  balance  the  oxidizing  and  reducing  equations  the 
amount  of  oxygen  absorbed  or  liberated  must  be  known  for  each 
substance  present.  For  instance,  a  molecule  of  potassium  dichro- 
mate  breaks  up  as  follows: 

K2Cr207=K20  +  Cr203  +  30, 

liberating  three  atoms  of  oxygen.  Potassium  permanganate  breaks 
up  so  as  to  give  five  atoms  of  oxygen  for  every  two  molecules  of 
the  salt,  as  follows: 

2KMn04  =K20  +  2MnO  +  50. 

Amongst  reducing  substances  two  atoms  of  ferrous  iron  take  up 
one  atom  of  oxygen  to  form  ferric  iron  while  one  atom  of  stan- 
nous  tin  takes  up  one  atom  of  oxygen  or  its  equivalent.  Two 
molecules  of  potassium  permanganate  giving  five  atoms  of  oxygen 
are  therefore  able  to  oxidize  ten  atoms  of  ferrous  iron.  If  the 
iron  is  present  as  ferrous  sulphate  one  molecule  of  sulphuric  acid 
will  be  required  to  convert  two  atoms  of  ferrous  iron  into  ferric 
iron  as  follows: 

2FeS04+H2S04+0=Fe2(S04)3+H20.     .    .    .     (1) 

As  five  atoms  of  oxygen  are  given  by  the  two  molecules  of  potas- 
sium permanganate  we  must  multiply  each  member  of  the  equa- 
tion by  five,  giving: 


4  +  5H2S04+50=5Fe2(S04)3  +  5H20.      ,    ;    .  (2) 

The  potassium  and  manganese  of  the  potassium  permanganate  are 
also  converted  into  sulphates,  two  molecules  of  potassium  per- 
manganate requiring  three  molecules  of  sulphuric  acid  as  follows: 


2KMn04+3H2S04=K2S04+2MnS04  +  3H20  +  50.      .     .  (3) 


CALCULATION  OF   VOLUMETRIC  DETERMINATIONS.      499 
Combining  equation  (2)  and  (3),  we  obtain  the  complete  equation 
10FeS04+ 2KMn04+ 8H2S04  =5Fe2(S04)3+ K,S04+ 2MnS04 


The  free  oxygen  disappears  from  both  equations  since  it  is  present 
in  the  first  member  in  the  potassium  permanganate  and  in  the 
second  as  water. 

The  oxidation  of  stannous  chloride  with  potassium  dichromate 
can  also  be  written  in  two  equations  which  may  then  be  combined 
into  one.  The  potassium  dichromate  liberates  oxygen  in  the 
presence  of  .  reducing  agent  according  to  the  following  equation  : 


K2Cr207+8HCl=2KCl  +  2CrCl3+30  +  4H20.      .     .     .  (1) 

The  stannous  chloride  in  the  presence  of  acid  and  an  oxidizing 
agent  reacts  as  follows: 

+  2HCl+0=SnCl4+H20  ......  (2) 


As  one  molecule  of  potassium  dichromate  liberates  three  atoms  of 
oxygen  equation  (2)  must  be  multiplied  by  three,  giving: 


=3SnCl4+3H20.      .-.    .    .     .(3) 

By  combining  equations    (1)   and  (3)  we  obtain    the  complete 
equation  as  follows  : 

K,Cra07+  3SnCl2+  14HC1  =2KC1+  2CrCl3+  3SnCl4+  7H20. 

CALCULATION   OF   VOLUMETRIC   DETERMINATIONS. 

As  volumetric  solutions  are  generally  made  on  the  normal 
basis  of  strength  the  calculation  of  the  results  is  quite  simple. 
Roundabout  and  very  cumbersome  methods  of  calculation  are 
nevertheless  often  used. 

756.  Standard  Solutions.  —  When  solutions  such  as  hydrochloric 
or  sulphuric  acid  have  been  standardized  by  weighing  precipi- 
tates of  silver  chloride  or  barium  sulphate  the  weight  of  the 
acid  radicle  present  in  the  precipitate  may  be  computed.  From 
this  weight  the  corresponding  weight  of  acid  may  be  com- 
puted, and  finally  the  ratio  of  this  weight  to  the  theoretical 
weight  for  a  normal  solution  may  be  calculated.  The  same 


500  STOICHIOMETRY. 

result  may,  however,  be  obtained  by  a  single  calculation.  As 
a  liter  of  normal  hydrochloric  acid  contains  the  gram  molecular 
weight  of  the  acid  it  must  give  a  gram  molecular  weight  of  sil- 
ver chloride  according  to  the  equation, 

HC1  +  AgN03  =  AgCl  +  HN03. 

If  10  c.c.  of  the  acid  has  been  taken  for  the  determination 
the  silver  chloride  precipitate  should  weigh  1.4338  or  I/  100th  of 
the  gram  molecular  weight.  The  ratio  of  this  number  to  the 
weight  found  will  give  the  strength  of  the  acid  in  terms  of  normal. 
For  example,  if  1.6728  grams  of  AgCl  were  obtained,  the  acid  is 

(1   A7OQ\ 
i'4oQQJ-     To   dilute   the   acid   to   exact  strength   there 

must  be  added  166.7  c.c.  of  water  to  one  liter  of  the  acid.    If  the 

-I    4QQC  \ 

'  -00  ),   the   number   .85712   is  obtained 


and  the  dilution  must  be  made  by  diluting  857.12  c.c.  of  the 
strong  acid  to  one  liter. 

757.  Acidimetry  and  Alkalimetry.  —  Having  obtained  a  normal 
acid  solution  the  calculation  of  the  amount  of  any  substance 
titrated  is  equally  simple.  For  all  univalent  bases  titrated  one 
liter  of  the  acid  is  equal  to  the  gram  molecular  weight  of  the  base. 
For  example,  one  liter  of  a  normal  acid,  whether  it  is  sulphuric 
or  hydrochloric,  is  equal  to  17.034  grams  of  ammonia,  while  one 
c.c.  is  equal  to  one-thousandth  of  this  amount  or  .017034  gram. 
If  10  c.c.  of  acid  has  been  used  to  titrate  a  given  amount  of  ammo- 
nia, there  is  present  .17034  gram  of  NH3.  If  this  ammonia  has 
been  formed  from  the  nitrogen  contained  in  a  substance  analyzed, 
the  amount  of  nitrogen  may  be  computed  directly,  since  one  liter 
of  a  normal  acid  will  be  equivalent  to  14.01  grams  of  nitrogen  in 
the  form  of  ammonia.  If,  for  example,  the  ammonia  has  been 
obtained  from  an  ammonium  salt,  such  as  ammonium  sulphate, 
the  value  of  the  normal  acid  in  terms  of  this  salt  is  obtained  from 
its  molecular  weight,  half  of  which  is  taken  since  each  molecule 
contains  two  molecules  of  ammonia. 

The  value  of  a  standard  acid  in  terms  of  a  given  salt  will  some- 
times vary  with  the  indicator  used,  as  already  explained  in  Chap- 
ter XX,  page  250.  If  disodic  phosphate  is  titrated  with  a  strong 


CALCULATION  OF   VOLUMETRIC  DETERMINATIONS.      501 

acid,  using  methyl  orange  as  the  indicator,  the  following  reaction 
will  take  place: 

Na2HP04+HCl  =NaCl+NaH2P04. 

A  liter  of  normal  acid  will  therefore  be  equal  to  a  gram  molecular 
weight  of  disodic  phosphate,  which  will  be  358.4  grams  of  the  crys- 
tallized salt.  While  the  disodic  phosphate  is  alkaline  to  methyl 
orange  it  is  neutral  to  phenolphthalein,  to  which  the  trisodium  salt 
is  alkaline.  If  the  latter  salt  is  titrated  with  acid,  using  phenol- 
phthalein as  the  indicator,  the  following  reaction  takes  place: 

Na3P04+  HC1  =Na2HP04+  NaCl. 

One  liter  of  a  normal  acid  is  therefore  equal  to  the  gram  molecular 
weight  of  this  salt  or  380.44  grams  of  the  crystallized  salt.  A 
mixture  of  trisodic  and  disodic  phosphates  may  therefore  be  ana- 
lyzed by  titration  with  a  standard  acid,  using  phenolphthalein  and 
methyl  orange  as  the  indicators.  The  method  of  calculation  is 
seen  from  the  following  illustration:  One  gram  of  a  sample  of 
disodic  phosphate  was  titrated  with  fifth-normal  acid.  With 
phenolphthalein  1.50  c.c.  of  acid  were  used,  and  when  methyl 
orange  had  been  added  13.5  c.c.  acid  were  required  to  again  give 
the  acid  reaction.  The  acid  used  with  the  phenolphthalein  rep- 
resents the  amount  of  trisodic  phosphate  present,  1  c.c.  of  fifth- 
normal  acid  being  equal  to  .07608  gram  of  this  salt. 

IOTT^     1.  50  X.  07608 
Percentage  of  Na3P04.12H20  =  -  •    OOQ  —  =11.41. 


The  13.50  c.c.  of  acid  used  with  the  methyl  orange  neutralized  the 
disodic  phosphate  present  in  the  original  material  and  that  pro- 
duced by  the  titration  with  phenolphthalein.  The  acid  used  for 
the  latter  purpose  will  be  exactly  equal  to  that  used  with  the 
phenolphthalein,  in  this  case  1.50  c.c.  The  acid  necessary  to 
neutralize  the  disodic  phosphate  originally  present  will  be  12.00 
c.c.  (13.50-1.50).  The  percentage  of  Na2HP04.12H20  will  be 

12.00  X.  0717 


502  STOICHIOMETRY. 

758.  Percentage  Given  by  Number  of  Cubic  Centimeters  of 
Acid  Used. — Frequently  it  is  convenient  to  use  a  solution  of 
such  a  strength  that  when  a  given  weight  of  substance  is 
taken  the  number  of  cubic  centimeters  used  in  the  titration 
shall  be  equal  to  the  percentage  of  a  given  constituent.  For 
this  purpose  the  solution  must  be  made  of  such  a  strength 
that  100  c.c.  are  equal  to  the  amount  of  substance  to  be 
weighed  out,  provided  the  substance  is  absolutely  pure.  If  sodium 
carbonate  is  the  substance  to  be  titrated  and  one  gram  is  the 
amount  to  be  weighed  out,  100  c.c.  of  the  acid  must  be  equal  to 
1  gram  of  pure  sodium  carbonate.  As  100  c.c.  of  a  normal  acid 
are  equal  to  5.3  grams  of  sodium  carbonate,  the  acid  required  must 
be  -|§rd  normal.  If  an  acid  double  this  strength  is  used  the  num- 
ber of  cubic  centimeters  used  must  be  multiplied  by  2  to  give  the 
percentage.  The  amount  of  substance  to  be  weighed  out  may 
also  be  so  chosen  that  the  percentage  shall  be  equal  to  the  number 
of  cubic  centimeters  used  in  a  titration.  The  acid  may  then  be  of 
any  desired  strength.  The  amount  of  substance  weighed  out  must 
in  every  case  be  equal  to  the  weight  of  the  pure  substance,  which 
will  just  neutralize  100  c.c.  of  the  acid.  In  the  case  of  a  fifth-nor- 
mal acid  100  c.c.  are  equal  to  1.06  grams  of  pure  sodium  carbonate. 
If  this  amount  of  the  impure  article  be  weighed  out  and  titrated 
with  the  acid  the  number  of  cubic  centimeters  used  will  be  equal 
to  the  percentage  of  Na2C03  present.  This  is  evident  from  the 
consideration  that  while  1.06  grams  of  the  pure  compound  will 
require  100  c.c.,  any  decrease  in  the  purity  of  the  substance  will 
produce  a  proportionate  decrease  in  the  amount  of  acid  used  in 
its  titration. 

759.  Oxidation  and  Reduction  Titrations. — In  calculating  the 
results  obtained  by  means  of  oxidizing  and  reducing  solutions 
the  amount  of  oxygen  absorbed  or .  liberated  by  the  substances 
acted  upon  must  always  be  kept  in  mind.  Equivalent  weights 
of  reducing  or  oxidizing  substances  absorb  or  give  up  the  same 
amount  of  oxygen.  It  has  already  been  shown  that  31.63  grams 
of  KMn04  contain  the  same  amount  (8  grams)  of  available  oxygen 
as  49.05  grams  of  K2Cr207.  A  liter  of  solution  containing  either 
of  these  -amounts  of  oxidizing  substances  is  a  normal  solution. 
120.97  grams  of  iodine  liberate  the  same  amount  of  oxygen  to 


CALCULATION  OF   VOLUMETRIC  DETERMINATIONS.       503 

reducing  substances,  and  dissolved  in  a  liter  give  a  normal  oxidiz- 
ing solution.  These  different  amounts  of  oxidizing  substances 
are  therefore  equal  to  each  other,  because  the  same  amount  of 
free  oxygen  is  furnished  by  each. 

The  method  of  shortening  calculations  by  applying  this  princi- 
ple may  be  seen  from  the  following  examples:  A  potassium  per- 
manganate solution  which  had  been  standardized  by  means  of 
pure  iron  wire  was  used  to  standardize  a  sodium  thiosulphate 
solution.  The  standardization  of  the  potassium  permanganate 
solution  with  iron  takes  place  according  to  the  following  equation: 

2KMn04+  10FeS04+  8H2S04  =K2S04+  5Fe2(S04)3+ 2MnS04 

+  8H20. 

Two  molecules  of  potassium  permanganate  are  therefore  equal  to 
ten  atoms  of  iron.  A  measured  volume  of  the  permanganate 
solution  was  acidified  with  hydrochloric  acid  after  the  addition  of 
potassium  iodide.  Iodine  was  liberated  according  to  the  following 
equation : 

KMn04 + 8HC1  +  5KI  =  51  +  6KC1  +  MnCl2  +  4H20. 

The  iodine  liberated  is  then  titrated  with  sodium  thiosulphate 
solution.  By  this  equation  two  molecules  of  potassium  perman- 
ganate are  equal  to  ten  atoms  of  iodine.  As  by  the  first  equation 
the  same  amount  of  potassium  permanganate  is  equal  to  ten  atoms 
of  iron,  one  atom  of  iron  is  equal  to  one  atom  of  iodine;  that  is, 
55.9  parts  of  iron  are  equal  to  126.97  parts  of  iodine.  As  1  c.c.  of 
the  potassium  permanganate  solution  was  found  equal  to  .01115 
gram  of  iron  the  value  of  1  c.c.  in  iodine  is  obtained  from  the 
following  proportion: 

55.9  :  126.85  ::  .01115:* 
x  =  .  0253  gram. 

As  25  c.c.  of  the  potassium  permanganate  solution  required  23  c.c. 
of  the  thiosulphate  solution  to  titrate  the  iodine  liberated,  the 

25  X  0253 

iodine  value  of  1  c.c.  of  the  latter  solution  is  equal  to ^ ,  or 

£& 

.0275  gram. 


504  STOICHIOMETRY 

760.  lodometric  Titrations. — In  a  similar  manner  the  value  of 
the  thiosulphate  solution  in  terms  of  any  substance  it  may  be  used 
to  titrate  may  be  calculated.  The  intermediate  reactions  are  used 
simply  to  find  the  number  of  iodine  atoms  equivalent  to  one  mole- 
cule of  the  substance  titrated.  For  example,  manganese  dioxide 
may  be  boiled  with  hydrochloric  acid,  the  chlorine  evolved  being 
absorbed  in  potassium  iodide,  and  the  iodine  liberated  titrated 
with  the  thiosulphate  solution.  The  following  reactions  take 
place : 

Mn02+4HCl  =MnCl2+Cl2+2H20 

C12+2KI=2KC1+I2 
I2+Na2S203   =2NaI+Na2S406. 

One  molecule  of  manganese  dioxide  is  therefore  equal  to  two  atoms 
of  iodine.  The  value  of  1  c.c.  of  the  sodium  thiosulphate  solution 
in  terms  of  manganese  dioxide  is  obtained  from  the  following  pro- 
portion: 

I2  :  Mn02  : :  .0275  :  x 
253.7:     87     : :  .0275  :  x 
£=.00943 


APPENDIX. 


REAGENTS. 

IT  is  important  that  the  reagents  used  should  be  of  known  defi- 
nite strength  and  that  the  strength  should  be  indicated  on  the 
bottles.  So  far  as  possible  also  the  reagents  should  be  of  uniform 
strength.  This  is  not  always  possible  on  account  of  the  solubilities 
of  the  salts  employed.  The  strength  is  most  easily  indicated  by 
the  normal  system  of  nomenclature.  Chemically  pure  material 
should  always  be  used.  The  impurities  which  may  be  present 
and  the  tests  to  be  applied  are  given  in  the  notes  on  pp.  32-35. 
Many  salts  when  in  solution,  especially  those  that  react  alkaline, 
act  on  the  glass  becoming  contaminated  with  silica  and  other 
constituents  of  the  glass.  Only  recently  prepared  solutions  of 
such  reagents  should  be  used.  It  is  therefore  advisable  to  keep 
the  dry  salts  at  hand  and  make  up  the  solutions  only  as  required 
for  immediate  use. 

REAGENTS  OF  FIVE  TIMES  NORMAL  STRENGTH.    LABEL,  5  N. 

Sulphuric  Acid,  H2S04 Equivalent  =49 

Ordinary  concentrated  acid,  sp.  gr.  1.84,  has  a  strength  thirty- 
six  times  normal,  and  may  be  designated  36  N.  One  volume  of 
strong  acid  diluted  with  6  volumes  of  water  =  5  N. 

Nitric  Acid,  HN03 Equivalent  =63 

Concentrated  acid,  sp.  gr.  1.40  =  15  N.  One  volume  of  strong 
acid  diluted  with  2  volumes  of  water  =5  N. 

Hydrochloric  Acid,  HC1 Equivalent  =36.5 

Concentrated  acid,  sp.  gr.  1.20  =  13  N.  Five  volumes  of  strong 
acid  diluted  with  8  volumes  of  water  =5  N. 

505 


506  APPENDIX. 

Acetic  Acid,  H.C2H302 Equivalent  =60 

Glacial  acid,  M.P.  10°  C.;  =  17  N.  One  volume  of  glacial  acid 
diluted  with  2J  volumes  of  water =5  N. 

Potassium  Hydroxide,  KOH Equivalent  =56 

280  grams  dissolved  in  water  and  diluted  to  one  liter  =5  N. 

Sodium  Hydroxide,  NaOH Equivalent  =40 

200  grams  dissolved  in  water  and  diluted  to  one  liter  =5  N. 

Ammonia  (solution  in  water,  NH4OH) Equivalent  =35 

The  strong  solution,  sp.  gr.  0.90  =  15  N.  One  volume  of  the 
strong  solution  diluted  with  2  volumes  of  water =5  N. 

Ammonium  Sulphide,  (NH4)2S Equivalent  =34 

600  c.c.  of  5N  ammonia  are  saturated  with  sulphuretted 
hydrogen;  this  gives  hydrogen  ammonium  sulphide,  NH4HS. 
This  is  made  up  to  1  liter  by  adding  5N  ammonia.  (This 
reagent  is  slowly  decomposed  by  atmospheric  oxygen,  ammo- 
nia is  evolved,  and  yellow  ammonium  sulphide,  (NH4)2S2,  is 
formed.) 

Sodium  Sulphide,  Na2S Equivalent  =39 

"200  grams  of  sodium  hydroxide  are  dissolved  in  800  c.c.  of 
water.  400  c.c.  of  this  solution  are  saturated  with  sulphuretted 
hydrogen  and  the  remaining  half  added,  together  with  water 
sufficient  to  make  the  volume  up  to  1  liter.  When  hydrogen 
sodium  sulphide,  NaHS,  is  required,  the  sodium  hydroxide  is 
simply  saturated  with  sulphuretted  hydrogen,  without  the  further 
addition  of  sodium  hydroxide. 

Ammonium  Chloride,  NH4C1 Equivalent  =53.5 

267.5  grams  of  the  salt  dissolved  in  water  and  diluted  to  one 
liter- 5  N. 

Ammonium  Carbonate,  (NH4)2C03 Equivalent  =48 

200  grams  of  ammonium  sesquicarbonate  (commercial  carbon- 
ate) dissolved  in  350  c.c.  of  5  N  ammonia,  and  diluted  with 
water  to  one  liter  =  5  N. 

Ammonium  Acetate,  NH4C2H302 Equivalent  =77 

To  300  c.c.  of  glacial  acetic  acid  (17  N)  an  equal  volume  of 
water  is  added,  the  acid  neutralized  with  strong  ammonia  and 
diluted  to  one  liter  =5  N. 


REAGENTS.  507 


REAGENTS  OF  NORMAL  STRENGTH.    LABEL,  N. 

The  following  normal  reagents  are  prepared  by  dissolving  the 
equivalent  weight  in  grams  of  the  various  salts  in  water,  and 
diluting  to  one  liter.  In  all  cases  the  nearest  whole  number  to 
the  equivalent  weight  may  be  taken  as  sufficiently  exact. 

Barium  chloride,  BaCl2.2H20 Equivalent  weight  122.0 

Disodium  phosphate,  HNa2PO4.12H20.          "  "        119.3 

Lead  acetate,  Pb(C2H302)2.3H20 "  "        189 . 5 

"  Magnesia  mixture  "  (MgCl2.  (NH4C1)2,  and  NH4OH). 
68  grams  of  MgCl2.6H20,  together  with  165  grams  of  ammo- 
nium chloride,  are  dissolved  in  300  c.c.  of  water;  300  c.c.  of  5  N 
ammonia  are  added,  and  the  solution  diluted  with  water  to  one 
liter. 

Stannous  chloride,  SnCl2.2H20 Equivalent  weight  112.5 

112  grams  of  the  salt  are  dissolved  in  200  c.c.  of  5  N  hydro- 
chloric acid  and  the  solution  diluted  with  water  to  one  liter. 
Fragments  of  granulated  tin  should  be  placed  in  the  solution. 


REAGENTS  OF  VARIOUS  STRENGTHS. 

Ammonium  oxalate,  (NH4)2C204.2H20.  Equivalent  weight  80.0 

N 

40  grams  dissolved  in  one  liter  =  —  solution. 

A 

Molybdate  Solution. — The  molybdate  solution  is  made  as  fol- 
lows: 75  grams  of  ammonium  molybdate  are  dissolved  in  500  c.c. 
of  water  with  the  addition  of  a  little  ammonia  if  necessary.  If 
still  turbid,  the  solution  is  filtered  and  then  poured  with  constant 
stirring  into  500  c.c.  of  a  mixture  of  250  c.c.  of  concentrated  nitric 
acid  (sp.  gr.  1.40)  and  250  c.c.  of  water.  The  solution  may  also 
be  made  from  molybdic  oxide  by  dissolving  60  grams  in  440  c.c.  of 
water  and  60  c.c.  strong  ammonia  (sp.  gr.  0.90),  and  pouring  the 
solution  into  500  c.c.  of  nitric  acid  diluted  as  already  directed. 
The  freshly  made  molybdate  solution  must  be  allowed  to  stand  in 
a  warm  place  for  several  days.  The  clear  solution  is  decanted  or 
filtered  off  for  use.  Large  amounts  of  the  solution  should  not  be 


503  APPENDIX. 

made  up  at  one  time,  since  it  will  not  keep  more  than  several 
months,  as  the  molybdic  acid  is  slowly  precipitated. 

Mercuric  chloride,  HgCl2 Equivalent  weight  135.5 

N 

27  grams  dissolved  in  one  liter =-  solution. 

5 

Silver  nitrate,  AgN03 Equivalent  weight  170 . 0 

N 

3.4  grams  dissolved  in  100  c.c.  =7-  solution. 

o 

Potassium  dichromate,  K2Cr207 Equivalent  weight  294.5 

100  grams  are  dissolved  in  one  liter  of  water. 


TABLE  I.     INTERNATIONAL  ATOMIC  WEIGHTS 
FOR   1909* 


Name. 

Symbol. 

Atomic 
Weight. 

Name. 

Smybol. 

Atomic 
Weight. 

Aluminium 

Al 
Sb 
A 
As 
Ba 
Bi 
B 
Br 
Cd 
Cs 
Ca 
C 
Ce 
Cl 
Cr 
Co 
Cb 
Cu 
Dy 
Er 
Eu 
F 
Gd 
Ga 
Ge 
Gl 
Au 
He 
H 
In 
I 
Ir 
Fe 
Kr 
La 
Pb 
Li 
Lu 
Mg 
3  n 
Hg 

27.1 
120.2 
39.9 
75.0 
137.37 
208.0 
11.0 
79.92 
112.40 
132.81 
40.09 
12.00 
140.25 
35.46 
52.1 
58.97 
93.5 
63.57 
162.5 
167.4 
152.0 
19.0 
157.3 
69.9 
72.5 
9.1 
197.2 
4.0 
1.008 
114.8 
126.92 
193.1 
55.85 
81.8 
139.0 
207.10 
7.00 
174.0 
24.32 
54.93 
200  0 

Molybdenum  
Neodymium     . 

Mo 
Nd 
Ne 
Ni 
N 
Os 
0 
Pd 
P 
Pt 
K 
Pr 
Ra 
Rh 
Rb 
Ru 
Sm 
Sc 
Se 
Si 

A 

Sr 
S 
Ta 
Te 
Tb 
Tl 
Th 
Tm 
Sn 
Tl 
W 
U 
V 
Xe 
Yb 

Yt 
Zn 
Zr 

96.0 
144.3 
20.0 
58.68 
14.01 
190.9 
16.00 
106.71 
31.0 
195.0 
39.10 
140.6 
226.4 
102.9 
85.45 
101.7 
150.4 
44.1 
79.2 
28.3 
107.88 
23.00 
87.62 
32.07 
181.0 
127.5 
159.2 
204.0 
232.42 
168.5 
119.0 
48.1 
184 
238.5 
51.2 
128 
172.0 

89.0 
65.37 
90.6 

Antimony  .  .   ..  .   .  * 
A  rtron 

Neon  

Nickel       

Barium  

Nitrogen  

Bismuth. 

Boron 

Bromine               « 

Cadmium              . 

Caesium        •  .     . 

Platinum  

Calcium               .  . 

Potassium    

Carbon 

Praseodymium  .  .  . 
Radium     

Chlorine  

Rhodium  

Chromium  ... 

Cobalt  

Samarium  

Copper 

Scandium  .... 

Dysprosium 

Silicon 

Europium     . 

Silver    

Fluorine 

Sodium 

Gadolin  ium 

Strontium 

Gallium       

Sulphur  

Germanium 

Tantalum  

Tellurium  

Gold 

Terbium   .... 

HeliuTn  .    .  . 

Thallium  

Hydrogen  .... 

Indium  

Iodine  

Tin  

Iridium    

Iron 

Tungsten 

Krypton  
Lanthanum 

Uranium  

Vanadium    •  •  .  . 

Lead  
Lithium      ...    . 

Xenon    .    .  . 

Ytterbium  

Lutecium. 

1    (Neoyt  terbium). 
Yttrium 

Magnesium 

Manganese 

Zinc              . 

Mercury  

Zirconium     

*  Compiled  by  the  International  Committee  on   Atomic  Weights  con- 
sisting of  F.  W.  Clarke,  W.  Ostwald,  T.  E.  Thorpe,  and  G.  Urbain. 

509 


£10  TABLES. 

TABLE  II.— CHEMICAL  FACTORS  AND  THEIR  LOGARITHMS. 


Atomic 
Weight. 

Weighed. 

Required 

Factor. 

Per  Cent. 

Loga- 
rithms.* 

Aluminium  .... 
Antimony  

27.1 

120.2 
75.0 

137.37 

208.0 

79.92 
112.40 

40.09 

12.00 
35.46 

52.1 

58.97 
C3.57 

Al,03 

A1P04 
ii 

SbaO4 

AsaS3 

MgaAsaO7 
tt 

« 

BaSO4 
(i 

14 

« 

BaCOs 

it 

o 
« 

BaCrO4 
ii 

Bi,03 
BiOCl 

AgBr 
(i 

CdO 

CdS 

u 

CaO 
« 

CaCO3 
u 

CaSO* 

CO2 
AgCl 

Ag 

u 

Cr3O, 
« 
it 

PbCrO4 
ii 

<« 
tt 

BaCrO4 
11 
ii 
(t 
Co 
CoSO4 

14 

2CoSO4.3K2SO4 

14 

Cu 

Al 

Al 
A13O, 
Sb 
As 
As,O3 
As 
As,09 
AsaO, 
Ba 
BaCla 
BaCla.2H20 
BaO 
Ba 
BaO 
C 
CO2 
Ba 
BaO 
Bi 
Bi 
Bia03 
Br 
HBr 
Cd 
Cd 
CdO 
Ca 
CaCO, 
Ca 
CaO 
Ca 
CaO 
C 
Cl 
HCI 
Cl 
HCI 
Cr 
CrO, 
Cr04 
Cr 
CrO3 
CrO4 
CraO3 
Cr 
CrO3 
CrO4 
Cr2O3 
CoO 
Co 
CoO 
Co 
CoO 
CuO 
CuSO4.5H2O 

53.03 
22.20 
41.85 
78.97 
60.92 
80.42 
48.29 
63.74 
74.04 
58.85 
89.23 
104.66 
65.71 
69.61 
77.71 
06.08 
22.29 
54.20 
60.51 
89.65 
80.17 
89.42 
42.56 
43.11 
87.54 
77.80 
88.88 
71.47 
178.44 
40.06 
56.04 
29.44 
41.20 
27.27 
24.74 
25.44 
32.87 
33.80 
68.46 
131.54 
152.57 
16.12 
30.97 
35.92 
23.55 
20.55 
39.49 
45.80 
30.02  ' 
127.14 
38.04 
48.36 
14.16 
18.00 
125.17 
392.83 

9.72455 
9.34625 
9.62170 
9.89749 
9.78478 
9.90536 
9.68383 
9.80441 
9.86947 
9.76975 
9.95054 
0.01982 
9.81760 
9.84261 
9.89046 
8.78390 
9.34817 
9.73396 
9.78181 
9.95257 
9.90399 
9.95142 
9.62896 
9.63461 
9.94220 
9.89099 
9.94879 
9.85415 
0.25150 
9.C0265 
9  74850 
9.46899 
9.61484 
9.43573 
9.39337 
9.40543 
9.51680 
9.52886 
9.83546 
0.11905 
0.18345 
9.  20737 
9.  49096 
9.55536 
9.37191 
9.31291 
9.59650 
9.66090 
9.47745 
0.10426 
9.  58019 
9.68445 
9.15107 
9.25534 
0.09750 
0.59420 

Cadmium  .... 

Carbon    

Chromium  
Cobalt  

Copper              . 

*  The-10  after  each  of  the  logarithms  has  been  omitted. 


TABLES.  511 

CHEMICAL  FACTORS  AND  THEIR  LOGARITHMS-  (Continued). 


Atomic 
Weight. 

Weighed. 

Required. 

Factor. 

Per  Cent. 

Loga- 
rithms. 

Fluorine  

19.0 

1.008 
126-92 

55.85 
207.10 

24.32 
54.93 

200.0 

58.68 
14.01 

CuO 

Cu28 

H 

CuCNS 
CaF2 

(4 

H20 
Agl 

PdI2 
Fe203 
FeP04 

a 

PbO 
PbO2 
PbS 

PbS04 
PbCl2 
PbCr04 

MgO 
Mg2P207 

MgS04 

Mn3O4 

»  i 

MnS 

(  t 

Mn2P2O7 

1  1 

MnSO4 
« 

Hg 
HgS 

a 

HgCl 

(t 

Ni 
NiO 
NiSO4 

(NH4)2PtCl8 

«  i 
C20H16N4.HNO 

Cu 
Cu 
CuO 
Cu 
CuO 
F 
HF 
H 
I 
HI 
HI 
I 
Fe 
FeO 
Fe 
FeO 
Pb 
Pb 
Pb 
PbO 
Pb 
PbO 
Pb 
PbO 
Pb 
PbO 
Mg 
Mg 
MgO 
Mg 
MgO 
Mn 
MnO 
MnO2 
Mn 
MnO 
Mn 
MnO 
Mn 
MnO 
HgO 
Hg 
HgCl2 
HgO 
Hg 
HgO 
NiO 
Ni 
Ni 
NiO 
N 
NH3 
NH4 
HNO3 
N205 

79.89 
79.86 
99.96 
52.26 
65.41 
48.66 
51.24 
11.19 
54.06 
54.48 
70.97 
70.40 
69.94 
89.98 
37.02 
47.63 
92.83 
86.62 
86.59 
93.28 
68.31 
73.59 
74.49 
80.25 
64.08 
69.03 
60.32 
21.85 
36.22 
20.20 
33.49 
72.03 
93.01 
113.98 
63.14 
81.53 
38.70 
49.98 
36.38 
46.97 
108.00 
86.18 
116.74 
93.07 
84.94 
91.73 
127.26 
78.58 
37.92 
48.26 
6.31 
7.68 
8.13 
15.47 
12.87 

9.90250 
9.90232 
9.99981 
9.71814 
9.81564 
9.68718 
9.70964 
9.04884 
9.73283 
9.73627 
9.85704 
9.84760 
9-84475 
9.95416 
9.56848 
9.67789 
9.96768 
9.93760 
9.93747 
9.96979 
9.83449 
9.86681 
9.87211 
9.90443 
9.80671 
9.83903 
9.78044 
9.33938 
9.55894 
9.30538 
9-52493 
9.85749 
9.96851 
0.05685 
9.80029 
9.91131 
9.58774 
9.69876 
9.56083 
9.67185 
0.03342 
9.93541 
0.06722 
9.96883 
9.92911 
9.96253 
9.10471 
9.89529 
9.57886 
9.68357 
8.80024 
8  88514 
8.91004 
9.18952 
9.10958 

Hydrogen.  
Iodine  

Lead  .... 

Magnesium  .  .  . 
Manganese  .  .  - 

Mercury.    .  . 

Nickel  .  .  . 

Nitrogen  .  .  . 

512  TABLES. 

CHEMICAL  FACTORS  AND  THEIR  LOGARITHMS— (Continued). 


Atomic 
Weight. 

Weighed. 

Required. 

Factor. 

Per  (  ent. 

Loga- 
rithms.* 

Platinum  

195.0 

Pt 

N 

14.37 

9.15744 

ti 

NH, 

17.47 

9.24234 

i  < 

NH4 

18.50 

9.26724 

Phosphorus  .... 

31.0 

Mg2P307 

P 

27.85 

9.44478 

P04 

85.34 

9.93114 

It 

P.05 

63.78 

9.80468 

Molybdenum... 

96.0 

NH4),PO4.12MoO, 

P 

1.65 

8.21787 

" 

P04 

5.06 

8.70423 

(i 

F.O, 

3.78 

8.57777 

Potassium  

39.10 

KC1 

K 

52.44 

9.71967 

(i 

K2O 

63.17 

9.80051 

K,PtCl6 

K 

16.09 

9.20661 

" 

K20 

19.38 

9.28745 

K2S04 

K 

44.87 

9.65199 

n 

K2O 

54.06 

9.73283 

Selenium  

79.2 

Se 

SeO2 

140.40 

0.14737 

Silicon  

28.3 
107.88 

Si02 
AgCl 

Si 
Ag 

46.93 
75.26 

9.67147 
9.87657 

Silver  

AgBr 

.  & 

Ag 

57.44 

9.75924 

Agl 

Ag 

45.95 

9.66224 

23.00 

Ag2S 
Nad 

Ag 

Na 

87.06 
39.34 

9.93982 
9.59487 

Na2O 

53.03 

9^72451 

Na2SO4 

Na 

32.38 

9.51026 

<  < 

Na20 

43.64 

9.  63989 

Na2C03 

Na 

43.40 

9.63745 

<. 

Na20 

58.49 

9.76708 

Strontium  

87.62 

SrCO, 

Sr 

59.35 

9.77345 

" 

SrO 

70.19 

9.84G29 

SrS04 

Sr 

47.70 

9.67852 

" 

SrO 

56.41 

9.75136 

Sulphur  

32.07 

BaS04 

S 

13.74 

9.13793 

11 

S02 

27.45 

9.43S48 

(i 

S03 

34.30 

9.53530 

« 

S04 

41.16 

9.61442 

" 

H2S04 

42.02 

9.C2345 

Tellurium  

127.5 

Te 

Te02 

125.09 

0.09725 

.      Sn 

SnO 

113.44 

0.05478 

M 

SnO2 

126.89 

0.10343 

Tin  

119.0 

SnO, 

Sn 

78.81 

9.89657 

Titanium  

48.1 

k  '"  v^2 

Ti02 

Ti 

60.05 

9.77852 

Zinc  

65.37 

ZnO 

Zn 

80.34 

9.90492 

Zn2P2Ot 

Zn 

42.90 

9.  63248 

ZnO 

53.40 

9.72756 

ZnS 

Zn 

67.00 

9.82664 

it 

ZnO 

83.51 

9.92172 

TABLES  OF  LOGAKITHMS  AND 
ANTILOGARITHMS. 


514 


TABLES. 


TABLE  III.— LOGARITHMS. 


Natural 
Numbers. 

0 

1 

o 

3 

4 

5 

C 

• 

r 

8 

9 

PROPORTIONAL  PARTS. 

1 

9 

3 

4 

5 

6 

7 

8 

9 

10 

0000 

0043 

0086 

0128 

0170 

0212 

0253 

0294 

0334 

0374 

4 

8 

12 

17 

21 

2529 

33 

37 

11 

0414 

0453 

0492 

05310569 

0607 

0645 

0682 

0719 

0755 

4 

8 

11 

15 

19 

2326 

30 

34 

12 

0792 

0328 

0864 

08990934 

0969 

1004 

1038 

10721106 

3 

7 

10  14 

17 

21  24  28 

31 

13 

1139 

1173 

1206 

1239  1271 

1303 

1335 

1367 

1399  1430 

3 

6 

1013 

16 

19  23  26 

29 

14 

1461 

1492 

1523 

1553  1584 

1614 

1644 

1673 

1703  1732 

3 

G 

9 

12 

15 

1821  24 

27 

i 

15 

1761 

1790 

1818 

1847 

1875 

1903 

1931 

1959 

1987 

2014 

3 

6 

8 

11 

14 

17  20  22 

25 

16 

2041 

2068 

2095 

2122 

214S 

2175 

2201 

2227 

2253 

2279 

3 

5 

8 

11 

13 

16  18  21 

24 

17 

2304 

2330 

2355 

2380 

2405 

2430 

2455 

2480 

2504 

2529 

2 

5 

7 

10 

12 

15  17  20 

22 

18 

2553 

2577 

2601 

2625 

2648 

2672 

2695 

2718 

2742 

2765 

2 

5 

7 

9 

12 

14  16  19 

21 

19 

2788 

2810 

2833 

2856 

2878 

2900 

2923 

2945 

2967 

2989 

2 

4 

7 

9 

11 

13  16  18 

20 

20 

30103032 

3054 

3075  3096 

3118 

3139 

3160 

3181 

3201 

2 

4 

6 

8 

11 

1315 

17  19 

21 

32223243 

3263 

3284  3304 

3324 

3345 

3365 

3385 

3404 

2 

4 

6 

8 

10 

12  14  16  18 

22 

3424  3444 

3464 

348313502 

3522 

3541 

3560 

3579 

3598 

2 

4 

6 

8 

10 

12  14  15  17 

23 

36173636 

3655 

3674!3692 

3711 

3729 

3747 

3766 

3784 

2 

4 

6 

7 

9 

11 

13  15  17 

24 

3802 

3820 

3838 

38563874 

3892 

3909 

3927 

3945 

3962 

2 

4 

5 

7 

911 

12 

1416 

25 

3979 

3997 

4014 

4031 

4048 

4065 

4082 

4099 

4116 

4133 

2 

8 

5 

7 

0 

10 

12 

14 

15 

26 

41504166 

4183 

4200  4216 

4232 

4249 

4265 

4231 

4298 

2 

3 

5 

7 

8 

10 

11 

1315 

27 

43144330 

4346 

43624378 

4393 

4409 

4425 

4440 

4456 

2 

3 

5 

8 

8 

911 

13  14 

28 

4472 

4487 

4502 

4518  4533 

4548 

4564 

4579 

4594 

4609 

2 

3 

5 

6 

8 

911  1214 

29 

4624 

4639 

4654 

4669  4683 

4698 

4713 

4728 

4742 

4757 

1 

3 

4 

6 

7 

9 

10  12  13 

30 

4771 

4786 

4800 

4814 

4829 

4843 

4857 

4871 

4886 

4900 

3 

4 

6 

7 

9 

10 

11 

13 

31 

4914 

4928 

4942 

4955  4969 

4983 

4997 

5011 

5024 

5038 

3 

4 

G 

7 

8 

10 

11  12 

32 

5051 

5065 

5079 

5092!  5105 

5119 

5132 

5145 

5159 

5172 

3 

4 

5 

7 

8 

9 

11  12 

33 

5185 

5198 

5211 

5224  5237 

5250 

5263 

5276 

5289 

5302 

3 

4 

5 

6 

8 

9 

1012 

34 

5315 

5328 

5340 

5353 

5366 

5378 

5391 

5403 

5416 

5428 

3 

4 

5 

6 

8 

9 

1011 

35 

5441 

5453 

5465 

5478 

5490 

5502 

5514 

5527 

5539 

5551 

1 

2 

4 

5 

6 

7 

9 

1011 

36 

5563 

5575 

5587 

5599 

5611 

5623 

5635 

5647 

5658 

5670 

1 

2 

4  5 

6 

7 

8 

1011 

37 

56  2 

5694 

5705 

5717 

5729 

5740 

5752 

5763 

5775 

5786 

1 

2 

3  5 

6 

7 

8 

910 

38 

5798 

5809 

5821 

5832 

5843 

5855 

5866 

5877 

5888 

5899 

1 

2 

3 

5 

6 

7 

8 

9  10 

39 

5911 

5922 

5933 

5944 

5955 

5966 

5977 

5988 

5999 

6010 

1 

2 

3 

4 

5 

7 

8 

9  10 

40 

6021 

6031 

6042 

6053 

6064 

6075 

6085 

6096 

6107 

6117 

1 

2 

3 

4 

5 

6 

8 

9  10 

41 

6128 

6138 

6149 

6160 

6170 

6180 

6191 

6201 

6212 

6222 

1 

2 

3 

4 

5 

6 

7 

8  9 

42 

6232 

6243 

6253 

6263 

6274 

6284 

6294 

6304 

6314 

6325 

1 

2 

3 

4 

5 

6 

7 

8  9 

43 

6335 

6345  6355  6365 

6375 

6385 

6395 

6405 

6415 

6425 

1 

2 

3 

4 

5 

G 

7 

8  9 

44 

6435 

6444 

6454 

6464 

6474 

6484 

6493 

6503 

6513 

6522 

1 

2 

3 

4 

5 

6 

7 

8  9 

45 

6532 

6542 

6551 

6561 

6571 

6580 

6590 

6599 

6609 

6618 

1 

2 

3 

4 

8 

6 

7 

8  9 

46 

6628 

6637 

6646 

6656 

6665 

6675 

6684 

6693 

6702 

6712 

1 

2 

3 

4 

5 

6 

7 

7  8 

47 

6721 

6730 

6739 

6749 

6758 

6767 

6776 

6785 

6794 

6803 

1 

2 

3 

4 

5 

5 

6 

7  8 

48 

6812 

6821 

6830 

6839 

6848 

6857 

6866  6875 

6884 

6893 

1 

2 

3 

4 

4 

5 

u 

7  8 

49 

6902 

6911 

6920 

6928 

6937 

6946 

6955 

6964 

6972 

6981 

1 

2 

3 

4 

4 

5 

6 

7  8 

50 

6990 

6998 

7007 

7016 

7024 

7033 

7042 

7050 

7059 

7067 

1 

2 

3 

3  4 

5 

6 

7 

0 

51 

7076 

7084 

7093 

7101 

7110 

7118 

7126 

7135 

7143 

7152 

1 

2 

3 

3 

4 

5 

6  7 

8 

52 

7160 

7168 

7177 

7185 

7193 

7202 

7210 

7218 

7226 

7235 

1 

2 

2 

3 

4 

5 

6  7 

7 

53 

7243 

7251 

7259 

7267 

7275 

7284 

7292 

7300 

7308 

7316 

1 

2 

2 

3 

4 

6  6 

•- 

54 

7324 

7332 

7340 

7348 

7356 

7364 

7372 

7380 

7388 

7396 

1 

2 

2 

3 

4 

• 

6  6 

7 

TABLES. 
LOGARITHMS. 


515 


in 

PROPORTIONAL  PARTS. 

ll 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

|» 

1 

9 

3 

4 

fi 

6 

7 

8 

9 

55 

7404 

74127419 

7427 

7435 

7443 

7451 

7459 

7466 

7474 

1 

2 

2 

3 

4 

5 

5 

6 

7 

56 

7482 

7490  7497 

7505 

7513 

7520 

7528 

7536 

7543 

7551 

1 

2 

2 

3 

4 

5 

5 

6 

7 

57 

7559 

7566  7574 

7582 

7589 

7597 

7604 

7612 

7619 

7627 

1 

2 

2 

4 

5 

5 

6 

7 

58 

7634 

7642  7649 

7657 

7664 

7672 

7679 

7686 

7694 

7701 

1 

2 

3 

4 

4 

5 

6 

7 

59 

7709 

7716  7723 

7731 

7738 

7745 

7752 

7760 

7767  7774 

1 

2 

3 

4 

4 

6 

6 

7 

60 

7782 

7789  7796 

7803 

7810 

7818 

7825 

7832 

7839  7846 

1 

2 

3 

4 

4 

5 

6 

6 

61 

7853 

7860  7868 

7875 

7882 

7889 

789: 

7903 

7910  7917 

1 

2 

3 

4 

4 

5 

6 

6 

62 

7924 

7931 

7938 

7945 

7952 

7959 

7966 

7973 

7980  7987 

1 

2 

3 

3 

4 

5 

6 

6 

63 

7993 

8000  8007  8014 

8021 

8028 

8035 

8041 

80488055 

1 

1 

2 

3 

3 

4 

5 

5 

6 

64 

8062 

806980758082 

8089 

8096 

8102 

8109 

81168122 

1 

1 

2 

3 

3 

4 

5 

5 

6 

65 

8129 

8136 

8142 

8149 

8156 

816281698176 

8182  8189 

1 

1 

2 

3 

3 

4 

5 

5 

6 

66 

8195 

8202  8209  8215 

8222 

822882358241 

82488254 

1 

1 

2 

a 

3 

4 

5 

5 

6 

67 
68 

8261 
8325 

8267  8274  8280 
833183388344 

8287 
8351 

82938299830683128319 
8357  8363  8370  8376  8382 

1 
1 

2 

2 

3 
3 

3 
3 

4 
4 

5 

4 

5 
5 

6 
6 

69 

8388 

839584018407 

8414 

8420  8426  8432  8439  8445 

1 

2 

2 

3 

4 

4 

5 

6 

70 
71 

8451 
8513 

8457  8463  8470!8476 
8519852585318537 

8482  8488  8494  8500*8506 
8543  8549  8555  8561  8567 

1 

2 
2 

2 
2 

3 
3 

4 
4 

4 
4 

5 

5 

6 
5 

-  72 

8573 

8579858585918597 

8603  8609  8615  8621  8627 

1 

2 

2 

3 

4 

4 

fi 

5 

73 

8633 

8639  8645  8651 

8657 

8663  8669  8675  8681  8686 

1 

2 

2 

3 

4  4 

5 

5 

74 

8692 

8698  8704 

8710 

8716 

87228727873387398745 

1 

2 

2 

3 

4|4 

5 

5 

75 

8751 

8756 

8762 

8768  8774 

8779878587918797 

8802 

2 

2 

3 

3 

4 

5 

5 

76 

8808 

8814 

8820  8825  8831 

8837  8842  8848  8854  8859 

2 

2 

3 

3 

4 

5 

5 

'77 

8865 

8871 

8876  8882  8887 

8893  8899  8904  8910  8915 

2 

2 

3 

3 

4 

4 

5 

78 

8921 

8927 

8932 

89388943 

8949  8954  8960  8965  8971 

2 

2 

3 

3 

4 

4 

5 

79 

8976 

8982 

89S7 

89938998 

9004  9009  9015  9020  9025 

2 

2 

3 

3 

4 

4 

5 

80 

9031 

9036 

9042 

9047  9053 

9058  9063  9069  9074 

9079 

1 

2 

2 

3 

3  4 

4 

5 

.  81 

9085 

9090 

9096  9101  9106 

9112911791229128 

9133 

1 

2 

2 

3 

3 

4 

4 

5 

82 

9138 

9143 

914991549159 

91659170917591809186 

1 

2 

2 

3 

3 

4 

4 

5 

83 

9191 

9196 

920192069212 

9217  9222  9227  9232  9238 

1 

2 

2 

3 

3 

4 

4 

5 

84 

9243 

9248 

9253  9258  9263 

9269  9274  9279 

9284  9289 

1 

2 

2 

3 

3 

4 

4 

5 

85 

9294 

9299 

9304  9309  9315 

9320  9325  9330 

9335  9340 

1 

2 

2 

3 

3 

4 

4 

5 

86 

9345 

9350  9355  9360:9365 

9370  9375  9380 

9385  9390 

I 

2 

2 

3 

3 

4 

4 

5 

87 

9395 

9400940594109415 

9420  9425  9430 

9435  9440 

0 

1 

2 

2 

3 

3 

4 

4 

88 

9445 

9450  9455  9460 

9465 

9469  9474  9479 

94849489 

0 

1 

2 

2 

3 

8 

4 

4 

89 

9494 

9499  9504 

9509 

9513 

9518  9523  9528 

95339538 

0 

1 

2 

2 

3 

3 

4 

4 

90 

9542 

9547  9552 

9557 

9562 

9566  9571  9576 

9581 

9586 

0 

1 

2 

2 

3 

3 

4 

4 

91 

9590 

9595960096059609 

9614  9619  9624 

9628|9633 

0 

1 

2 

2 

3 

3 

4 

4 

92 

9638 

96439647 

9652  9657 

9661  9666  9671 

9675 

9680 

0 

1 

2 

2 

3 

3 

4 

4 

93 

9685 

96899694 

96999703 

970S  9713  9717 

9722 

9727 

0 

1 

2 

2 

3 

3 

4 

4 

94 

9731 

9736  9741 

97459750 

9754  9759  9763 

9768 

9773 

0 

1 

2 

2 

3 

3 

4 

4 

95 

9777 

9782  9786 

9791  9795 

9800  9805  9809 

9814 

9818 

0 

1 

1 

2 

2 

3 

3 

4 

4 

96 

9823 

9827  9832 

983619841 

9845  9850  9854  9859 

9863 

0 

1 

1 

2 

2 

3 

3 

4 

4 

97 

9868 

9872  9877 

9881*9886 

9890  9894  9899,9903 

9908 

0 

1 

1 

2 

2 

3 

3 

4 

4 

93 

9912 

9917  9921  9926  9930  9934  9939  9943  9948 

9952 

0 

1 

1 

2 

2 

3 

3 

4 

4 

99 

•—  

9956 

9961  9965  9969  9974 

9978  9983  9987 

9991 

9996 

0 

1 

1 

2 

2 

3 

3 

3 

4 

516 


TABLES. 
TABLE  IV.— ANTILOGARITHMS. 


Logarithms. 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

PROPORTIONAL  PARTS. 

1 

2 

3 

4 

5 

6 

7 

8 

9 

.00 

1000 

1002 

1005 

1007 

1009 

1012 

1014 

1016 

1019 

1021 

0 

0 

1 

1 

1 

Q 

2 

2 

.01 

1023 

1026 

1028 

1030 

1033 

1035 

1038 

1040 

1042 

1045 

0 

0 

1 

1 

1 

o 

4 

2 

2 

.02 

1047 

1050 

1052 

1054 

1057 

1059  1062 

1064 

1067 

1069 

0 

0 

1 

1 

2 

2 

2 

.03 

1072 

1074 

1076 

1079  1081 

1084 

1086 

1089 

1091 

1094 

0 

0 

1 

1 

2 

2 

2 

.04 

1096 

1099 

1102 

1104 

1107 

1109 

1112 

1114 

1117 

1119 

0 

1 

1 

2 

2 

2 

2 

.05 

1122 

1125 

1127 

1130 

1132 

1135 

1138 

1140 

1143 

1146 

0 

1 

1 

2 

2 

2 

2 

.06 

1148 

1151 

1153 

1156 

1159 

1161 

1164 

1167 

1169 

1172 

0 

1 

1 

2 

2 

2  2 

.07 

1175 

1178 

1180 

1183 

1186 

1189 

1191 

1194 

1197 

1199 

0 

1 

1 

2 

2 

2  2 

.08 

1202 

1205 

1208 

1211 

1213 

1216 

1219 

1222 

1225 

1227 

0 

1 

1 

2 

2 

2 

3 

.09 

1230 

1233 

1236 

1239 

1242 

1245 

1247 

1250 

1253 

1256 

0 

1 

1 

2 

2 

2 

3 

.10 

1259 

1262 

1265 

1268 

1271 

1274 

1276 

1279 

1282 

1285 

0 

1 

1 

2 

2 

2 

3 

.11 

1288 

1291 

1294 

1297 

1300 

1303 

1306 

1309 

1312 

1315 

0 

1 

2 

2 

2 

2 

3 

.12 

1318 

1321 

1324 

1327 

1330 

1334 

1337 

1340 

1343 

1346 

0 

1 

2 

2 

2 

2 

3 

.13 

1349 

1352 

1355 

1358 

1361 

1365 

1368 

1371 

1374 

1377 

0 

1 

2 

2 

2 

3 

3 

.14 

1380 

1384 

1387 

1390 

1393 

1396 

1400 

1403 

1406 

1409 

0 

1 

2 

2 

2 

3 

3 

.15 

1413 

1416 

1419 

1422 

1426 

1429 

1432 

1435 

1439 

1442 

0 

1 

2 

2 

2 

3 

3 

.16 

1445 

1449 

1452 

1455  1459 

1462 

.1466 

1469 

1472 

1476 

0 

1 

2 

2 

2 

3 

3 

.17 

1479 

1483 

1486 

1489  1493 

1496 

1500 

1503 

1507 

1510 

0 

1 

2 

2 

2 

3 

3 

.18 

1514 

1517 

1521 

1524 

152* 

1531 

1535 

1538 

1542 

1545 

0 

1 

2 

2 

2 

3 

3 

.19 

1549 

1552 

1556 

1560 

1563 

1567 

1570 

1574 

1578 

1581 

0 

1 

2 

2 

3 

3 

3 

.20 

1585 

1589 

1592 

1596 

1600 

1603 

1607 

1611 

1614 

1618 

0 

1 

1 

2 

2 

3 

3 

3 

.21 

1622 

1626 

1629 

1633 

1637 

1641 

1644 

1648 

1652 

1656 

0 

1 

2 

2 

2 

3 

3 

3 

.22 

1660 

1663 

1667 

1671 

1675 

1679 

1683 

1687 

1690 

1694 

0 

1 

2 

2 

2 

3 

3 

3 

.23 

1698 

1702 

1706 

1710 

1714 

1718 

1722 

1726 

1730 

1734 

0 

1 

2 

2 

2 

3 

3 

4 

.24 

OR 

1738 

770 

1742 

709 

1746 

17QC 

1750 

1  701 

1754 

•I  7QC 

1758 

7QQ 

1762 

-IOAO 

1766 

1807 

1770 
QI  i 

1774 

1Q1  fi 

0 

1 

2 

2 

2 

3 

3 

4 

pZQ 

.26 

1  1  O 

1820 

tQft 

1824 

1  /OO 

1828 

i  /yi 
1832 

i  <yo 
1837 

.  /yy 
1841 

loUo 

1845 

loUi 

1849 

.Oil 

1854 

J.O  AU 

1858 

0 

1 

2 

2 

3 

3 

3 

4 

.27 

1862 

1866 

1871 

1875 

1879 

1884 

1888  1892 

1897 

1901 

0 

1 

2 

2 

3 

3 

3 

4 

.28 

1905 

1910 

1914 

1919 

1923 

1928  1932  1936 

1941 

1945 

0 

1 

2 

2 

3 

3 

4 

4 

.29 

1950 

1954 

1959 

1963 

1968 

1972 

1977  1982 

1986 

1991 

0 

1 

2 

2 

3 

3 

4 

4 

.30 

1995 

2000 

2004 

2009 

2014 

2018 

2023  2028 

2032 

2037 

0 

1 

2 

2 

3 

3 

4 

4 

.31 

2042 

2046 

2051 

2056 

2061 

2065  2070 

2075 

2080 

2084 

0 

] 

2 

2 

3 

3 

4 

4 

.32 

2089 

2094 

2099 

2104 

2109 

2113 

2118 

2123 

2128 

2133 

0 

1 

2 

2 

3 

3 

4 

4 

.33 

2138 

2143 

2148 

2153 

2158 

2163 

2168 

2173 

2178 

2183 

0 

1 

2 

2 

3 

3 

4 

4 

.34 

2188 

2193 

2198 

2203 

2208 

2213 

2218 

2223 

2228 

2234 

1 

1 

2 

2 

3 

3 

4 

4 

5 

.35 

2239 

2244 

2249 

2254 

2259 

22652270 

2275 

2280 

2286 

1 

1 

2 

2 

3 

3 

4 

4 

5 

.36 

2291 

2296 

2301 

2307 

2312 

2317 

2323 

2328 

2333 

2339 

1 

1 

2 

2 

3 

3 

4 

4  5 

.37 

2344 

2350 

2355 

2360 

2366 

2371 

2377 

2382 

2388 

2393 

1 

2 

2 

3 

3 

4 

4  5 

.38 

2399 

2404 

2410 

2415 

2421 

2427 

2432 

2438 

2443 

2449 

2 

2 

3 

3 

4 

4  5 

.39 

2455 

2460 

2466 

2472 

2477 

2483 

2489 

2495 

2500 

2506 

2 

2 

3 

3 

4 

5  5 

.40 

2512 

2518 

2523 

2529 

2535 

2541 

2547 

2553 

2559 

2564 

2 

2 

3 

4 

4 

5  5 

.41 

2570 

2576 

2582 

2588 

2594 

2600 

2606 

2612 

2618 

2624 

2 

2 

3 

4 

4 

5  5 

.42 

2630 

2636 

2642 

2649 

2655 

2661 

2667 

2673 

2679 

2685 

2 

2 

3 

4 

4 

5  0 

.43 

2692 

2698 

2704 

2710 

2716 

2723 

2729 

2735 

2742 

2748 

2 

3 

3 

i 

4 

5  6 

.44 

2754 

2761 

2767 

2773 

2780 

2786 

2793 

2799 

2805 

2812 

2 

3 

3 

4 

4 

j  6 

.45 

2818 

2825 

2831 

2838 

2844 

2851 

2858 

2164 

2871 

2877 

2 

3 

3 

4 

5 

5  6 

.46 

2884 

2891 

2897 

2904 

2911 

29172924 

2931 

2938 

2944 

2 

3 

3 

4 

5 

5  6 

.47 

2951 

2958 

2965  2972 

2979 

2985|2992 

2999 

3006 

3013 

2 

3 

3 

4 

5 

5  6 

.48 

3020 

3027 

30343041 

3048 

3055'  3062 

3069 

3076 

3083 

1 

1 

2 

3 

4 

4 

5 

6  6 

.49 

3090 

3097 

31053112 

3119 

31263133 

3141 

3148 

3155 

1 

1 

2 

3 

4 

4 

5 

6 

6 

TABLES. 
ANTILOGARITHMS. 


517 


1 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

PROPORTIONAL  PARTS. 

1  2 

3 

5 

4 

7 

.|. 

.50 

3162 

53170 

3177 

3184 

t319 

3199 

320€ 

3214 

322] 

3228 

j  j 

f 

e!7 

.51 

3236 

,3243 

3251 

325£ 

3266 

3273 

3281 

3289 

329C 

13304 

2 

2 

4 

I 

6  7 

.52 

3311 

3319 

3327 

3334 

3342 

3350 

3357 

3365 

3373  3381 

f 

2 

4 

5 

6  7 

.53 

3388 

3396 

3404 

3412 

3420 

3428 

3436 

3443 

3451  3459 

m 
4 

2 

4 

6 

6  7 

.54 

3467 

3475 

3483 

3491 

3499 

3508 

3516 

3524 

3532 

3540 

2 

2 

4 

5 

6 

6  7 

.55 

3548 

3556 

3565 

3573 

3581 

3589 

3597 

3606 

3614 

3622 

2 

0 

4 

4 

6 

7  7 

.56 

3631 

3639 

3648 

3656 

3664 

3673 

3681 

3690 

3698 

3707 

2 

3 

4 

| 

6 

7  8 

.57 

3715 

3724 

3733 

3741 

3750 

3758 

3767 

3776 

3784 

3793 

2 

3  ' 

4 

5 

6 

7 

1 

8 

.58 

3802 

3811 

3819 

3828 

3837 

3846 

3855 

3864 

3873 

3882 

2 

3  ' 

4 

5 

6 

7 

8 

.59 

3890 

3899 

3908 

3917 

3926 

3936 

3945 

3954 

3963 

3972 

2 

3  < 

5 

5 

6 

7 

8 

.60 

981 

3990 

3999 

4009 

4018 

4027 

4036 

4046 

4055 

4064 

2 

3  « 

5 

6 

6 

7 

8 

.61 

074 

4083 

4093 

4102 

4111 

4121 

4130 

4140 

4150 

4159 

2 

3  ' 

5 

6 

7 

8 

Q 

.62 

169 

4178 

4188 

4198 

4207 

4217 

4227 

4236 

4246 

4256 

2 

3  * 

5 

6 

7 

8 

Q 

.63 

266 

4276 

4285 

4295 

4305 

4315 

4325 

4335 

4345 

4355 

2 

3  ^ 

5 

6 

7 

8 

9 

.64 

365 

4375 

4385 

4395 

4406 

4416 

4426 

4436 

4446 

4457 

2 

3  ' 

5 

6 

7 

8 

Q 

.65 

467 

4477 

4487 

4498 

4508 

4519 

4529 

4539 

4550 

4560 

2 

3  ' 

5 

6 

7 

8 

9 

.66 

571 

4581 

4592 

4603 

4613 

4624 

4634 

4645 

4656 

4667 

2 

3  4 

5 

6 

7 

9 

10 

.67 

677 

4688 

4699 

4710 

4721 

4732 

4742 

4753 

4764 

4775 

2 

3  4 

5 

7 

8 

9 

10 

.68 

786 

4797 

808 

4819 

4831 

4842 

4853 

4864 

4875 

4887 

2 

3  A 

6 

7 

8 

9'lO 

.69 

898 

4909 

920 

4932 

4943 

4955 

4966 

4977 

4989  5000 

2 

3  I 

6 

7 

8 

910 

•70 

012 

5023 

035 

5047 

5058 

5070 

5082 

5093 

51055117 

2 

4 

6 

7 

8 

911 

.71 

129 

5140 

152  5164 

5176 

5188 

5200 

5212 

52245236 

2 

4 

6 

7 

8 

0  11 

.72 

248 

5260 

272  5284 

5297 

5309 

5321 

5333 

5346535 

2 

4 

6 

8 

91011 

.73 

^370 

5383 

5395  5408 

5420 

5433 

5445 

5453 

5470  5483 

3 

4 

6 

8 

91011 

.74 

495 

5508 

521  5534 

5546 

5559 

572 

5585 

5598 

5610 

3 

4 

6 

8 

9 

10  12 

.75 

623 

5636 

649  5662 

5675 

5689 

5702 

715 

5728 

5741 

3 

4 

7 

8 

9 

1012 

.76 

754 

5768 

781  5794 

5808 

5821 

834 

848 

5861 

5875 

3 

4 

7 

8 

9 

11  12 

.77 

888 

5902 

916  5929 

5943 

5957 

970 

984 

5998 

6012 

3 

4 

7 

8 

0 

11 

12 

.78 

026 

6039 

05316067 

6081 

095 

109 

124 

6138 

6152 

3 

4 

7 

8 

0 

11 

13 

.79 

166 

6180 

194 

6209 

6223 

237 

252 

6266 

6281 

6295 

3 

4 

7 

9 

0 

11  13 

.80 

3310 

6324 

3339 

6353 

6368 

6383 

397 

6412 

6427 

6442 

3 

4  6 

7 

9 

0 

12 

13 

.81 

457 

6471 

3486 

6501 

6516 

531 

546 

6561 

6577 

6592 

3 

5  : 

8 

9 

1 

12 

14 

.82 

607 

6622 

637 

6653 

6668 

683 

699 

6714 

6730 

6745 

3 

> 

8 

9 

1 

l£ 

14 

.83 

761 

6776 

792 

6808 

6823 

839 

3855 

6871 

6887 

6902 

3 

5  6 

8 

9 

1 

13 

14 

.84 

918 

6934 

950 

6966 

6982 

998 

015 

7031 

7047 

7063 

5  6 

8 

10 

1 

13 

15 

.85 

079 

7096 

112 

7129 

7145 

161 

178 

7194 

7211 

7228 

5  7 

8 

10 

2 

13 

15 

.86 

244 

7261 

278 

7295 

7311 

328 

345 

7362 

7379 

7396 

5  7 

8 

10 

2 

13 

15 

.87 

413 

7430 

447 

7464 

7482 

499 

516 

7534 

7551 

7568 

5  7 

9 

10 

2 

14 

16 

.88 

586 

7603 

621 

7638 

7656 

674 

691 

7709 

7727 

7745 

. 

5  7 

9 

11 

2 

14 

16 

.89 

762 

7780 

798 

7816 

7834 

852 

870 

7889 

7907 

7925 

4 

5  7 

9 

11 

3 

14 

16 

.90 

943 

7962 

980 

7998 

8017 

035 

054 

8072 

3091 

8110 

6  7 

9 

11 

3 

15 

17 

.91 

128 

8147 

166 

8185 

8204 

222 

241 

8260 

3279 

8299 

6  8 

9 

11 

3 

15 

17 

.92 

318 

8337 

356 

8375 

8395 

414 

433 

8453 

3472 

8492 

6  8 

10 

12 

4 

15 

17 

.93 

511 

8531 

551 

8570 

8590 

610 

630 

8650 

3670 

8690 

4 

6  8 

10 

12 

4 

16 

18 

.94 

710 

8730 

750 

8770 

8790 

810 

831 

8851 

3872 

8892 

4 

6  8 

10 

12 

4 

16 

18 

.95 

913 

8933 

954 

8974 

8995 

016 

036 

9057 

3078 

9099 

4 

6  8 

10 

12 

5 

L7 

19 

.96 

120 

9141 

162 

9183 

9204 

226 

247 

9268 

3290 

9311 

6  8 

11 

13 

5 

17 

19 

.97 

333 

9354 

376 

9397 

9419 

441 

462 

9484 

3506 

9528 

7  9 

11 

13 

5 

17 

20 

.98 

550 

9572 

594 

9616 

9638 

661 

683 

9705 

3727 

9750 

4 

7  9 

11 

13 

3 

18 

20 

.99 

772 

9795 

817 

9840 

9863 

886 

908 

9931 

3954 

9977 

7  9 

11 

14 

6 

18 

20 

518 


TABLES. 


TABLE  V.— SPECIFIC  GRAVITY  OF  AMMONIA  SOLUTIONS  AT  15°  C. 
LUNGE  AND  WIERNIK.* 


Snecific 
Gravity 
15° 
*4T 

in  Vacuo. 

Per  Cent 
NH3, 
Grams. 

1  Liter 
Contains 
NH3, 
Grams. 

Correction 
of  Specific 
Gravity 
for  ±1* 

Specific 
Gravity 

4° 
in  Vacuo. 

Per  Cent 
NH3, 
Grams. 

1  Liter 
Contains 
NH3, 
Grams. 

Correction 
of  Specific 
Gravity 
for  ±  1°. 

1.000 

0.00 

0.0 

0.00018 

0.940 

15.63 

146.9 

0.00039 

0.998 

0.45 

4.5 

0.00018 

0.938 

16.22 

152.1 

0.00040 

0.996 

0.91 

9.1 

0.00019 

0.936 

16.82 

157.4 

0.00041 

0.994 

1.37 

13.6 

0.00019 

0.934 

17.42 

162.7 

0.00041 

0.992 

1.84 

18.2 

0.00020 

0.932 

18.03 

168.1 

0.00042 

0.990 

2.31 

22.9 

0.00020 

0.930 

18.64 

173.4- 

0.00042 

0.988 

2.80 

27.7 

0.00021 

0.928 

19.25 

178.6 

0.00043 

0.986 

3.30 

32.5 

0.00021 

0.926 

19.87 

184.2 

0.00044 

0.984 

3.80 

37.4 

0.00022 

0.924 

20.49 

189.3 

0.00045 

0.982 

4.30 

42.2 

0.00022 

0.922 

21.12 

194.7 

0.00046 

0.980 

4.80 

47.0 

0.00023 

0.920 

21.75 

200.1 

0.00047 

0.978 

5.30 

51.8 

0.00023 

0.918 

22.39 

205.6 

0.00048 

0.976 

5.80 

56.6 

0.00024 

0.916 

23.03 

210.9 

0.00049 

0.974 

6.30 

61.4 

0.00024 

0.914 

23.68 

216.3 

0.00050 

0.972 

6.80 

66.1 

0.00025 

0.912 

24.33 

221.9 

0.00051 

0.970 

7.31 

70.9 

0.00025 

0.910 

24.99 

227.4 

0.00052 

0.968 

7.82 

75.7 

0.00026 

0.903 

25.65 

232.9 

0.00053 

0.966 

8.33 

80.5 

0.00026 

0.906 

26.31 

238.3 

0.00054 

0.964 

8.84 

85.2 

0.00027 

0.904 

26.  93 

243.9 

0.00055 

0.962 

9.35 

89.9 

0.00028 

0.902 

27.65 

249.4 

0.00056 

0.960 

9.91 

95.1 

0.00029 

0.900 

23.33 

255.0 

0.00057 

0.958 

10.47 

100.3 

0.00030 

0.898 

29.01 

260.5 

0.00058 

0.956 

11.03 

105.4 

0.00031 

0.896 

29.69 

266.0 

0.00059 

0.954 

11.60 

110.7 

0.00032 

0.894 

30.37 

271.5 

0.00060 

0.952 

12.17 

115.9 

0.00033 

0.892 

31.05 

277.0 

0.00060 

0.950 

12.74 

121.0 

0.00034 

0.890 

31.75 

282.6 

0.00061 

0.948 

13.31 

126.2 

0.00035 

0.888 

32.50 

288.6 

0.00062 

0.946 

13.88 

131.3 

0.00036 

0.886 

33.25 

294.6 

0.00063 

0.944 

14.46 

136.5 

0.00037 

0.884 

34.10 

301.  4* 

0.00064 

0.942 

15.04 

141.7 

0.00038 

0.882 

34.95 

308.3 

0.00065 

*  Zeit.  f.  angew.  Chem.,  1889,  181. 


TABLES 


519 


TABLE  VI.-SPECIFIC  GRAVITY  OF  HYDROCHLORIC  ACID  AT  15°  C. 
LUNGE  AND  MARCHLEWSKI.* 


Specific  Grav- 

ityatf 

in  Vacuo. 

Degrees 
Baume.t 

Degrees 
Twaddell. 

100  Parts  by 
Weight  Contain 
Grams, 
HC1. 

1  Liter  Contains 
in  Kilograms, 
HC1. 

1.000 

0.0 

0 

0.16 

0.0016 

1.005 

0.7 

1 

1.15 

0.012 

1.010 

1.4 

2 

2.14 

0.022 

1.015 

2.1 

3 

3.12 

0.032 

1.020 

2.7 

4 

4.13 

0.042 

1.025 

3.4 

5 

5.15 

0.053 

1.030 

4.1 

6 

6.15 

0.064 

1.035 

4.7 

7 

7.15 

0.074 

1.040 

5.4 

8 

8.16 

0.085 

1.045 

6.0 

9 

9.16 

0.096 

1.050 

6.7 

10 

10.17 

0.107 

1.055 

7.4 

11 

11.18 

0.118 

1.060 

8.0 

12 

12.19 

0.129 

1.065 

8.7 

13 

13.19 

0.141 

1.070 

9.4 

14 

14.17 

0.152 

1.075 

10.0 

15 

15.16 

0.163 

1.080 

10.6 

16 

16.15 

0.174 

1.085 

11.2 

17 

17.13 

0.186 

1.090 

11.9 

18 

18.11 

0.197 

1.095 

12.4 

19 

19.06 

0.209 

1.100 

13.0 

20 

20.01 

0.220 

1.105 

13.6 

21 

20.97 

0.232 

1.110 

14.2 

22 

21.92 

0.243 

1.115 

14.9 

23 

22.86 

0.255 

1.120 

15.4 

24 

23.82 

0.267 

1.125 

16.0 

25 

24.78 

0.278 

1.130 

16.5 

26 

25.75 

0.291 

1.135 

17.1 

27 

26.70 

0.303 

1.140 

17.7 

28 

27.66 

0.315 

1  .  1425 

18.0 

28.14 

0.322 

1.145 

18.3 

29 

28.61 

0.32S 

1.150 

18.8 

30 

29.57 

0.340 

1.152 

19.0 

t 

29.95 

0.345 

1.155 

19.3 

3i 

30.55 

0.353 

1.160 

19.8 

32 

31.52 

0.366 

1.163 

20.0 

32.10 

0.373 

1.165 

20.3 

33 

32.49 

0.379 

1.170 

20.9 

34 

33.46 

0.392 

1.171 

21.0 

33.65 

0.394 

1.175 

21.4 

35 

34.42 

0.404 

1  180 

22.0 

36 

35.39 

0.418 

1.185 

22.5 

37 

36.31 

0.430 

1.190 

23.0 

38 

37.23 

0.443 

1.195 

23.5 

39 

38.16 

0.456 

1.200 

24.0 

40 

39.11 

0.469 

*Zeit.  angew.  Chem.,  1891,  135. 

t  Fur  the  American  Standard  Baumd  scale  and  the  specific-gravity  table  adopted  by  the 
Manufacturing  Chemists'  Association  of  the  United  States,  see  Van  Nostrand's  Chemical 
Annual. 


520 


TABLES. 


TABLE  VII.— SPECIFIC  GRAVITY  OF  NITRIC  ACID  AT  15  °C. 
LUNGE  AND  REY.* 


Specific 
Gravity 

-? 

in  Vacuo. 

Degrees 
Baume'.t 

Degrees 
Twaddell. 

100  Parts  by  Weight 
Contain,  Grams, 

1  Liter  Contains  in 
Kilograms, 

N205. 

HN03. 

NA,. 

HNOa. 

1.000 

0 

0 

o.os 

0.10 

0.001 

0.001 

1.005 

0.7 

1 

0.85 

1.00 

0.003 

0.010 

1.010 

1.4 

2 

1.62 

1.90 

0.016 

0.019 

1.015 

2.1 

3 

2.39 

2/0 

0.024 

0.028 

1.020 

2.7 

4 

3.17 

3.70 

0.033 

0.038 

1.025 

3.4 

5 

3.94 

4.60 

0.040 

0.047 

.030 

4.1 

6 

4.71 

5.50 

0.049 

0.057 

.035 

4.7 

7 

5.47 

6.3"> 

0.057 

0.066 

.040 

5.4 

8 

6.22 

7.26 

0.064 

0.075 

.045 

6.0 

9 

6.97 

8.13 

0  073 

0.035 

.050 

6.7 

10 

7.71 

8.99 

0  0  1 

0.094 

.055 

7.4 

11 

8.43 

».f4 

0.0^9 

0.104 

.060 

8.0 

12 

9.15 

10.63 

0.097 

0.113 

.065 

8.7 

13 

9.87 

11.51 

0.105 

0.123 

.070 

9.4 

14 

10.57 

12.33 

0.113 

0.132 

.075 

10.0 

15 

11.27 

13.15 

0.121 

0.141 

.OSO 

10.6 

16 

11.96 

13.95 

0.129 

0.151 

.OS5 

11.2 

17 

12.64 

14.74 

0.137 

0.160 

.090 

11.9 

18 

13.31 

15.53 

0.145 

0.169 

.095 

12.4 

19 

13.99 

16.32 

0.153 

0.179 

.100 

13.0 

20 

14.67 

17.11 

0.161 

0.188 

.105 

13.6 

21 

15.34 

17.^9 

0.170 

0.198 

.110 

14.2 

22 

16.00 

18.67 

0.177 

0.207 

.115 

14.9 

23 

16.67 

19.45 

0.186 

0.217 

.120 

15.4 

24 

17.34 

20.23 

0.195 

0.227 

.125 

16.0 

25 

18.00 

21.00 

0.202 

0.236 

.130 

16.5 

26 

18.66 

21.77 

0.211 

0.246 

.135 

17.1 

27 

19.32 

22.54 

0.219 

0.256 

.140 

17.7 

23 

19.93 

23.31 

0.223 

0.266 

.145 

18.3 

29 

20.64 

24.  OS 

0.237 

0.276 

.150 

18.8 

30     - 

21.29 

24.84 

0.245 

0.2«6 

.155 

19.3 

31 

21.94 

25.60 

0.254 

0.296 

.160 

19.8 

'32 

22.60 

26.36 

0.262 

0.306 

1.165 

20.3 

33 

23.25 

27.12 

0.271 

0.316 

1.170 

20.9 

34 

23.90 

27.88 

0.279 

0.326 

1.175 

21.4 

35 

24.54 

28.63 

0.238 

0.336 

1.180 

22.0 

36 

25.18 

29.33 

0.297 

0.317 

1.185 

22.5 

37 

25.83 

30.13 

0.306 

0.357 

1.190 

23.0 

38 

26.47 

30.88 

0.315 

0.367 

1.195 

23.5 

39 

27.10 

31.62 

0.324 

0.378 

1.200 

24.0 

40 

27.74 

32.36 

0.333 

0.388 

1.205 

24.5 

41 

23.36 

33.09 

0.342 

0.399 

1.210 

25.0 

42 

23.99 

33.82 

0.351 

0.409 

1.215 

25.5 

43 

29.61 

34.55 

0.360 

0.420 

1.220 

26.0 

44 

30.24 

35.23 

0.369 

0.430 

1.225 

26.4 

45 

30.88 

36.03 

0.378 

0.441 

1.230 

26.9 

46 

31.53 

36.78 

0.387 

0.452 

1.235 

27.4 

47 

32.17 

37.53 

0.397 

0.463 

1.240 

27.9 

48 

32.82 

38.29 

0.407 

0.475 

1.245 

28.4 

49 

33.47 

39.05 

0.417 

0.486 

1.250 

28.8 

50 

34.13 

39.82 

0.427 

0.498 

*  Zeit.  angew.  Chem.,  1891,  165. 

+  For  the  American  Standard  Baume'  scale  and  the  specific-gravity  table  adopted  by  the 
Manufacturing  Chemists'  Association  of  the  United  States,  see  Van  Nostrand  s  Chemical 
Annual. 


TABLES. 


521 


SPECIFIC  GRAVITY  OF  NITRIC  ACID  AT  15°  C— (Continued). 


Specific 
Gravity 

*v 

*n  Vacuo. 

Degrees 
Baume. 

Degrees 
Twaddell. 

100  Parts  by  Weight 
Contain,  Grams, 

I  Liter  Contains  in 
Kilograms, 

N205. 

HNO3. 

N.OS. 

HN03. 

1.255 

29.3 

51 

34.78 

40.53 

0.437 

0.509 

1.260 

29.7 

52 

35.44 

41.34 

0.447 

0.521 

1.265 

30.2 

53 

36.09 

42.10 

0.457 

0  5.3 

1.270 

30.6 

54 

36.75 

42.87 

0.467 

0.5^4 

1.275 

31.1 

55 

37.41 

43.64 

0.477 

0.556 

1.280 

31.5 

56 

38.07 

44.41 

0.487 

0.56J 

1.285 

32.0 

57 

38.73 

45.18 

0.498 

0.5  1 

1.290 

32.4 

58 

39.39 

45.95 

0.503 

0.593 

1.295 

32.8 

59 

40.05 

46.72 

0.519 

0.605 

1.300 

33.3 

60 

40.71 

47.49 

0.529 

0.617 

1.305 

33.7 

61 

41.37 

48.26 

0.540 

0.630 

1.310 

34.2 

62 

42.06 

49.07 

0.551 

0.643 

1.315 

34.6 

63 

42.76 

49.89 

0.562 

0.656 

1.320 

35.0 

64 

43.47 

50.71 

0.573 

0.669 

1.325 

35.4 

65 

44.17 

51.53 

0.5S5 

0.6S3 

1.330 

35.8 

66 

44.89 

52.37 

0.597 

0.697 

1.3325 

36.0 

665 

45.26 

52.80 

0.603 

0.704 

1.335 

36.2 

67 

45.62 

53.22 

0.609 

0.710 

1.340 

36.6 

68 

46.35 

54.07 

0.621 

0.725 

1.345 

37.0 

69 

47.03 

54.93 

0.633 

0.739 

1.350 

37.4 

70 

47.82 

55.79 

0.645 

0.753 

1.355 

37.8 

71 

48.57 

56.66 

0.653 

0.768 

1.360 

38.2 

72 

49.35 

57.57 

0.671 

0.783 

1.365 

38.6 

73 

50.13 

58.48 

0.6S4 

0.798 

1.370 

39.0 

74 

50.91 

59.39 

0.698 

0.814 

1.375 

39.4 

75 

51.69 

60.30 

0.711 

0.829 

1.380 

39.8 

76 

52.52 

61.27 

0.725 

0.846 

1.3833 

40.0 

53.08 

61.92 

0.735 

0.857 

1.385 

40.1 

77 

53.35 

62.24 

0.739 

0.862 

1.390 

40.5 

78 

54.20 

63.23 

0.753 

0.879 

1.395 

40.8 

79 

55.07 

64.25 

0.76S 

0.896 

1.400 

41.2 

80 

55.97 

65.30 

0.783 

0.914 

1.405 

41.6 

81 

56.92 

66.40 

0.800 

0.933 

1.410 

42.0 

82 

57.86 

67.50* 

0.816 

0.952 

1.415 

42.3 

83 

58.83 

68.63 

0.832 

0.971 

1.420 

42.7 

84 

59.83 

69.80 

0.849 

0.991 

1.425 

43.1 

85 

60.84 

70.98 

0.867 

1.011 

1.430 

43.4 

86 

61.86 

72.17 

0.885 

1.032 

1.435 

43.8 

87 

62.91 

73.39 

0.903 

1.053 

1.440 

44.1 

88 

64.01 

74.68 

0.921 

1.075 

1.445 

44.4 

89 

65.13 

75.98 

0.941 

1.098 

1.450 

44.8 

90 

66.24 

77.28 

0.961 

1.121 

1.455 

45.1 

91 

67.38 

78.60 

0.981 

1.144 

1.460 

45.4 

92 

68.56 

79.98 

1.001 

1.168 

1.465 

45.8 

93 

69.79 

81.42 

1.023 

1.193 

1.470 

46.1 

94 

71.06 

82.90 

1.045 

1.219 

1.475 

46.4 

95 

72.39 

84.45 

1.068 

1.246 

1.480 

46.8 

96 

73.76 

86.05 

1.092 

1.274 

1.485 

47.1 

97 

75.18 

87.70 

1.116 

1.302 

1.490 

47.4 

98 

76.80 

89.60 

1.144 

1.335 

1.495 

47.8 

99 

78.52 

91.60 

1.174 

1.369 

522 


TABLES. 


SPECIFIC  GRAVITY  OF  NITRIC  ACID  AT  15°  C.— (Continued). 
LUNGE  AND  REY. 


Specific 
Gravity 

Degrees 
Baum£. 

Degrees 
Twaddell. 

100  Parts  by  Weight 
Contain,  Grams, 

1  Liter  Contains  in 
Kilograms, 

at   4o 
in  Vacuo 

N205. 

HNO3. 

N205. 

HN03. 

1.500 

48.1 

100 

80.65 

94.09 

1.210 

1.411 

1.501 

.... 

. 

81.09 

94.60 

1.217 

1.420 

1.502 

.... 

.  .  . 

81.50 

95.03 

1.224 

1.42S 

1.503 

.... 

... 

81.91 

95.55 

1.231 

1.436 

1.504 

82.29 

96.00 

1.238 

1.444 

.505 

48*.4 

ioi 

82.63 

96.39 

1.244 

1.451 

.506 

.... 

82.94 

96.76 

1.249 

1.457 

.507 

83.26 

97.13 

1.255 

.     1.464 

.508 

48  '.5 

83.  53 

97.50 

1.260 

1.470 

.509 

.... 

83.87 

97.84 

1.265 

1.476 

.510 

48*.7 

102 

84.09 

93.10 

1.270 

1.481 

.511 

.... 

. 

84.  23 

93.32 

1.274 

1.486 

.512 

.... 

84.46 

98.53 

.277 

1.490 

.513 

.... 

... 

84.63 

98.73 

.280 

1.494 

.514 

? 

84.78 

98.90 

.233 

1.497 

.515 

49.6 

ios 

84.92 

99.07 

.237 

1.501 

.516 

.... 

85.04 

99.21 

.29 

1.504 

.517 

85.15 

99.34 

.292   , 

1.507 

.518 

... 

85.26 

99.46 

.294 

1.510 

.519 

85.35 

99.57 

.296 

1.512 

1.520 

49*.4 

104 

85.44 

99.67 

.299 

1.515 

TABLES 


523 


TABLE  VIII.— SPECIFIC  GRAVITY  OF  SULPHURIC  ACID. 
LUNGE,  ISLER,  AND  NAEF.* 


Specific 
Gravity 

Degrees 
Baume.f 

Degrees 
Twaddell. 

100  Parts  by  Weight 
Contain,  Grams, 

1  Liter  Contains  in 
Kilograms, 

at  40 
in  Vacuo. 

SO3. 

H2SO4. 

SO3. 

H2S04. 

1.000 

0 

0 

0.07 

0.09 

0.001 

0.001 

1.005 

0.7 

1 

0.68 

0.83 

0.007 

0.008 

1.010 

1.4 

2 

1.28 

1.57 

0.013 

0.016 

1.015 

2.1 

3 

1.88 

2.30 

0.019 

0.023 

1.020 

2.7 

4 

2.47 

3.03 

0.025 

0.031 

1.025 

3.4 

5 

3.07 

3.76 

0.032 

0.039 

1.030 

4.1 

6 

3.67 

4.49 

0.038 

0.046 

.035 

4.7 

7 

4.27 

5.23 

0.044 

0.054 

.040 

5.4 

8 

4.87 

5.96 

0.051 

0.062 

.045 

6.0 

9 

5.45 

6.67 

0.057 

0.071 

.050 

6.7 

10 

6.02 

7.37 

0.063 

0.077 

.055 

7.4 

11 

6.59 

8.07 

0.070 

0.085 

.060 

8.0 

12 

7.16 

8.77 

0.076 

0.093 

.065 

8.7 

13 

7.73 

9.47 

O.OS2 

0.102 

1.070 

9.4 

14 

8.32 

10.19 

0.089 

0.109 

1.075 

10.0 

15 

8.90 

10.90 

0.096 

0.117 

1.080 

10.6 

16 

9.47 

11.60 

0.103 

0.125 

1.085 

11.2 

17 

10.04 

12.30 

0.109 

0.133 

1.090 

11.9 

18 

10.60 

12.99 

0.116 

0.142 

1.095 

12.4 

19 

11.16 

13.67 

0.122 

0.150 

1.100 

13.0 

20 

11.71 

14.35 

0.129 

0.158 

1.105 

13.6 

21 

12.27 

15.03 

0.136 

0.166 

1.110 

14.2 

22 

12.82 

15.71 

0.143 

0.175 

1.115 

14.9 

23 

13.36 

16.36 

0.149 

0.183 

1.120 

15.4 

24 

13.89 

17.01 

0.156 

0.191 

1.125 

16.0 

25 

14.42 

17.66 

0.162 

0.199 

1.130 

16.5 

26 

14.95 

18.31 

0.169 

0.207 

1.135 

17.1 

27 

15.48 

18.96 

0.176 

0.215 

.140 

17.7 

28 

16.01 

19.61 

0.183 

0.223 

.145 

18.3 

29 

16.54 

20.26 

0.189 

0.231 

.150 

18.8 

30 

17.07 

20.91 

0.196 

0.239 

.155 

19.3 

31 

17.59 

21.55 

0.203 

0.248 

.160 

19.8 

32 

18.11 

22.19 

0.210 

0.257 

.165 

20.3 

33 

18.64 

22.83 

0.217 

0.266 

.170 

20.9 

34 

19.16 

23.47 

0.224 

0.275 

.175 

21.4 

35 

19.69 

24.12 

0.231 

0.283 

.180 

22.0 

36 

20.21 

24.76' 

0.238 

0.292 

1.185 

22.5 

37 

20.73 

25.40 

0.246 

0.301 

1.190 

23.0 

38 

21.26 

26.04 

0,253 

0.310 

1.195 

23.5 

39 

21.78 

26.68 

0.260 

0.319 

1.200 

24.0 

40 

22.30 

27.32 

0.268 

0.328 

1.205 

24.5 

41 

22.82 

27.95 

0.275 

0.337 

1.210 

25.0 

42 

23.33 

28.58 

0.282 

0.346 

1.215 

25.5 

43 

23.84 

29.21 

0.290 

0.355 

1.220 

26.0 

44 

24.36 

29.84 

0.297 

0.364 

.225 

26.4 

45 

24.88 

30.48 

0.305 

0.373 

.230 

26.9 

46 

25.39 

31.11 

0.312 

0.382 

.235 

27.4 

47 

25.88 

31.70 

0.320 

0.391 

.240 

27.9 

48 

26.35 

32.28 

0.327           0.400 

.245 

28.4 

49 

26.83 

32.86 

0.334           0.409 

.250            28.8 

50 

27.29 

33.43 

0.341           0.418 

.255 

29.3 

51 

27.76 

34.00 

0.348 

0.426 

.260            29.7 

52 

28.22 

34.57 

0.356 

0.435 

*Zeit.  angew.  Chem.,  1890,  131;    Chem.  Ind.,  1883,  39. 

+  For  the  American  Standard  Baumd  scale  and  the  specific-gravity  table  adopted  by  the 
Manufacturing  Chemists'  Association  of  the  United  States,  see  Van  Nostrand's  Chemical 
Annual. 


524 


TABLES. 


SPECIFIC  GRAVITY  OF  SULPHURIC  ACID— (Continued). 


Specific 
Gravity 

Degrees 
Baume\ 

Degrees 
TwaddeU. 

100  Parts  by  Weight 
Contain,  Grams, 

1  Liter  Contains  in 
Kilograms  , 

at  40 
in  Vacuo. 

S03. 

H2SO-4. 

SO3. 

H2S04. 

1.265 

30.2 

53 

28.69 

35.14 

0.363 

0.444 

1.270 

30.6 

54 

29.15 

35.71 

0.370 

0.454 

1.275 

31.1 

55 

29.62 

36.29 

0.377 

0.462 

1.280 

31.5 

56 

30.10 

36.87 

0.385 

0.472 

1.285 

32.0 

57 

30.57 

37.45 

0.393 

0.4S1 

1.290 

32.4 

58 

31.04 

38.03 

0.400 

0.490 

1.295 

32.8 

59 

31.52 

38.61 

0.40S 

0.500 

1.300 

33.3 

60 

31.99 

39.19 

0.416 

0.510 

1.305 

33.7 

61 

32.46 

39.77 

0.424 

0.519 

1.310 

34.2 

62 

32.94 

40.35 

0.432 

0.529 

1.315 

34.6 

63 

33.41 

40.93 

0.439 

0.538 

1.320 

35.0 

64 

33.88 

41.50 

0.447 

0.548 

1.325 

35.4 

65 

34.35 

42.08 

0.455 

0.557 

1.330 

35.8 

66 

34.80 

42.66 

0.462 

0.567 

1.335 

36.2 

67 

35.27 

43.20 

0.471 

0.577 

1.340 

36.6 

68 

35.71 

43.74 

0.479 

0.586 

.345 

37.0 

69 

36.14 

44.28 

0.486 

0.596 

.350 

37.4 

70 

36.58 

44.82 

0.494 

0.605 

.355 

37.8 

71 

37.02 

45.35 

0.502 

0.614 

.360 

38.2 

72 

37.45 

45.88 

0.509 

0.624 

.365 

38.6 

73 

37.89 

46.41 

0.517 

0.633 

.370 

39.0 

74 

38.32 

46.94 

0.525 

0.643 

.375 

39.4 

75 

38.75 

47.47 

0.533 

0.653 

.380 

39.8 

76 

39.18 

48.00 

0.541 

0.662 

.385 

40.1 

77 

39.62 

48.53 

0.549 

0.672 

.390 

40.5 

78 

40.05 

49.06 

0.557 

0.682 

.395 

40.8 

79 

40.48 

49.59 

0.564 

0.692 

.400 

41.2 

80 

40.91 

50.11 

0.573 

0.702 

.405 

41.6 

81 

41.33 

50.63 

0.581 

0.711 

.410 

42.0 

82 

41.76 

51.15 

0.589 

0.721 

.415 

42.3 

83 

42.17 

51.66 

0.597 

0.730 

.420 

42.7 

84 

42.57 

52.15 

0.604 

0.740 

.425 

43.1 

85 

42.96 

52.63 

0.612 

0.750 

1.430 

43.4 

86 

43.36 

53.11 

0.620 

0.759 

1.435 

43.8 

87 

43.75 

53.59 

0.628 

0.769 

1.440 

44.1 

88 

44.14 

54.07 

0.636 

0.779 

1.445 

44.4 

89 

44.53 

54.55 

0.643 

0.789 

1.450 

44.8 

90 

44.92 

55.03 

0.651 

0.798 

1.455 

45.1 

91 

45.31 

55.50 

0.659 

0.808 

1.460 

45.4 

92 

45.69 

55.97 

0.667 

0.817 

1.465 

45.8 

93 

46.07 

56.43 

0.675 

0.827 

1.470 

46.1 

94 

46.45 

56.90 

0.683 

0.837 

1.475 

46.4 

95 

46.83 

57.37 

0.691 

0.846 

1.480 

46.8 

96 

47.21 

57.83 

0.699 

0.856 

1.485 

47.1 

97 

47.57 

58.28 

0.707 

0.865 

1.490 

47.4 

98 

47.95 

58.74 

0.715 

0.876 

1.495 

47.8 

99 

48.34 

59.22 

0.723 

0.885 

1.500 

48.1 

100 

48.73 

59  .  70 

0.731 

0.896 

1.505 

48.4 

101 

49.12 

60.18 

0.739 

0.906 

1.510 

48.7 

102 

49.51 

60.65 

0.748 

0.916 

1.515 

49.0 

103 

49.89 

61.12 

0.756 

0.926 

1.520 

49.4 

104 

50.28 

61.59 

0.764 

0.936 

1  525 

49.7 

105 

50.66 

62.06 

0.773 

0.946 

1  530 

50.0 

106 

51.04 

62.53 

0.781 

0.957 

TABLES. 


525 


SPECIFIC  GRAVITY  OF  SULPHURIC  ACID— (Continued). 


Specific 
Gravity 
15° 
«*lo- 
in  Vacuo. 

Degrees 
Baumd. 

Degrees 
Twaddell. 

100  Parts  by  Weight 
Contain,  Grams, 

1  Liter  Contains  in 
Kilograms, 

S03. 

H2S04. 

SO3. 

H2SO,. 

1.535 

50.3 

107 

51.43 

63.00 

0.789 

0.967 

1.540 

50.6 

108 

51.78 

63.43 

0.797 

0.977 

.545 

50.9 

109 

52.12 

63.85 

0.805 

0.987 

.550 

51.2 

110 

52.46 

64.26 

0.813 

0.996 

.555 

51.5 

111 

52.79 

64.67 

0.821 

1.006 

.560 

51.8 

112 

53.12 

65.08 

0.829 

1.015 

.565 

52.1 

113 

53.46 

65.49 

0.837 

1.025 

.570 

52.4 

114 

53.80 

65.90 

0.845 

1.035 

.575 

52.7 

115 

54.13 

66.30 

0.853 

1.044 

.580 

53.0 

116 

54.46 

66.71 

0.861 

1.054 

.585 

53.3 

117 

54.80 

67.13 

0.869 

1.064 

.590 

53.6 

118 

55.18 

67.59 

0.877 

1.075 

.595 

53.9 

119 

55.55 

68.05 

0.886 

1.085 

.600 

54.1 

120 

55.93 

68.51 

0.895 

1.096 

.605 

54.4 

121 

56.30 

6S.97 

0.904 

1.107 

.610 

54.7 

122 

56.63 

69.43 

0.913 

1.118 

.615 

55.0 

123 

57.05 

69.89 

0.921 

1.123 

.620 

55.2 

124 

57.40 

70.32 

0.930 

1.139 

.625 

55.5 

125 

57.75 

70.74 

0.938 

1.150 

.630 

55.8 

126 

58.09 

71.16 

0.947 

1.160 

.635 

56.0 

127 

58.43 

71.57 

0.955 

1.170 

.640 

56.3 

128 

58.77 

71.99 

0.964 

1.181 

.645 

56.6 

129 

59.10 

72.40 

0.972 

1.192 

.650 

56.9 

130 

59.45 

72.82 

0.981 

1.202 

.655 

57.1 

131 

59.73 

73.23 

0.9S9 

1.212 

.660 

57.4 

132 

60.11 

73.64 

0.993 

1.222 

.665 

57.7 

133 

60.46 

74.07 

1.007 

1.233 

.670 

57.9 

134 

60.82 

74.51 

1.016 

1.244 

.675 

58.2 

135 

61.20 

74.97 

1.025 

1.256 

.680 

58.4 

136 

61.57 

75.42 

1.034 

1.267 

.685 

58.7 

137 

61.93 

75.86 

1.043 

1.278 

.690 

58.9 

138 

62.29 

76.30 

1.053 

1.289 

.695 

59.2 

139 

62.64 

76.73 

1.062 

1.301 

1.700 

59.5 

140 

63.00 

77.17 

1.071 

1.312 

1.705 

59.7 

141 

63.35 

77.60 

1.030 

1.323 

1.710 

60.0 

142 

63.70 

78.04 

1.019 

1.334 

1.715 

60.2 

143 

64.07 

78.48 

1.099 

1.346 

1.720 

60.4 

144 

64.43 

78.92 

.103 

1.357 

1.725 

60.6 

145 

64.78 

79.36 

.118 

.369 

1.730 

60.9 

146 

65.14 

79.80 

.127 

.381 

1.735 

61.1 

147 

65.50 

80.24 

.136 

.392 

1.740 

61.4 

148 

65.86 

80.68 

.146 

.404 

1.745 

61.6 

149 

66.22 

81.12 

.156 

.416 

.750 

61.8 

150 

66.58 

81.56 

.165 

.427 

.755 

62.1 

151 

66.94 

82.00 

.175 

.439 

.760 

62.3 

152 

67.30 

82.44 

.185 

.451 

.765 

62.5 

153 

67.65 

82.88 

.194 

.463 

.770 

62.8 

154 

68.02 

83.32 

.204 

.475 

.775 

63.0 

155 

68.49 

83.90 

.216 

.489 

.780 

63.2 

156 

68.98 

84.50 

.228 

.504 

.785 

63.5 

157 

69.47 

85.10 

.240 

.519 

.790 

63.7 

158 

69.96 

85.70 

.252 

.534 

.795 

64.0 

159 

70.45 

86.30 

.265 

1.549 

.800 

64.2 

160 

70.94 

86.90 

1.277 

1  .  564 

526  TABLES. 

SPECIFIC  GRAVITY  OF  SULPHURIC  ACID— (Continued). 


Specific 
Gravity 

a*  -40- 
in  Vacuo. 

Degrees 
Baume\ 

Degrees 
Twaddell. 

100  Parts  by  Weight 
Contain,  Grams, 

1  Liter  Contains  in 
Kilograms, 

SO3. 

H2S04. 

SO3. 

H2SO4. 

1.805 

64.4 

161 

71.50 

87.60 

1.291 

.581 

1.810 

64.6 

162 

72.08 

88.30 

.305 

.598 

1.815 

64.8 

163 

72.69 

£9.05 

.319 

.621 

1.820 

65.0 

164 

73.51 

90.05 

.338 

.639 

1.821 

73.63 

90.20 

.341 

.643 

1.822 

65.1 

73.80 

90.40 

.345 

.647 

1.823 

.... 

73.96 

90.60 

.348 

.651 

1.824 

65.2 

74.12 

90.80 

.352 

.656 

1.825 

.... 

ies 

74.29 

91.00 

.356 

.661 

1.826 

65'.3 

74.49 

91.25 

.360 

.666 

1.827 

.... 

74.69 

91.50 

.364 

.671 

1.828 

Q5A 

74.86 

91.70 

.368 

.676 

1.829 

75.03 

91.90 

.372 

.681 

1.830 

iee 

75.19 

92.10 

.376 

.685 

1.831 

65  '.5 

75.35 

92.30 

1.380 

.690 

1.832 

75.53 

92.52 

1.384 

.695 

1.833 

65!  6 

75.72 

92.75 

1.388 

.700 

1.834 

.... 

, 

75.96 

93.05 

1.393 

1.706 

1.835 

65!7 

i67 

76.27 

93.43 

1.400 

1.713 

1.836 

.... 

76.57 

93.80 

1.406 

1.722. 

1.837 

.... 

76.90 

94.20 

1.412 

1.730 

1.838 

65^8 

77.23 

94.60 

1.419 

1.739 

1.839 

77.55 

95.00 

.426 

.748 

1.840 

65'.9 

ies 

78.04 

95.60 

.436 

.759 

1.8405 

.... 

78.33 

95.95 

.451 

.765 

1.8410 

79.19 

97.00 

.453 

.7,6 

1.8415 

.... 

79.76 

97.70 

.469 

.799 

1.8410 

.... 

80.16 

98.20 

.476 

.808 

1.8405 

.... 

. 

80.57 

98.70 

.483 

1.816 

1.8400 

.... 

80.98 

99.20 

.490 

1.825 

1.8395 

.... 

81.18 

99.45 

.494 

1.830 

1.8390 

.... 

81.39 

99.70 

.497 

1.834 

1.8385 



81.59 

99.95 

.500 

1.838 

TABLE    IX.— VAPOR   TENSION    OF   WATER   IN    MILLIMETERS    OF 

MERCURY. 

MAGNUS. 


Temp. 

Millimeters. 

Temp. 

Millimeters. 

Temp. 

Millimeters. 

Temp. 

Millimeters. 

0 

4.525 

8 

7.964 

15 

12.677 

23 

20.909 

4-1 

4.867 

9 

8.525 

16 

13.519 

24 

22.211 

2 

5.231 

10 

9.126 

17 

14.409 

25 

23.5-2 

3 

5.619 

11 

9.756 

1$ 

15.351 

26 

25.026 

4 

6.032 

12 

10.421 

19 

16.345 

27 

26.547 

5 

6.471 

13 

11.130 

20 

17.396 

23 

28.148 

6 

6.939 

14 

11.882 

21 

18.505 

29 

29>32 

7 

7  436 

22 

19  .  675 

30 

31.602 

TABLES. 


527 


TABLE  X.— DENSITY  OF  WATER. 

ROSSETTI. 


Tempera- 
ture. 

Density  (Density 
at  0°  =  1). 

Volume  (Volume 
atO°  =  l). 

Density  (Density 
at4°  =  l). 

Volume  (Volume 
at4°  =  l). 

-10 

0.998274 

1.001729 

0.998145 

.001858 

-  9 

0.998556 

1.001449 

0.998427 

.001575 

-  8 

0.998814 

1.001191 

0.998685 

.001317 

-  7 

0.999040 

1.000963 

0.998911 

.001089 

-  6 

0.999247 

1.000756 

0.999118 

.000883 

-  5 

0.999428 

1.000573 

0.999298 

.000702 

-  4 

0.999584 

1.000416 

0.999455 

.000545 

-  3 

0.999719 

1.000281 

0.999590 

.000410 

-  2 

0.999832 

1.000168 

0.999703 

.000297 

-  1 

0.999926 

1.000074 

0.999797 

.000203 

0 

1.000000 

1.000000 

0.999871 

.000129 

+  1 

1.000057 

0.999943 

0.999928 

.000072 

2 

1.000098 

0.999902 

0.999969 

.000031 

3 

1.000120 

0.999880 

0.999991 

.000009 

4 

1.000129 

0.999871 

1.000000 

.000000 

5 

1.000119 

0.999881 

0.999990 

.000010 

6 

1.000099 

0.999901 

0.999970 

.000030 

7 

1.000062 

0.999938 

0.999933 

.000067 

8 

1.000015 

0.999985 

0.999886 

.000114 

9 

0.999953 

1.000047 

0.999824 

.000176 

10 

0.999876 

1.000124 

0.999747 

.000253 

11 

0.999784 

1.000216 

0.999655 

.000345 

12 

0.999678 

.000322 

0.999549 

.000451 

13 

0.999559 

.000441 

0.999430 

.000570 

14 

0.999429 

.000572 

0.999299 

.000701 

15 

0.999289 

.000712 

0.999160 

.000841 

16 

0.999131 

.000870 

0.999002 

.000999 

17 

0.998970 

.001031 

0.998841 

.001160 

18 

0.998782 

.001219 

0.998654 

.001348  i 

19 

0.998588 

1.001413 

0.998460 

.001542 

20 

0.998388 

1.001615 

0.998259 

.001744  - 

21 

0.998176 

1.00182S 

0.998047 

.001957 

22 

0.997956 

1.002048 

0.997828 

.002177 

23 

0.997730 

1.002276 

0.997601 

.002405 

24 

0.997495 

1.002511 

0.997367 

.002641 

25 

0.997249 

1.002759 

0.997120 

.002888 

26 

0.996994 

1.003014 

0.996866 

.003144 

27 

0.996732 

1.003278 

0.996603 

.003408 

28 

0.996460 

.003553 

0.996331 

.003682 

29 

0.996179 

.003835 

0.996051 

.003965 

30 

0.99589 

.00412 

0.99577 

.00425 

31 

0.99560 

.00442 

0.99547 

.00455 

32 

0.99530 

.00473 

0.99517 

.00486 

33 

0.99498 

.00505 

0.99485 

.00518 

34 

0.99465 

.00538 

0.99452 

.00551 

35 

0.99431 

.00572 

0.99418 

.00586 

36 

0.99396 

1.0060S 

0.99383 

.00621 

37 

0.99360 

1.00645 

0.99347 

.00657 

38 

0.99323 

1.00682 

0.99310 

.00694 

39 

0.99286 

1.00719 

0.99273 

.00732 

40 

0.99248 

1.00757 

0.99235 

.00770 

41 

0.99210 

1.00796 

0.99197 

.00809 

42 

0.99171 

1.00  36 

0.99158 

.00849 

528 


TABLES. 
DENSITY  OF  WATER—  (Continued). 


Tempera- 
ture. 

Density  (Density 
atO°  =  l). 

Volume  (Volume 
at  0°  =  1). 

Density  (Density 
at  4°  =  1). 

Volume  (Volume 
at4°  =  l). 

43 

44 

0.99131 
0.99091 

1.00876 
1.00917 

0.99118 
0.99078 

1.00889 
1.00929 

45 

0.99050 

1.00953 

0.99037 

1.00971 

46 

0.99009 

1.01001 

0.98996 

1.01014 

47 

0.98967 

1.01044 

0.98954 

1.01057 

48 

0.98923 

1.01088 

0.98910 

1.01101 

49 

0.98878 

1.01134 

0.98865 

1.01148 

50 

0.98832 

1.01182 

0.98819 

1.01195 

51 

0.98785 

1.01230 

0.98772 

1.01243 

52 

0.98737 

1.01279 

0.98725 

1.01292 

53 

0.98689 

1.01328 

0  .98677 

1.01341 

54 

0.9S642 

1.01377 

0  98629 

1.01390 

55 

0.98594 

1.01426 

0  98531 

1.01439 

56 

0.98547 

1.01475 

0.98534 

1.01488 

57 

0.93499 

1.01524 

0.98486 

1.01537 

58 

0.98450 

1.01574 

0.08437 

1.01587 

59 

0.9S401 

1.01625 

0.98388 

1.01638 

60 

0.98350 

1.01678 

0.98338 

1.01691 

61 

0.98299 

1.01731 

0.93286 

1.01744 

62 

0.98247 

1.01785 

0.98234 

1.01798 

63 

0.98194 

1.01839 

0.98182 

1.01852 

64 

0.98140 

1.01895 

0.98128 

1.01908 

65 

0.98036 

1.01951 

0.98074 

1.01964 

66 

0.93032 

1.02003 

0.98019 

1.02021 

67 

0.97977 

1.02065 

0.97954 

1.02078 

68 

0.97921 

1.02124 

0.97903 

1.02137 

69 

0.97864 

1.02183 

0.97851 

1.02196 

70 

0.97807 

1.02243 

0.97794 

1.02256 

71 

0.97749 

1.02303 

0.97736 

1.02316 

72 

0.97690 

1.02365 

0.97677 

1.02378 

73 

0.97631 

1.02427 

0.97618 

1.02440 

74 

0.97571 

1.02490 

0.97553 

1.02503 

75 

0.97511 

1.02553 

0.97498 

1.02566 

76 

0.97450 

1.02617 

0.97438 

1.02630 

77 

0.97389 

1.02681 

0.97377 

1.02694 

78 

0.97328 

1.02745 

0.97316 

1.02758 

79 

0.97267 

1.02809 

0.97255 

1.02322 

80 

0.97206 

1.02874 

0.97194 

1.02887 

81 

0.97145 

1.02939 

0.97132 

1.02952 

82 

0.97033 

1.03005 

0.97070 

1.03018 

83 

0.97020 

1.03072 

0.97007 

1.03085 

84 

0.96956 

1.03139 

0.96943 

1.03153 

85 

0.96892 

1.03207 

0.96879 

1.03221 

86 

0.96323 

1.03276 

0.96315 

1.03289 

87 

0.96764 

1.03345 

0.96751 

1.03358 

88 

0.96699 

1.03414 

0.96687 

1.03427 

89 

0.96634 

1.03484 

0.96622 

1.03497 

90 

0.96563 

1.03554 

0.96556 

1.03567 

91 

0.96502 

1.03625 

0.96490 

1.03638 

92 

0.96435 

1.03697 

0.96423 

1.03710 

93 

0.96363 

1.03770 

0.96356 

1  03782 

TABLES. 
DENSITY  OF  WATER— (Continued). 


529 


Tempera- 
ture. 

Density  (Density 
atO°  =  l). 

Volume  (Volume 
at  0°  =  1). 

Density  (Density 
at  4°  =  1). 

Volume  (Volume 
at  4°  =  1) 

94 

0.96300 

1.03844 

0.9628& 

1.03-56 

95 

0.96231 

1.03918 

0.96219 

1.03931 

96 

0.96161 

1.03993 

0.96149 

1.04006 

97 

0.96091 

1.04069 

0.96079 

1.040*2 

93 

0.96020 

1.04145 

0.96008 

1.04158 

99 

0.95949 

1.04222 

0.95937 

1.04235 

100 

0.95879 

1.04299 

0.95866 

1.04312 

INDEX. 


Accelerators,  use  of,  in  Parr  calorim- 
eter, 401. 

Accuracy,  limit  of,  4,  122. 
Acetic   acid,   reagent,   preparation  of, 

506. 

Acetyl  value  of  fats  and  oils,  447. 
determination  of,  447. 
distillation  process  for,  448. 
filtration  process  for,  448. 
table  of  values  of,  461. 
Acid, 

best  for  general  use,  255. 
boric,  titration  of,  293. 
carbonic.     See  carbon  dioxide, 
chromic,  titration  of,  334. 
free,  in  fats  and  oils,  determina- 
tion of,  438. 
hydrochloric.       See  hydrochloric 

acid. 

oxalic.     See  oxalic  acid, 
phosphoric.     See  phosphoric  acid, 
standard,  most  desirable  strength 

for  general  use,  256. 
sulphuric.     See  sulphuric  acid, 
sulphurous,  oxidation  of,  306. 
Acid  radicles,  secondary,  electrolytic 

reactions  of,  200. 
Acids, 

determination  of,  94. 
normal,  247. 
standard,  258. 

dilution  of,  to  exact  strength, 

262,  265. 
standardization  of,  by 

anhydrous  oxalic  acid,  260. 
crystallized  oxalic  acid,  259. 
evaporation  with  ammonia, 

261,  263. 
gravimetric     methods,     261 , 

265,  272. 

potassium  dichromate,  260. 
"         tetroxalate,260. 
sodium  carbonate,  258,  264. 
specific  gravity,  261,  271. 
titration  of,  by  iodometric  meth- 
ods, 343. 


Acids,  weak,  titration  of,  255. 

titration  of  base  in  salts  of. 

253. 
Acidimetry,  247. 

calculation  of  titrations  in,  500. 
Acidity  of  fats  and  oils,  437. 

determination  of,  438. 
of  nutrient  gelatin,  adjusting  of, 

Ex.  72,  425. 
Air, 

collection  of  samples  of,  302,  305. 
determination  of  carbon  dioxide 
in,  301,  Ex.  55,  304. 

calculation  of  results  of,  305. 
displacement  in  weighing, 
correction  for,  11,  234. 
effect  of,  on  weighing,  11. 
drying  of,  by  calcium  chloride,  45. 
exclusion   of,    from   ferrous   solu- 
tions, 312. 

leaks,  testing  for,  184. 
Albuminoid    ammonia,   determination 

of,  411,  416. 

Alcohol,  absolute,  absorption  of  ben- 
zine vapor  by,  467. 

methyl,   distillation  of  boric 

acid  with,  116. 

Alkali,  caustic,  action  on  glass  by,  53, 60. 
precipitation  of  copper,  man- 
ganese,  nickel,  and   cobalt 
by,  60. 

cyanides,  analysis  of,  by  iodomet- 
ric methods,  342. 
Alkalies, 

action  on  glass  by,  60. 
contamination      of      precipitates 

with,  60. 

determination  of,  in  silicates,  190, 
195. 

in  feldspar,  195. 
in  water,  431. 

separation  of,  from  each  other,  187. 
from  magnesium,  186. 
from  zinc,  145. 
standard,  279. 
titration  of,  265. 

531 


532 


INDEX. 


Alkalimetric  method  o?  titrating  phos- 
phoric acid,  360. 
Alkalimetry,   calculation  of   titrations 

in,  500. 

Alkaline  earth,  carbonates,  purification 
of,  30,  33. 

metals,  separation  from  zinc, 

145. 

Alkaline  solutions,  method  of  keeping, 
289. 

standard,  not  permanent,  279. 
Alkalinity  of  water,  268. 

determination  of,  269. 
Alloys,  analysis  of,  121. 
brass  or  bronze,  146. 
Britannia  metal,  142. 
determination  of  silver  in,  355. 
German  silver,  148. 
manganes  e-phosphorus-bronze, 

149. 

Rose's  metal,  129. 
silver  coin,  123. 
soft  solder,  128. 
taking  samples  of,    for   analysis. 

153. 

type-metal,  140. 
Aluminium, 

determination  of,  in  chromite,  164. 
in  cinnabar,  175. 
in  dolomite,  181. 
in  feldspar,  194. 
in  potash  alum,  Ex.  10,  54. 
in  salts  of  volatile  and  organic 

acids,  71. 
in  silicates,  189. 
in  smaltite,  170. 
in  water,  432. 
electrolytic  separation  from  lead, 

222. 

hydroxide,  precipitation  of,  51. 
effect  of  organic  matter  on,  54. 
washing  and  ignition  of,  52, 

54. 
oxide,  determination  of  silica  in, 

53. 

pure  compounds  of,  31. 
separation,  as  basic  acetate  from 
manganese,  cobalt,  nickel,  and 
zinc,  157. 

as  basic  carbonate  from  man- 
ganese, cobalt,  nickel,  and 
zinc,  159. 
as    hydroxide,    from    cobalt, 

nickel,  and  zinc,  156. 
from  chromium,  159,  164. 
from  iron,  160,  162. 
from  zinc,  145. 
volumetric  from  iron,  162. 
Ammeter,  205. 
Ammonia, 

albuminoid,  determination  of,  in 
water,  411,  416. 


Ammonia,   determination  of,   as   am- 
monium chloride,  105. 

as  platino-chloride,  106. 

in  ammonium   chloride,   Ex. 

52,  282. 
free,  determination  of,  in  water, 

411,  413. 

free  water,  preparation  of,  412. 
reagent,  preparation  of,  506. 
solution,  standard,  412. 
specific  gravity  of,  table  of,  518. 
standard  solutions  of,  282. 
not  permanent,  279. 
titration  of,  287. 
Ammonium, 

acetate,  reagent,  506. 
carbonr.te,  precipitation  of  metals 
by,  64. 

reagent,  506. 

solution   of   arsenic   sulphide 

in,  134. 

chloride,  determination  of  ammo- 
nia as,  105. 

pure,  preparation  of,  32. 
reagent,  506. 

hydroxide,  precipitation  of  metals 
by,  51. 

reagent,  506. 

oxalate,  precipitation  of  calcium 
by,  66,  67. 

reagent,  preparation  of,  507. 
testing  for  purity,  32. 
platino-chloride,  determination  of 

ammonia  as,  103. 
salts,  removal  of,  85. 
sulphate,  pure,  preparation  of,  33. 
sulphocyanate,  standard   solution 

of,  355. 

Analysis,  limit  of  accuracy  in,  122. 
taking  of  sample  for,  152. 
technical,  364. 

Analytical  equations,  balancing  of,  496. 
Anions,  definition  of,  197. 
Anode   or  cathode,   rotation    of,   209, 

217,  221,  225. 
Anodes,  platinum,  205. 
Anthracite  coal,  definition  of,  391,  395. 
Antilogarithms,  table  of,  516. 
Antimony,   determination   of,    as   sul- 
phide, 78. 

by  iodometric  methods,  343. 
in  bronze  or  brass,  147. 
in  pyrites,  173 
in  smaltite,  169. 
in  type-metal,  141. 
in  Wood's  metal,  131. 
electrolytic   separation,   from   ar- 
senic and  tin,  137. 

from  lead,  222. 
oxide,  analysis  of,  by  iodometric 

methods,  342. 
pure  compounds  of,  31. 


INDEX. 


533 


Antimony,    separation,    from    arsenic 
and  tin,  134. 

by  H.  Rose's  method,  136. 
from  copper,  214. 
from  lead,  127. 
from  tin  by  F.  W.  Clarke's 
method,  134,  144. 

by  reduction  with  iron, 

136,  141. 
from  zinc,  145. 
Apparatus, 

calibration  of,  226. 
volumetric,  227. 

Arms  of  balance,  determination  of  rela- 
tive length  of,  Ex.  2,  15. 
Arsenic, 

compounds,  solution  of,  139. 
determination,  as  magnesium  py- 
roarsenate,  88. 

as  sulphide,  78,  81. 
in  arsenious  oxide,  Ex.  21,  81. 
in  brass  or  bronze,  147. 
in  pyrites,  173. 
in  smaltite,  169. 
in  soft  solder,  128. 
in  type-metal,  142. 
in  Wood's  metal,  131. 
electrolytic  separation  from  anti- 
mony, 137. 

pure  compounds  of,  31. 
separation,  as  arsenious  chloride, 
138. 

from  antimony  by  H.  Rose's 

method,  136. 

from  antimony  and  tin,  134. 
from  copper,  214. 
from  tin  by  F.  W.  Clarke's 

method,  134,  144. 

Arsenic  acid,  separation  of,  from  phos- 
phoric acid,  118. 

Arsenious  chloride,  distillation  of,  139. 
oxide,  analysis  of,  by  iodometric 
methods,  342. 

determination  of  arsenic  in, 

Ex.  21,  81. 
oxidation  of,  306. 
percentage  of  arsenic  in,  81. 
pure,  testing  of,  33. 
standardization      of      iodine 

solutions  with,  339,  341. 
Arsenites,  oxidation  of,  306. 
Arsenopyrite,  analysis  of,  Ex.  36,  172. 
Ash,  determination  of,   in  coal,  391, 

393. 

Atomic  weights,  limit  of  accuracy  in, 
486. 

international  table  of,  509. 
limit  of  accuracy  in,  486. 
Available  oxygen,  307. 

determination,  in  manganese  ores, 
322,  Ex.  58,  325. 

in  potassium  die  iromate,  307. 


Available    oxygen,  determination,   in 
potassium  permanganate,  307. 

in  pyrolusite,  Ex.  58,  325. 

B.  coli,  in  water,  test  for,  424. 
present  in  intestines,  407. 
test  for,  in  water,  Ex.  73,  428. 
Bacteria  present  in  water,  423. 
Bacterial  count,  in  water,  424,  Ex  72 
425. 

preparation  of,  nutrient  agar- 
agar  for,  Ex.  72,  127. 
nutrient  gelatin  for,  Ex. 
,     72,  425. 
Bacteriological  examination  of  water, 

423, 

Balance,  centre  of  gravity  of,  9. 
construction  of,  7. 
determination  of  relative  length  of 

arms,  Ex.  2,  15. 
maximum  load  of,  15. 
parts  of,  7. 
rules  for  using,  20, 
sensibility  of,  9. 

determination  of,  Ex.  1,  15. 
testing  for  equality  of  arms,  13. 
Balancing  of  equations,  496. 
Barium,  carbonate,  ignition  of,  66,  71. 
pure,  preparation  of,  33. 
weighing  of,  60,  71. 
chloride,  determination  of  barium 
in,  Ex.  18,  75. 

of  water  in,  Ex.  8,  44. 
percentage  of  barium  in,  76. 
pure,  testing  of,  33. 
reagent,  preparation  of,  507. 
determination,  as  chromate,  89. 
in  barium  chloride,  Ex.  18, 75. 
in  salts  of  organic  acids,  71. 
in  salts  of  volatile  acids,  76. 
hydroxide,  standard  solutions  of, 
281. 

titration  of,  with  oxalic  acid, 

303,  305. 

use  of,  in  determining  carbon 
dioxide  in  gases,  301,  303, 
304. 
precipitation,  as  carbonate,  64. 

as  sulphate,  73,  75. 
pure  compounds  of,  31. 
separation  as  chromate  from  cal- 
cium and  strontium,  178. 

as  sulphate  from  magnesium 

and  the  alkalies,  177. 
from  calcium  as  nitrate,  180. 
from  calcium  and  strontium, 

176. 

from  lead,  127. 
from  the  alkali  metals,  176. 
sulphate,  determination  of  sulphur 
in,  103. 

dissociation  of,  21. 


534 


INDEX. 


Barium,  sulphate,  ignition  of,  74. 

properties  of,  73. 

thiosulphate,    standardization    of 
iodine  solutions  with,  338,  340. 
weighing  as  chromate,  179. 
Bases  present  in  fats  and  oils,  436. 
Basicity  of  acids  with  indicators,  251. 
Baume*  scale,  276. 

American  Standard,  277. 
Beam  of  balance,  7. 
Benzene  vapor,  absorption  by  absolute 

alcohol,  467. 

Berthelot,  bomb  calorimeter  of,  396. 
Bismuth, 

determination,  as  oxide,  130. 
as  oxychloride,  92,  127. 
as  sulphide,  78,  79. 
hi  Britannia  metal,  143. 
in  Rose's  metal,  130. 
in  salts  of  volatile  and  organic 

acids,  71. 

in  Wood's  metal,  131. 
oxide,  pure,  preparation  of,  33. 
oxychloride,  properties  of,  92. 
pure  compounds  of,  31. 
separation,  from  copper,  214. 
from  lead  as  chloride,  126. 
from  zinc,  145. 

Bituminous  coal,  definition  of,  391, 395. 
Bleaching  powder, 

active  substance  in,  346. 
determination  of  available   chlo- 
rine in,  Ex,  67,  347. 
iodometric  titration  of,  347. 
sampling  of,  347. 
titration  with  sodium  arsenite,  347. 
Bolting-cloth,  use  of ,  in  sifting,  155. 
Bomb  calorimeter  of  Berthelot,  396. 
Borates,  solution  of,  116. 
Borax,  anhydrous,  fusion  of  carbonates 

with,  111. 

Boric  acid,  apparatus  for  determina- 
tion of,  115. 

decomposition      of      organic 

matter  containing,  295. 
determination     of,     by     the 

method  of  Gooch,  115. 
distillation   of,   with  methyl 

alcohol,  116. 
titration  of,  293. 

by  Jones'  mannitol  meth- 
od, 294. 
by  Thompson's  glycerine 

method,  293. 
weighing  of,  116. 

oxide,  decomposition  of  silicates 
by,  190. 

purification  of,  191. 
volatilization  of,  191. 
Brass,  analysis  of,  Ex.  31,  146. 
Britannia  metal,  analysis  of,  Ex.  30, 
142. 


Britannia  metal,  decomposition  of,  by 

means  of  chlorine,  142. 
British  Thermal  Unit,  definition  of,  396. 
Bromates,  analysis  of,  by  iodometric 

methods,  343. 
Bromine, 

absorption  of  hydrogen  sulphide 
by,  369. 

of  "  ilium  inants"  by,  466. 
determination,  as  silver  bromide, 
95. 

in  metallic  salts,  95. 
distillation  of,  99. 
free,  determination  of,  by  iodo- 
metric methods,  343. 
separation  from  chlorine  and  io- 
dine, 97. 

from  hydrocyanic  acid,  100. 
from  iodine  as  palladous  io- 
dide, 96. 
Bronze,  analysis  of,  Ex.  31,  146. 

manganese-phosphorus,     analysis 

of,  Ex.  33,  149. 
B.  typhus  in  water,  424. 
Bun  sen, 

conditions  for  explosion  of  hydro- 
gen, 468. 

distilling  apparatus  of,  344. 
filter-pump,  24. 
method  of  producing  pure  oxygen, 

468. 

valve,  311. 
Burettes, 

calibration  of,  Ex.  47,  244. 

by  Morse-Blalock  bulbs,  238. 
curve  of  corrections  for,  Fig.  41. 

243. 

errors  in  reading,  229. 
Hempel  gas,  472. 
irregularities  of  bore,  241 
reading  of,  228. 
total  capacity  of,  240. 
use  of  floats  with,  229. 
Winkler  modified  gas,  472. 
Butter  fat,  table  of  Polenske  values  of, 
459. 

Cadmium, 

absorption  of  hydrogen  sulphide 
by  alkaline  solution  of,  369,  372. 
carbonate,  properties  of,  68. 

ignition  of,  69. 
determination,  as  sulphide,  78. 

in  Wood's  metal,  131. 
electrolytic  separation,  from  cop- 
per, 214. 

from  lead,  222. 
iodide,  pure,  testing  of,  33. 
oxide,  ignition  of,  69. 
pure  compounds  of,  31 . 
separation,  from  copper,  130,  131. 

from  zinc,  145. 


INDEX. 


535 


Cadmium,    precipitation    by    sodium 

carbonate,  68. 
Calcium,  carbonate,  64. 

determination  of  calcium  in, 

Ex.  15,  67. 
ignition  of,  65. 
percentage  of  calcium  oxide 

in,  68. 

precipitation  of  carbon  diox- 
ide as,  299. 

pure,  preparation  of,  33. 
weighing  of,  65. 

chloride,  fused,  drying  properties 
of,  44,  45. 

effect  of  temperature  on  dry- 
ing properties  of,  45. 
determination,  in  calcium  carbon- 
ate, Ex.  15,  67. 
in  cinnabar,  175. 
in  dolomite,  181. 
in  salts  of  organic  acids,  71. 
in  salts  of  volatile  acids,  76. 
in  water,  432. 

OZalate,  conversion  into  carbonate 
and  oxide,  67,  68. 

precipitation  and  digestion  of, 

66,  67. 

titration  of,  328. 
oxide,  weighing  of  boric  acid  with, 

117. 
phosphate,  standardization  with, 

359. 

precipitation,    as    carbonate,    64, 
195. 

by  ammonium  oxalate,  66, 67. 
pure  compounds  of,  31. 
separation,  as  nitrate  from  barium, 
180. 

as    nitrate    from    strontium, 

179. 

as  oxalate  from  the  alkalies, 
magnesium,    barium,    and 
strontium,  176,  181. 
as  sulphate  from  the  alkalies 

and  magnesium,  177. 
from  barium  as  chromate,  178. 
from  barium  and  strontium, 

176. 

from  the  alkali  metals,  176. 
volumetric  determination  of,  328. 
weighing,  as  carbonate,  65,  68. 

as  oxide,  65,  68,  71. 
Calculation,  by  logarithms,  487. 

of  acidimetry  and  alkalimetry,  500. 

of  calibration  of  weights,  17. 

of  determinations  of  iron  as  ferric 

oxide,  59,  60. 

of  iodometric  titrations,  504. 
of  oxidation  and  reduction  titra- 
tions, 502. 

of  results  of  analysis  of  illuminat- 
ing-gas, 481. 


Calculation,  of  titrations  with  acids, 
248. 

of  volumetric  determinations,  499. 
of  water  analysis,  432. 
Calibration,  233. 

of  burettes,  Ex.  47,  244. 

by  means  of  Morse-Blalock 

bulbs,  235. 

by  weighing  water,  234. 
of  flasks,  Ex.  46,  244. 

by  the  Morse-Blalock  bulbs, 

238. 

by  weighing  water,  233. 
temperature  of  water  used  in, 

237. 

of  Morse-Blalock  bulbs,  236. 
of  volumetric  apparatus,  226. 
of  weights,  Ex.  3,  16. 

calculation  of  results  of,  17. 
Calorie,  definition  of,  395. 
Calorific  value,  of  coal,  determination 
of,  Ex.  71 ,  403. 
of  fuels,  determination  of,  395. 

effect  of  composition  on,  395. 
Calorimeter,  Parr,  397. 
Calorimeters,  description  of,  396. 
Cannel-coal,  definition  of,  391,  395. 
Capsule,  Hofmeister,  use  of,  290. 
Carbon, 

combined,  determination  of,  383. 
combustion  of,  in  a  stream  of  oxy- 
gen, 377. 

with  chromic  acid,  380. 
condition  in  which  present  in  iron 

and  steel,  3G4. 

determination  by  the  colorimetric 
method  of  Eggertz,  383. 

in  chrome  steel  or  iron  high  in 

silica,  381. 

in  iron  and  steel,  374. 
direct  weighing  of,  377. 
filtration  of,  on  a  platinum  boat, 
377. 

on  a  carbon  filter,  378. 
fixed,  determination  of,   in  coal, 

393. 

graphitic,  determination  of,  382. 
oxidation  of,  by  chromic  acid,  374. 

apparatus  for,  374. 
separation   from   iron   by   coppej 

solution,  376. 

total,  determination  of,  374. 
Carbon  dioxide, 

absorption,  by  caustic  potash  solu- 
tion, 464. 

by  soda-lime,  114. 
in  caustic  potash,  114,  183. 
determination  of,  109. 

by  fusion  of  carbonates  with 

borax,  111,  182. 
by  fusion  of  carbonates  with 
microcosmic  salt.  111. 


536 


INDEX. 


Carbon  dioxide,  determination  of,  by 
loss,  by  laboratory  apparatus,  110. 
by  the   Schrotter  appa- 
ratus, 109. 
by    the    apparatus    of    Fre- 

senius.  112,  183. 
in  air,  301,  Ex.  55,  304. 

calculation  of  results  of, 

304. 

in  bottled  waters,  300. 
in  dolomite,  182. 
in  gases,  300. 

by  Pettenkofer's  method, 

301. 

in  illuminating-gas,  478. 
in  water,  296. 

by  Pettenkofer's  method, 
298. 

free,  296. 

semi-combined,  297. 
total,  298. 
direct  weighing  of,  112,  183. 

in  dolomite,  183. 
drying  and  absorption  of,  114. 
effect  on  indicators,  250. 
precipitation  of,  266,  268. 

as  calcium  carbonate,  299. 
purification  and  absorption  of ,  378. 
titration  of,  265,  267,  293. 
Carbon  di-sulphide,  extraction  of  sul- 
phur by,  79. 

Carbon  monoxide,  absorbents  for,  464. 
Carbonates,  determination  of    carbon 
dioxide  in,  by  fusion  with  borax  or 
microcosmic  salt,  111. 
Cathions,  definition  of,  197. 
Cathode,  rotation  of.  209,  217,  221, 225. 
Cathodes,  platinum  dishes  as,  205. 

roughening  of,  205. 

Caustic  potash,  bulbs,  Geissler,  use  of, 
114,  183. 

standard  alcoholic  solution  of.  281, 
437. 

See  potassium  hydroxide. 
Caustic  soda.     See  sodium  hydroxide. 
Cells,  primary  and  storage,  203. 
Centre  of  gravity  of  balance,  9. 
Centrifugal  machine,  separation  of  lead 

peroxide  by,  389. 

Chalcopyrite,  analysis  of,  Ex.  26,  172. 
Characteristic  of  logarithms,  method  of 

finding,  363. 

Chemical  examination  of  water,  410. 
Chlorates,  analysis  of,  by  iodometric 

methods,  343. 

Chloric  acid,  determination  of,  100, 343. 
Chlorides, 

determination  of,  in  water,  421 . 
titration  of,  with  silver  nitrate, 

350,  351. 
Chlorine, 

absorption  of,  by  silver-foil,  379. 


Chlorine,  available,  determination  of, 
in  bleaching  powder,  346,  Ex.  67. 
347. 

decomposition  of  Britannia  metal 

by,  142. 

determination,  as  silver  chloride, 
94,  265. 

in  metallic  salts,  95. 
free,  determination  of,   by   iodc- 

metric  methods,  343. 
normal  in  water,  429. 
oxidation  of  sulphur  compounds, 

with,  104. 

separation  from  bromine  and  io- 
dine, 97. 

from  hydrocyanic  acid,  100. 
from  iodine  as  palladous  io- 
dide, 96. 
from  iodine  as  thallous  iodide, 

97. 

Cholesterol  and  phytosterol,  detection 
of  in  fats  and  oils,  151. 

melting-points  of,  452. 
Chromate  of  silver  as  indicator,  350. 
Chromates,  analysis  of,  by  iodometric 

methods,  343. 
Chrome-iron  ore,     decomposition     of, 

with  sodium  peroxide,  333. 
Chromic  acid,  oxidation  of  carbon  by, 
374,  380. 

titration  of,  334. 

Chromite,  analysis  of,  Ex.  34,  163. 
Chromium,  chromic,  precipitation  of, 
as  hydroxide,  51. 

determination,  as  barium  or  lead 
chromate,  89. 

in  iron  ores,  333,  Ex.  63,  333. 

in  chromite,  163. 

in  salts  of  volatile  and  organic 

acids,  71 
in  smaltite,  170. 

hydroxide,  effect  of  organic  matter 
on  precipitation  of,  54. 

washing  and  ignition  of,  52. 
oxidation   with   chlorine   or   bro- 
mine, 160. 

with  potassium  nitrate,  159. 
with   sodium    peroxide,    160, 

163. 
oxide,  determination  of  silica  in, 

53. 

pure  compounds  of,  31. 
separation,    as     hydroxide     from 
cobalt,  nickel,  and  zinc,  156. 
from  aluminium,  160,  164. 
from  iron,  160,  163. 
from  iron,  aluminium,  man- 
ganese, cobalt,  nickel,  and 
zinc,  159. 
from  zinc,  145. 

Chromous  chloride,  absorption  of  oxy- 
gen by,  464. 


INDEX. 


537 


Cinnabar,  analysis  of,  Ex.  37,  174. 
Clarke's  method  of  separation  of  ar- 
senic and  antimony  from  tin,  134. 
Classen's  method  of  determining,  zinc, 
224. 

copper,  215. 

Cleaning  mixture  for  volumetric  appa- 
ratus, 240. 
Coal, 

anthracite,  definition  of,  391 . 
bituminous,  definition  of,  391. 
cannel-,  definition  of,  391. 
determination  of,  coke,  391,  393. 
ash,  391 ,  393. 

calorific  value  of,  Ex.  71,  403. 
fixed  carbon,  393. 
moisture,  390,  392. 
sulphur,  392,  393. 

by    means    of    the    Parr 

calorimeter,  394. 
volatile  combustible  matter, 

391,  393. 
forms  in  which  sulphur  is  present 

in,  392. 
proximate  analysis  of,  390,  Ex.  70, 

392. 

Cobalt,  determination  of,  in  salts  of 
volatile  acids,  76. 

electrolytic,  determination  of,  21 9. 
by  ammonia  or  Fresenius 
and  Bergmann's  meth- 
od, 220. 

by  ammonium  oxalate  or 
Classen's  method,  219. 
separation,  from  copper,  214. 
from  lead,  222. 
from  manganese,  167. 
oxide,  ignited,  composition  of,  61. 
precipitation  with  caustic  alkali, 

60. 

pure  compounds  of,  31. 
separation,  as  sulphide  from  man- 
ganese, 166. 
from  iron,  160. 
from  iron  and  aluminium,  157. 
from    iron,    aluminium,    and 

chromium,  156,  157. 
from  manganese  and  nickel, 

166. 

from  nickel,  as  tri-potassium 
cobaltic  nitrite,  167,  171. 
by  means   of   nitroso-/?- 

naphthol,  168. 
from  zinc,  145. 
Cobaltous  hydroxide,  properties  of,  61. 

oxide,  properties  of,  61. 
Cochineal,  use  as  indicator,  251,  253. 
Coke,  definition  of,  391. 

determination  of,  391,  393. 
Color, 

comparison  of,  in  carbon  determi- 
nations. 384. 


Color,    determination    of,    in    water, 

409. 

Combustible  matter,  volatile,  determi- 
nation of,  in  coal,  391,  393. 
Composition  of  fats  and  oils,  436. 
Conditions  in  which  water  is  held,  39. 
Congo  red,  as  indicator,  251. 
Constant  weight,  41. 
Constitution,  water  of,  determination 

of,  40. 

Copper,  compounds  of,  conversion  into 
sulphides,  78. 

determination,  as  sulphide,  78,  80. 
by  iodometric  methods,  343. 
in  brass  or  bronze,  147. 
in  Britannia  metal,  143. 
in  cinnabar,  175. 
in  copper  pyrites,  174. 
in  copper  sulphate,  Ex.  12,  62, 

Ex.  20,  80. 
in  Rose's  metal,  130. 
in  salts  of  volatile  and  organic 

acids,  71. 

in  a  silver  coin,  124,  219. 
in  type-metal,  141. 
in  Wood's  metal,  132. 
electrolytic,    deposition,    from    a 
nitric  or  sulphuric  acid  solution, 
213,  Ex.  41,  217. 

from  an  ammonium  ox- 
alate solution  accord- 
ing to  Classen,  215. 
determination  of,  213. 

in  copper  sulphate,  Ex. 

40,  215. 
in  a  nickel  coin,  Ex.  44, 

220. 

in  a  silver  coin,  219. 
in  ores,  214. 

separation,  from  lead,  222. 
from  other  metals,  214. 
from  silver,  217. 
hydroxide,  properties  of,  61. 
oxide,  ignited,  composition  of,  61. 
use  of,  in  Kieldahl  digestion, 

283. 

precipitation,  as  cuprous  sulpho- 
cyanate,  78. 

with  caustic  alkali,  60,  62. 
pure  compounds  of,  31,  33. 
separation,  from  iron,  160. 

from  zinc,  145. 
sulphate, 

determination,  of  copper  in, 
40,  Ex.  12,  62,  Ex.  20,  80, 
215. 

of  water  in,  Ex.  7,  41. 
percentage  of  copper  in,  62. 
80. 

water  in,  42,  50. 
pure,  preparation  of,  33,  Ex. 
4,  36. 


538 


INDEX. 


Correction  factors  for  the  Parr  calorim- 
eter, 402. 

Counterpoise,  use  of,  in  weighing,  14. 
Crucible,  Gooch,  filtration  with,  25. 

Rose,  use  of,  80. 
Crystallization,  determination  of  water 

of,  40. 

Cubic  centimeter,  definition  of,  232. 
Culture  medium,  for  bacterial  count, 
preparation  of,  Ex.  72,  425. 

sterilization  of,  Ex.  72,  426. 
Cupric  hydroxide,  properties  of,  61 . 
Cuprous  chloride,  absorption  of  carbon 
monoxide  by.  464. 

ammoniacal  solution  of,  465. 
hydrochloric  acid  solution  of,  465. 
preparation  of,  465. 
Current,  electrical, 

method  of  obtaining,  202,  203. 
reduction,  of  voltage  of,  202. 

of  strength  by  incandescent 

lamps,  202. 

significance  of  density  of,  in  elec- 
trolytic methods,  200. 
Cyanides,  titration  of,  with  silver  ni- 
trate, 351,  354. 

Cyanogen,  determination  of,  in  potas- 
sium cyanide,  Ex.  68,  354. 
Cylinders,    platinum,     as    electrodes, 
206. 

Daniell  gravity  cell,  203. 
Decantation,  washing  by,  26. 
Density,    current,    significance    of,    in 
electrolytic  methods,  200. 
of  water,  table  of,  527. 
Deshay's  method  of  determining  man- 
ganese, 388. 
Desiccator,  use  of,  19. 
Digestion-flasks,  Kjeldahl,  284. 
Digestion  of  precipitates,  23. 
Dihydric  potassium  phosphate,  stand- 
ardization by,  358. 

Dimethyl  amidobenzine,  use  of,  as  indi- 
cator, 251. 
Dishes,  platinum,  as  cathodes,  205. 

roughening  of,  205. 

Disodium    phosphate,    standardization 
by,  358. 

reagent,  preparation  of,  507. 
Distillation,  apparatus,  of  Bunsen,  344. 
of  Mohr,  345. 
of  Kjeldahl,  285. 
of  arsenious  chloride,  139. 
Dolomite,  analysis  of,  Ex.  38,  180. 
calculation  of  formula  of,  495. 
Double  salts,  preparation  of,  29. 
Drown's  method  of  determining  silicon 

in  iron,  366. 

Drying-agents  for  gases,  44. 
Dudley,  C.  B.,  on  coal  analysis,  390. 
Duplicates,  agreement  of,  5. 


Economizing  time,  124. 
Edison,  Lalande  cell,  204. 

primary  cell,  204. 

Eggertz,  colorimetric  method  for  com- 
bined carbon,  383. 
Electric  motors,  use  of,  for  rotation  of 

electrodes,  212. 

Electrical  charge  on  ions,  amount  of, 
198. 

potential  of,  199. 

Electrification,  error  in  weighing  by,  14. 
Electrodes,  for  rotation,  209. 
platinum  cylinders  as,  206. 
platinum  gauze  cylinders  as,  207. 
rotation  of,  207. 
Electrolysis, 

ionic  theory  of,  197. 
of  warm  solutions,  208. 
Electrolytic, 

determination  of  metals,  213. 
methods,  197. 

significance  of  current  density 

in,  200. 

secondary  reactions,  of  acid  radi- 
cles, 200. 

of  metals,  201. 

separation  of  metals,  by  current 
density,  200. 

by  potential,  199,  205. 
stands,  205,  207. 

Elements,  difficulty  of  complete  sepa- 
ration of,  121. 
Equations, 

analytical,  496. 
balancing  of,  496. 
metathetical,  497. 
oxidation  and  reduction,  497. 
Errors, 

determination  of  amount  of,  323. 
in  reading,  burettes,  229. 

pipettes,  231. 
in  weighing,  14. 
of  graduation,  232,  233. 
sources  of,   in  water  determina- 
tions, 40. 
unavoidable,  5. 

Frythrosene,  use  of,  as  indicator,  270. 
Eschka's  method  of  determining  sul- 
phur in  coal,  392,  393. 
Ether  value  of  fats  and  oils,  440. 

Factors,  from  a  single  precipitate,  490. 

method  of  calculating,  487. 

table  of,  510. 
Factor  weights,  method  of  obtaining, 

492. 

Faraday's  law,  198. 
Fats  and  oils, 

acetyl  value  of,  447,  461. 

acidity  of,  437. 

analysis  of,  436. 

chemical  composition  of,  436. 


INDEX. 


539 


Fats  and  oils,  detection  of  phytosterol 
and  cholesterol  in,  451. 

determination  of  fatty  acids  of, 

455. 

ether  value  of,  440. 
free  acid  in,  determination  of,  438. 
Hehner  value  of,  443,  459. 
identification  of,  456. 
iodine  value  of,  443,  457. 
Kottstorfer  value  of,  439,  459. 
Maumene  number  of,  449,  460. 
melting-point  of  fatty  acids  of,  451 . 
physical  tests  on,  448. 
Polenske  value  of,  441. 

determination  of,  442. 
refraction,  index  of,  449. 
Reichert       and       Reichert-Meissl 

value,  440,  459. 
specific  gravity  of,  448. 
specific  temperature  reaction  of, 

449,  461. 
Fatty  acids,  determination  of,  455. 

melting-point  of,  451. 
Feldspar,  analysis  of,  Ex.  39,  192. 
Ferric,  chloride,  analysis  of,  by  iodo- 
metric  methods,  343. 

sulphocyanate,  as  indicator,  350. 
Ferrous,  ammonium   sulphate,  prepa- 
ration of,  29. 

pure,  testing  of,  33. 
standardization     of     dichro- 
mate  solutions  by,  331, 332. 
of    permanganate    solu- 
tions by,  313,  316. 
iron,  determination  of,  in  ores,  Ex. 

57,  320. 

salts,  determination  of  available 
oxygen  in  pyrolusite  by,  322, 
326. 

silicate,  decomposition  of,  319. 
sulphate,  purification  of,  30. 
titanate,  decomposition  of,  319. 
Filter-paper, 
ashless,  54. 
ignition  of,  52,  56. 

on  platinum  wire,  58. 
Filter-pump,  Bunsen,  use  of,  24. 
Filtration,  24. 

by  suction,  24. 
with  Gooch  crucible,  25. 
Flash  point  of  oils,  determination  of, 

452. 
Flasks,  standard,  227. 

calibration  of,  Ex.  46,  244. 

by  Morse-Blalock  bulbs,  238. 
by  weighing  water,  233. 
for  delivering  capacity,  234. 
Floats  for  burettes,  229. 
Flue  gases, 

interpretation  of  results  of  analy- 
sis of,  484. 
taking  sample  of,  484. 


Ford's  method  for  the  determination  of 
manganese,  385. 

Formulae  of  salts  and  isomorphous  mix- 
tures, calculation  of,  493. 

Forster's  modification  of  the  Kjeldahl 
method,  288. 

Fractional  weights,  8. 

Fresenius,  apparatus  for  determining 
carbon  dioxide,  according  to,  112. 

Fuels,  determination  of  calorific  value 
of,  395. 

Fuming  nitric  acid,  oxidation  of  sul- 
phur compounds  with,  102. 

Funnel,  hot-water,  28. 

Gas,  analysis,  apparatus  for,  471. 

double  absorption  pipette  for, 
475. 

method  of  filling,  475. 
explosion  pipette  for,  476. 
general  methods  of,  462. 
HempeFs  apparatus  for,  472. 
Orsat  apparatus  for,  481. 
manipulation  of,  483. 
pipette  for  explosion  with  hot 

platinum  wire,  477. 
simple  absorption  pipette  for, 

474. 
burette,  simple,  472. 

Winkler's  modified,  472. 
explosive,  definition  of,  467. 
illuminating-,  analysis  of,  Ex.  72, 
478. 

determination  of,  carbon  di- 
oxide in,  478. 

carbon  monoxide  in,  479. 
hydrogen  in,  480. 
"illuminants"  in,  479. 
methane  in,  480. 
oxygen  in,  479. 

non-explosive,  definition  of,  467. 
pressure,  regulation  of,  471. 
temperature,  regulation  of,  471. 
Gases, 

confining  liquids  for,  471 . 
determination  of  carbon  dioxide 

in,  300. 

drying-agents  for,  44. 
Geissler  caustic-potash  bulbs,  use  of, 

114,  183. 
Gelatinous  precipitates,  filtration  of,  by 

means  of  paper  pulp,  27,  54. 
General  operations,  21. 
German  silver,  analysis  of,  Ex.  32,  148,. 
Glass,  expansion  of,  232. 

solution  of,  in  ammonia,  53. 
Glycerine,   use  of,    in   titrating  boric 

acid,  293. 

Gold,  determination  of.  in  silver  coin. 
123. 

electrolytic    separation    of,    from 
lead,  222. 


540 


INDEX. 


Gooch,  crucible,  use  in  filtering  by  suc- 
tion, 25. 

determination   of   boric    acid    by 

method  of,  115. 
Graduation,  errors  of,  232. 
Graphitic  carbon,  determination  of,  382. 
Gravimetric  methods,  advantage  of,  2. 
definition  of,  1,  226. 
standardization  of  acids  by, 

261,  263,  265,  272. 
Greiss,    method    of,    for    determining 

nitrites  in  water,  416. 
Gunning's  modification  of  the  Kjeldahl 

method,  287. 

Gunning-Jodlbauer  modification  of  the 
Kjeldahl  method,  288. 

Halogens,  determination,  94. 
as  silver  salts,  94. 
in  metallic  salts,  95. 
separation  of,  96. 

from  hydrocyanic  acid,  100. 
Hanus'  monobromide  solution,  444. 
Hardness, 

permanent,   determination  of,   in 

water,  269,  Ex.  50,  269. 
temporary,   determination  of,   in 

water,  268,  Ex.  50,  269. 
Heat  of  combustion,  definition  of,  396. 
Hehner  value  of  fats  and  oils,  443. 

table  of,  459. 

Hempel,  double  absorption  pipette  for 
gases,  475. 

method  of  filling,  475. 
explosion  pipette,  476. 
gas,  apparatus,  472. 

burettes,  472. 
simple     absorption     pipette     for 

gases,  474. 

Hildebrand,W.  F.,  on  coal  analysis,  390. 
Hofmeister  capsule,  use  of,  290. 
Hot-water  funnel,  28. 
Hydriodic  acid,  pure  compounds  of,  32. 
Hydrobromic  acid,  pure  compounds  of, 

32. 

Hydrocarbons,   formation  of,   in  solu- 
tion of  iron  and  steel,  364. 

heavy,  absorbents  for,  466. 
Hydrochloric  acid, 

absorption  of,  by  copper  sulphate, 

379. 
fifth-normal,  preparation  of,  Ex. 

48,263. 

gas,  generation  of,  37,  140. 
normal,  247. 
pure  compounds  of,  32. 
reagent,  preparation  of,  505. 
specific  gravity  of,  table  of,  519. 
standard,  advantages  of,  255. 
preparation  of,  263,  437. 
Hydrocyanic    acid,    determination   of, 
100. 


Hydrocyanic  acid,  separation  of,  from 

the  halogens,  100. 

Hydrogen,  basis  for  computing  atomic 
weights,  486. 

gaseous,  combustion  by  palladium 
sponge,  469. 

determination  of,  467. 

in  illuminating-gas,  480. 
in  presence  of  methane, 

470,  480. 
explosion    of,    with    oxygen, 

467,  480. 

explosion  pipette  for,  476. 
ignition  of  sulphides  in,  80. 
slow  combustion  of,  468,  480. 
theory    of    combustion    with 

palladium  sponge,  469. 
peroxide,  absorption  of  hydrogen 
sulphide  by,  369. 

determination    of,    by    iodo- 

metric  methods,  343. 
pipette  for  combustion  with  hot 

platinum  wire,  477. 
sulphide,  formation  of,  in  solution 
of  iron  and  steel,  364. 

reduction  of  iron  by,  320. 
solutions  for  absorbing,  369. 
Hydrometers,  use  of,  275. 
Hygroscopic,  substances,  weighing  of, 
19. 

water,  determination  of,  40. 
Hypochlorites,    analysis    of,    by   iodo- 
metric  methods,  343. 

"  Illuminants," 

absorbents  for,  466. 
determination  of,  in  gas,  479. 
Illuminating-gas,  analysis  of,  Ex.  72, 

478. 
Incrustants,  269. 

determination  of,  270. 
Index  of  refraction,  determination  of, 

449. 
Indicator, 

iodine  as,  308. 

potassium,  ferrocyanide  as,  308. 

permanganate  as,  308. 
Indicators,  250. 

basicity  of  acids  with,  251. 
effect  of  carbon  dioxide  on,  250.  . 
for  oxidation  reactions,  307. 
for  precipitation  methods,  theory 

of,  349. 

properties  of  various,  251,  253. 
Indirect  determinations,  by  means  of 
silver-nitrate  solution,  353. 

calculation  of,  491. 
Industrial     processes,     controlled     by 

quantitative  analysis,  6. 
International  atomic  weights,  table  of, 

509. 
lod^te,  potassium  acid,  purity  of,  35. 


INDEX. 


541 


Iodine, 

determination,  as  silver  iodide,  95, 
99. 

in  metallic  salts,  95. 
distillation  of,  99. 
monobromide,  Hanus'  solution  of, 

444. 
monochloride,   Wijs'    solution  of, 

443. 

oxidizing  power  of,  306,  307. 
pure,  preparation  of,  34. 
reducing  solutions  used  with,  336. 
resublimed,     standardization     of 
thiosulphate  solution  with,  337, 
341. 

separauv  n,  from  chlorine  as  thal- 
lous  iodide,  97. 

from  chlor^.c  and  bromine  as 

palladous  iodide,  96. 
from  hydrocyanic  acid,  100. 
solution,   preparation  and  stand- 
ardization of,  Ex.  64,  339. 
titration  of,  with  sodium  thio- 
sulphate solution,  340. 
solutions,  conditions  of  use,  335. 
standard,  preparation  of,  33G. 
standardization  of,  336. 

by  arsenious  oxide,  339, 

341. 
by  barium  thiosulphate, 

338,  340. 
by  potassium  dichromate, 

337. 

by    potassium    perman- 
ganate, 337. 
by     resublimed     iodine, 

337,  341. 

by  the  iodates  of  potas- 
sium and  sodium,  338, 
341. 

solvents  of,  335. 
value  of  fats  and  oils,  443. 
determination  of,  445. 
table  of,  457. 

lodometric,  determination  of, 
antimony,  343. 
copper,  343. 
sulphur,  369. 
methods,  Chap.  XXV,  335. 

analysis    of,    oxidizing    sub- 
stances by,  342. 

reducing  agents  by,  342. 
sulphides  by,  342. 
titration,  of  acids  by,  294,  343. 
of  bleaching  powder  by, 

336. 

titrations,  calculation  of,  504. 
lonization, 

constant,  21. 

law  of,  21. 

of  barium  sulphate,  21 . 

of  metallic  salts,  197. 


Ions, 


amount  of  electrical  charge  on,  198. 
definition  of,  21,  197. 
potential  of  electrical  charge  on, 
199. 


Iron, 


analysis  of,  364. 
determination,  as  sulphide,  82. 
in  brass  or  bronze,  148. 
in  Britannia  metal,  143. 
in  chromite,  163. 
in  cinnabar,  175. 
in  dolomite,  181. 
in  feldspar,  194. 
in  German  silver,  148. 
in  ores,  318,  Ex.  57,  320,  331, 

Ex.  62,  333. 
in  pyrites,  174. 
in  salts  of  volatile  acids,  71. 
in  silicates,  189. 
in  smaltite,  169. 
in  soft-iron  wire,  Ex.  11,  57. 
in  soft  solder,  129. 
in  type-metal,  141. 
in  water,  432. 
in  Wood's  metal,  132. 
of  manganese  in, 

by  Deshay's  method,  388. 
by  Ford's  method,  385. 
by     Volhard's     method, 

387. 

of  phosphorus  in,  372. 
of  silicon  in,  365. 

after  solution  in  hydro- 
chloric acid,  366. 
by  Brown's  method,  366. 
of  sulphur  in,  367. 

by  solution  in  nitric  acid, 

367. 

of  total  carbon  in,  374. 
electrolytic    separation    of,    from 

copper,  214. 
from  lead,  222. 

ferric,  precipitation  as  hydroxide, 
51 . 

effect  of  organic  matter  on, 

54. 
washing  and  ignition  of,  52, 

58. 

in  zinc,  titration  of,  312. 
method  of  obtaining  sample  of,  for 

analysis,  3^5. 
oxide,  determination  of  silica  in, 

53. 

pig,  method  for  determining  sul- 
phur in,  3C8. 
pure  compounds  of,  31. 
reduction  of,  with  hydrogen  sul- 
phide, 194,  320. 

with  stannous  chloride,  319. 
with  sulphurous  acid,  320. 
with  zinc,  311,  316,  319. 


542 


INDEX. 


Iron,  separation,  as  basic  acetate  from 
manganese,  cobalt,  nickel,  and  zinc, 
157. 

as  basic  carbonate  from  man- 
ganese, cobalt,  nickel,  and 
,  zinc,  159. 

as    hydroxide    from    cobalt, 

nickel,  and  zinc,  156. 
by  extraction  with  ether  from 
chromium,    aluminium, 
manganese,  cobalt,  nickel, 
and  copper,  160,  169. 
by  J.  W.  Roth's  method,  160, 

169. 
from  aluminium  as  aluminate, 

162. 

from  manganese,  385. 
from  zinc,  132,  145. 
volumetric,  from  aluminium, 

102. 

solution  in  potassium  cupric  chlo- 
ride solution,  376. 
standardization  of  permanganate 
solution  by  standard  solution  of, 
310. 

volatilization  of  chloride,  381. 
volumetric  determination  of,  194, 
318. 

in       manganese-phosphorUs- 

bronze,   149. 

wire,  determination  of  iron  in,  Ex. 
11,  57. 

standardizations    of    dichro- 
mate  solutions  by,  330,  332. 
of  permanganate  solu- 
tions by,  309,  315. 

Jannasch  and  Aschoff,  separation  of 
halogens  according  to,  97. 

Jannasch  and  Heidenreich,  decomposi- 
tion of  silicates  according  to,  191. 

Jones'  mannitol  method  of  titrating 
boric  acid,  294. 

Kjeldahl, 

digestion,  284,  290. 
digestion-flasks,  284. 
distillation,  285,  291. 

apparatus  for,  286. 
method  of  determining  nitrogen, 
283. 

Forster's  modification  of,  288. 
determination    of    nitro- 
gen  in   potassium   ni- 
trate by,  Ex.  54,291. 
Gunning's     modification    of, 
287. 

determination  of  nitro- 
gen in  milk  by,  Ex.  53, 
289. 

Ounning-Jodlbauer  modifica- 
tion of,  288. 


Kjeldahl,  method  of,  substances  decom- 
posed with  difficulty  by,  289. 

Wilfarth's     modification     of, 

287. 

Knife-edge,  of  balance,  7. 
Kottstorfer  value  of  fats  and  oils,  439. 
determination  of,  439. 
table  of,  459. 

Lacmoid,  use  as  indicator,  251. 
Lamps,  incandescent,  reduction  of  cur- 
rent strength  by,  202. 
Langmuir's    method    of    determining 

rosin  in  shellac,  448. 
Lead, 

absorption  of  hydrogen  sulphide 
by  alkaline  solution  of,  369,  370 
371. 
acetate,   reagent,   preparation  of, 

507. 
carbonr.te,  G4. 

ignition  of,  04. 

determination  of,  as  chromate,  89. 
as  sulphate,  74. 
as  sulphide,  78. 
in  brass  or  bronze,  147. 
in  Britannia  metal,  143. 
in  lead  nitrate,  Ex.  17,  72. 
in  ores,  223. 
in  Rose's  metal,  129. 
in  salts  of  volatile  and  organic 

acids,  71,  77. 
in  soft  solder,  128. 
in    type-metal    as     chloride, 

140. 

in  Wood's  metal,  131. 
electrolytic    deposition    of,    201, 
213,  222. 

conditions  of,  222. 
separation  and  determination 

of,  213. 
separation    of,    from    silver, 

217. 

peroxide,  dissolving  off  platinum, 
223. 

oxidation  of  manganese  by, 

388. 
separation  of,  by  centrifugal 

machine,  389. 

precipitation  by  ammonium  car- 
bonate, 64. 

pure  compounds  of,  31. 
separation,  as  chloride  from  bis- 
muth, 126. 

as  sulphate,  126,  129. 

from  antimony  and  barium, 

127. 

of  silver  from,  123. 
sulphate,  determination  of  sulphur 
in,  104. 

ignition  of,  75. 
properties  of,  74. 


INDEX. 


543 


Limit  of  accuracy,  in  analysis,  122. 

in  atomic  weights,  486. 
Lindo-Gladding  method  of  determining 

potassium,  89. 
Linen  square,  use  of,  in  filtration,  24. 

use  of,  in  sifting,  155. 
Liquids,  taking  samples  of,  152. 
Liter, 

definition  of,  232. 
Mohr,  232. 

various  standards,  in  use,  232. 
Litmus,  use  of,  as  indicator,  251. 
Logarithms, 

division  by,  488. 
method  of  using,  487. 
multiplication  by,  488. 
table  of,  514. 

Lunge,  and  Marchlewski,  table  of  spe- 
cific  Tavity  of  hydrochloric  acid,  519. 
and  Rey,  table  of  specific  gravity 

of  nitric  acid,  520. 
and  Weirnik,  table  of  specific  grav- 
ity of  ammonia  solutions,  518. 
Isler  and  Naef,  table  of  specific 

gravity  of  sulphuric  acid,  523. 
Lunge's  nitrometer,  use  of,  in  standard- 
izing permanganate  solutions,  314, 
316. 

Magnesia  mixture,  117. 

preparation  of,  507. 
Magnesium,  ammonium  arsenate, 

determination  of  phosphoric 

acid  as,  117. 
ignition  of,  So,  87. 
precipitation  and  ignition  of, 

88. 

ammonium   phosphate,  precipita- 
tion of,  85,  87. 

carbonate,  determination  of  mag- 
nesium in,  Ex.  19,  77. 
determination,  as  phosphate,  85, 
87. 

in  chromite,  165. 
in  dolomite,  182. 
in  magnesium  carbonate,  Ex. 

19,  77. 
in  magnesium  sulphate,  Ex. 

23,  87. 

in  salts  of  organic  acids,  71. 
in  salts  of  volatile  acids,  71 , 76. 
in  water,  432. 

interference  of,  in  determination 
of  carbon  dioxide  in  water,  298. 
pure  compounds  of,  31. 
pyroarsenate,     determination     of 

arsenic  as,  88,  135. 
separation  from  the  alkalies,  186. 
by  means  of  barium  hydrox- 
ide, 187. 

by  means  of  mercuric  oxide, 
186. 


Magnesium,  sulphate,  determination  of 
magnesium  in,  Ex.  23,  87. 

of  water  in,  Ex.  9,  47. 
percentage  of  magnesium  in. 

88. 

pure,  preparation  of,  35. 
Magnus,    table    of    vapor    tension    of 

water,  526. 

Manganese,  ammonium  phosphate,  pre- 
cipitation of,  84. 

ignition  of,  86. 
carbonate,  properties  of,  69. 

ignition  of,  69. 

determination,  as  phosphate,  84. 
as  sulphide,  82. 
in  chromite,  164. 
in  cinnabar,  175. 
in  dolomite,  181. 
in  iron  and  steel,  385. 

Deshay's     method      for, 

388. 

Ford's  method  for,  385. 
Volhard's    method     for, 

387. 
in  potassium   permanganate, 

Ex.  22,  83. 

in  pyrolusite,  Ex.  59,  326. 
in  salts  of  organic  acids,  71. 
in  salts  of  volatile  acids,  71, 

76. 

in  smaltite,  171. 
volumetrically,  387,  388. 
dioxide,  analysis  of,  by  iodometric 
methods,  343. 

decomposition   of   potassium 

permanganate  by,  308. 
determination     of     available 
oxygen  in,  Ex.  65,  322,  344, 
Ex.  58,  325. 
precipitation  of,  385. 
electrolytic,  deposition  of,  201. 
separation,  from  cobalt  and 
nickel,  167,  171. 
from  copper,  214. 
from  lead,  222. 
hydroxide,  properties  of,  61. 
ores,  analysis  of,  322. 
oxidation  of,  to  permanganic  acid, 

388. 

oxide,  ignited,  composition  of,  61. 
phosphorus-bronze,     analysis    of, 

Ex.  33,  149. 

precipitation,  as  dioxide,  385. 
with  caustic  alkali,  60. 
with  sodium  carbonate,  68. 
protosesquioxide,   composition  of, 

69. 

pure  compounds  of,  31. 
separation,  from  chromium,  325. 
from  copper,  214. 
from  iron,  160,  385. 
from  iron  and  aluminium,  159. 


544 


INDEX. 


Manganese,     separation,     from     iron, 
aluminium,  and  chromium,  157. 
from  lead,  222. 
from  nickel  and  cobalt,  166, 

167,  171. 
from  zinc,  145. 

sulphide,   green,   precipitation  of, 
82. 

ignition  of,  83. 
the  two  sulphides  of,  81. 
volumetric  determination  in  man- 
ganese-phosphorus-bronze, 149. 
weighing  as  oxide,  69. 
Mannitol,  use  of,  in  titrating  boric  acid, 

294. 

Mantissa,  definition  of,  488. 
method  of  finding,  488. 
Marshand  U-tube,  46. 
Mass,  measurement  of,  9. 
Maiimene*  number,  determination  of, 

449. 

Maiimene"  numbers,  table  of,  460. 
Maximum  load  of  balance,  15. 
Medicus,  solution  of  lead  ores,  accord- 
ing to,  223. 

Melting-point  of  fatty  acids,  determi- 
nation of,  451 . 
Mercuric  chloride,  pure,  testing  of,  34. 

reagent,  508. 

Mercurous  chloride,  properties  of,  92. 
Mercury, 

as  confining  liquid  for  gases,  471 . 
determination,       as      mercurous 
chloride,  92. 

as  sulphide,  78,  174. 
by  distillation,  175. 
in  cinnabar,  174,  175. 
electrolytic    separation    of,    from 

lead,  222. 

pure  compounds  of,  31. 
pure,  testing  of,  34.  — * 
separation  from  copper,  214. 
from  silver,  123. 
from  zinc,  145. 

use  of,  in  Kjeldahl  digestion,  283. 
Metallic  oxides,  properties  of,  50. 
Metals, 

determination,  as  oxide,  50. 

as  phosphate,  chromate,  and 

chloride,  84. 
as  sulphate  and  as  sulphide, 

73. 

as  sulphide,  78. 
by  evaporation  with  sulphuric 

acid,  76. 
by  ignition  of  salts  of  volatile 

acids,  70. 

electrolytic  determination  of,  213. 
secondary  electrolytic  reactions  of, 

201. 

specific  heat  of,  401. 
taking  samples  of,  for  analysis,  153. 


Metals,  washing  and  drying  of  electro- 

lytically  deposited,  208. 
Metathetical   equations,   balancing  of, 

497. 
Methane,  combustion  of,  470. 

determination,    of    hydrogen    in 
presence  of,  470,»480. 

of,  in  illuminating-gas,  480. 
Methods  of  weighing  water,  39. 
Methyl  alcohol,  distillation  of  boric  acid 

with,  116. 
Methyl  chloride,  volatilization  of  boric 

acid  by,  191. 
Methyl  orr.nge, 

as  indicator,  use  of,  253. 
effect  of  carbon  dioxide  on,  250. 
theory  of  action  as  indicator,  349. 
Microcosmic  salt,  determination  of  car- 
bon dioxide  in  carbonates  by  means 
of,  111. 

standardization  by,  358. 
Milk,  determination  of  nitrogen  in,  by 
the  Kjeldahl-Gunning  method,  Ex. 
53,  289. 

weighing  sample  of,  290. 
Mineral  oils,  detection  of,  454. 
Minerals,  analysis  of,  152. 
arsenopyrite,  172. 
chalcopyrite,  172. 
chromite,  163. 
cinnabar,  174. 
dolomite,  180. 
feldspar,  192. 
pyrite,  172. 
smaltite,  169. 

calculation  of  formulas  of,  493. 
taking  samples  of,   for  analysis, 

153. 

Mohr,  distillation  apparatus  of,  345. 
Mohr's  salt,  standardization  of  perman- 
ganate solutions  by,  316. 
Moisture,  determination  of,  in  coal,  390, 

392. 

Molybdate  solution,  preparation  of ,  507. 
Morse-Blalock  bulbs, 

calibration  by  means  of,  235. 
calibration  of,  236,  Ex.  45,  242. 
burettes  by,  238. 
flasks  by,  238. 
description  of,  235. 
temperature  of  water  used  with, 

237. 
Mortars,  agate,  155. 

steel,  154. 

Motors  for  rotation  of  electrodes,  210. 
Mullers,  steel,  153. 

Naphthylamine  hydrochloride,  solution 

of,  for  determining  nitrites,  417. 
Neatness,  necessity  of,  3. 
Nessler,  solution,  preparation  of,  411. 
tubes,  412. 


INDEX. 


545 


Nesslerizing,  414. 
Nickel, 

ammonium    sulphate,   determina- 
tion of  nickel  in,  62. 
coin,  electrolytic  analysis  of,  Ex. 

44,  220. 

determination  of,  in  German  sil- 
ver, 148. 

in  nickel  ammonium  sulphate, 

Ex.  13,  62. 

in  salts  of  organic  acids,  71. 
in  salts  of  volatile  acids,  71, 

76. 

in  smaltite,  171 

electrolytic,  determination  of,  219. 
by  ammonia  or  Fresenius 
and  Bergmann's  meth- 
od, 220. 

by  ammonium  oxalate  or 
Classen's  method,  219. 
in  a  nickel  coin,  221. 
separation  from  copper,  214. 
from  lead,  222. 
from  manganese,  167. 
oxide,  ignited,  composition  of,  61. 
precipitation  of,  with  caustic  al- 
kali, 60,  62. 

pure  compounds  of,  31. 
separation,  as  sulphide  from  man- 
ganese, 166. 

from  cobalt,  as  tri-potassium 
cobaltic  nitrite,  167,  171. 
by  means   of   nitroso-/?- 

naphthol,  168. 
from  iron,  160. 
from    iron    and     aluminium, 

159. 
from    iron,    aluminium,    and 

chromium,  156,  157. 
from  manganese  and  cobalt, 

166. 

from  zinc,  145. 

Nickelous,  hydroxide,  properties  of,  61. 
oxide,  61. 

reduction  to  nickel,  63. 
Nitrates, 

'determination,    by    the    Kjeldahl 
method,  288. 
in  water,  418. 
of  nitrogen  in,  283,  288,  Ex. 

54,  291. 

of  N2O5in,  107. 
teduction  of,  by  Kjeldahl  method, 

283. 
Nitric  acid, 

determination    of,   by    means    of 
nitron,  106. 

in  nitrates,  107. 

fuming,  oxidation  of  sulphur  com- 
pounds with,  102. 
pure  compounds  of,  32. 
reagent,  preparation  of,  505. 


Nitric  acid,  removal  of,  in  sulphur  de- 
terminations, 104. 

specific  gravity,  table  of,  520. 
Nitrites,  determination  of,  in  water,  416. 
Nitrogen, 

determination,  105. 

by  the  Kjeldahl  method,  283. 
in  gases,  470. 
in  milk,  Ex.  53,  289. 
in  potassium  nitrate  by  the 
Kjeldahl  method,  Ex.  54, 
291. 

in  water,  411. 

organic,  conversion  of,  into  ammo- 
nium sulphate,  283. 
Nitrometer,  Lunge's,  use  of,  in  stand- 
ardization   of    permanganate    solu- 
tions, 314,  316. 
Nitron,  determination  of  nitric  acid  by 

means  of,  106. 
Nitrous  acid,  determination  of,  by  iodo- 

metric  methods,  343. 
Normal,  acids,  247. 

hydrochloric,  247. 
percentage  given  by  the  num- 
ber   of    cubic    centimeters 
used,  249. 
sulphuric,  247. 
chlorine  in  water,  429. 
oxidizing  solutions,  307. 
reducing  solutions,  307. 
solutions,  247. 

Note-book,  method  of  keeping,  3. 
Noyes,  Wm.  A.,  on  coal  analysis,  390. 
Nutrient  agar-agar  for  bacterial  count, 

preparation  of,  Ex.  72,  427. 
Nutrient  gelatin,  adjusting  acidity  of, 
Ex.  72,  425. 

preparation  of, 'Ex.  72,  425. 

Odor,  numerical  standards  for,  410. 

determination  of,  in  water,  410. 
Ohm's  law,  203. 
Oil  analysis,  454. 
Oils, 

determination  of  fatty  acids  of, 
455. 

of  flash  point  of,  452. 
of  viscosity  of,  453. 
identification  of,  456. 
mineral,  detection  of,  454. 
saponification  of,  454. 
See  also  fats  and  oils. 
Oil  testers,  use  of,  453. 
Operations,  general,  21. 
Ore-crushers,  154. 
Ores,  determination  of,  lead  in,  223. 

zinc  in,  225. 
pulverizing  of,  153. 
taking  samples  of,  for  analysis,153. 
Organic,  acids, 

ignition  of  salts  of,  71. 


546 


INDEX. 


Organic,  acids,  titration  of,  255. 

of  base  in  salts  of,  253. 
matter,  containing  boric  acid,  de- 
composition of,  295. 

in  water,   determination  of, 

419. 

removal  of,  54. 

Organic  sulphides,  produced  by  solu- 
tion of  steel  in  acids,  370. 
Orsat  apparatus,  for  gas  analysis,  481. 

manipulation  of,  483. 
Oxalates, 

electrolytic  decomposition  of,  201. 
solubility  of ,  121. 

Oxalic  acid,  anhydrous,  preparation  of, 
260. 

standardization  of  acids  with, 

260. 

crystallized,  standardization  of 
acids  with,  259. 

pure,  preparation  of,  259. 
determination  of  available  oxygen 

in  pyrolusite  by,  322,  325. 
pure  compounds  of,  32. 
pure,  testing  and  preparation  of, 

34,  259. 

separation   of    arsenic    and    anti- 
mony from  tin  by,  134. 
solution,  preparation  of,  420. 
standard  solution  of,  303. 

use  of,  in  determining  carbon 

dioxide  in  air,  303. 
standardization  of  permanganate 

solutions  by,  313. 
Oxidation, 

and  reduction  equations,  balanc- 
ing of,  497. 
and  reduction,  methods,  306. 

titrations,  calculation  of,  502. 
definition  of,  306. 
indicators  for,  307. 
Oxides,  metallic,  properties  of,  50. 
Oxidizing, 

solutions,  normal,  307. 
substances,  306. 

analysis    of,    by    iodometric 

methods,  342. 
Oxygen, 

available,  determination  of,  in 
manganese  ores,  322,  Ex.  58, 
325. 

in  pyrolusite,  Ex.  58,  325. 
in  potassium  dichromate,  307. 
in  potassium  permanganate, 

307. 
basis      for      computing      atomic 

weights,  496. 

determination  of,  in  illuminating- 
gas,  479. 

gaseous,  absorbents  for,  462. 
pure,  not  absorbed   by  phospho- 
rus, 463. 


Oxygen,  pure,   method    of    obtaining 
stream  of,  378. 

method  of  producing,  468. 
required  for  water,  determination 

of,  419. 

standardization  of  permanganate 
by  measurement  of,  314,  316. 

Palladium  sponge,  combustion  of  hy- 
drogen by,  469. 

Pslladous  iodide,  properties  of.  9G. 
Pans  of  balance,  7. 

Paper  pulp,  use  of,  in  filtration,  27,  54. 
Parr  calorimeter,  397. 

chemical    reactions    involved    in, 

397. 
construction    of    the    instrument, 

399. 

correction  factors  for,  402. 
determination,  of  calorific  value  of 
coal  by  means  of,  Ex.  71,  403. 
of  sulphur  in  coal  by  means 

of,  894. 

sodium  peroxide  for,  398. 
use  of  accelerators  in.  401. 
water  equivalent  of,  400. 
Pemberton's  alkalimetric  method  of  tit- 
rating phosphates,  360. 
Penfield's     method     of     determining 

water,  43. 
Percentage  given  by  number  of  cubic 

centimeters  of  acid  used,  249,  502. 
Persulphuric  acid,  analysis  of,  by  iodo- 
metric methods,  343. 
Pettenkofer's  method   of  determining 
carbon  dioxide  in  water,  298. 

in  gases,  301 . 

Phenacetolin,  use  of,  as  indicator,  251. 
Phenol,  use  of,  in  the  Kjeldahl  diges- 
tion solution,  283. 
Phenol-sulphuric  acid,  solution  of,  for 

determination  of  nitrates,  418. 
Phenolphthalein,  effect  of  carbon  diox- 
ide on,  250. 

use  of,  as  indicator,  251,  254. 
Phosphate  precipitates,  ignition  of,  86. 
Phosphates, 

titration   of,    with   permanganate 
solution,  362 

with  uranium  solution,  357. 
Phosphine,  formation  of,  in  solution  of 

iron  and  steel,  364. 

Phosphomolybdate,  direct  weighing  of, 
119. 

precautions    in    precipitation    of, 

118. 

precipitation  of,  119. 
separation  of  phosphoric  acid  as, 

118. 
titration  of,  360. 

by  Pemberton's  alkalimetric 
method,  3CO. 


INDEX. 


547 


Phosphomolybdate,  titration   of,  with 
permanganate  solution,  362. 

washing  of,  119. 
Phosphoric  acid, 

determination  of,  117,  357. 
precipitation    of,    as    magnesium 
ammonium  phosphate,  117, 120, 
361. 

pure  compounds  of,  32. 
separation  of  silicic   and   arsenic 

acids  from,  118. 

titration  of,  by  Pemberton's  meth- 
od, 360. 

by  the  uranium  method,  357. 
with  permanganate  solution, 

363. 
weighing    as    phosphomolybdate, 

119. 
Phosphorous  acid,  reduction  of  mercury 

by,  92. 
Phosphorus, 

condition  in  which  present  in  iron 

and  steel,  364. 

determination,  in  iron  and  steel, 
372. 

in    manganese  -  phosphorus  - 

bronze,  150. 

solution  of  iron  in  nitric  acid 
and  separation  of  silica  for, 
372. 
yellow,  absorption  of  oxygen  by, 

463. 

moulding  of,  into  sticks,  464. 
oxidation  of,  with  potassium  per- 
manganate, 373. 

pentoxide,  drying  properties  of,  44, 
46. 

method    of    filling    U- tubes 

with,  47. 
use  of,  in  Kjeldahl  digestion, 

283. 
pure,   oxygen    not    absorbed   by, 

463. 

separation  from  titanium,  373. 
Physical,  examination  of  water,  408. 

tests  of  fats  and  oils,  448. 
Phytosterol  and  cholesterol,  detection 
of,  in  fats  and  oils,  451 . 

melting-points  of,  452. 
Pipettes,  227. 

double  absorption  for  gases,  475. 

method  of  filling,  475. 
explosion,  476. 
for  combustion  with  hot  platinum 

wire,  477. 

method  of  using,  231. 
simple  absorption,  for  gases,  474. 
Platino-chloride,    ammonium,    proper- 
ties of,  103. 

potassium,  properties  of,  89. 
Platinum, 

anodes,  205. 


Platinum,  boat,  for  filtration  of  carbon 
on,  377. 

cone,  use  of,  24. 

crucible,  damaged  by  ignition  of 

phosphates,  87. 
cylinders,  as  electrodes,  206. 
dish,  precipitating  iron  in,  57. 
dishes,  as  cathodes,  205. 
gauze  cylinders  as  electrodes,  207. 
lead  peroxide  on,  dissolving  of,  223. 
tin  deposits  on,  dissolving  of,  221. 
utensils,  rules  for  using  and  clean- 
ing, 38. 

zinc  deposits  on,  dissolving  of,  224. 
Polenske  value,  of  fats  and  oils,  441. 
determination  of,  442. 
of  butter-fat,  table  of,  459. 
Policeman,  55. 
Post  of  balance,  7. 

Potash  alum,  calculation  of  formula  of, 
494. 

determination    of    aluminium   in, 

Ex.  10,  54. 

percentage  of  A12O3  in,  57. 
pure,  preparation  of,  34. 
Potassium, 

acid  iodate,  purity  of,  35. 

standardization    of    sodium- 
thiosulphate  solution  with, 
338,  341 . 
bromide,  pure,  testing  of  purity  of, 

35. 

chromate,  fusion  of  nitrates  with, 
107. 

use  of,  as  indicator,  350,  353. 
cyanide,  determination  of  cyano- 
gen in,  with  silver  nitrate,  351, 
Ex.  68,  354. 

fusion  of  stannic  phosphate 

with,  150. 

solution,   electrolytic  deposi- 
tion of  silver  from,  217. 
determination,  as  platino-chloride, 
89. 

by   Lindo-Gladding  method, 

89. 

in  feldspar,  196. 
in  presence  of  sodium,  353,491 . 
in  salts  of  volatile  acids,  76. 
dichromate,  indicator  for,  308. 
oxidizing  power  of,  307. 
properties  of,  330. 
pure,  preparation  of,  35,  330. 
reagent,  preparation  of,  508. 
solution,   standardization  of, 
by  ferrous  ammonium  sul- 
phate, 331,  332. 
by  iron  wire,  330,  332. 
standard  solution  of,  330. 
conditions  of  use  of,  330. 
preparation  of,   Ex.   61, 
332. 


548 


INDEX. 


Potassium,    dichromate,    standardiza- 
tion, of  acids  with,  260. 

of    sodium    thiosulphate 

solution  with,  337. 
ferricyanide,  as  indicator,  308. 

in  uranium  titration,  357. 
hydroxide,   absorption   of   carbon 
dioxide  by,  464. 

reagent,  preparation  of,  506. 
standard  alcoholic  solution  of, 

281,  437. 

iodide,  pure,  testing  of,  35. 
magnesium  sulphate,  preparation 

of,  29,  Ex.  5,  36. 

nitrate,  determination  of  nitrogen 
in,  by  the  Kjeldahl  method,  Ex. 
54,  291. 

purity  of,  35. 
standard  solution  of,  418. 
permanganate,  decomposition  of, 
by  manganese  dioxide,  308. 
determination  of  manganese 

in,  Ex.  22,  82. 
impurities  in,  309. 
its  own  indicator,  308. 
oxidizing  power  of,  306,  307. 
pure,  testing  of,  35. 
permanganate  solution, 

alkaline   for  water  analysis, 

412. 
determining   manganese   by, 

323. 
preparation  of,  309,  Ex.  56, 

315. 
removal  of  manganese  dioxide 

from,  309,  315. 
stability  of,  309. 
standardization  of,  309,  315. 
by  direct  measurement  of 

oxygen,  314,  316. 
by    ferrous    ammonium 

sulphate,  313,  316. 
by  oxalates,  313,  316. 
by  oxalic  acid,  313. 
by  pure  iron  wire,  309, 

315. 

by   standard    iron   solu- 
tions, 310. 

standardization     of     sodium 
thiosulphate  solution  with, 
337. 
titration  of  phosphoric  acid 

with,  362. 

platmo-chloride,  properties  of,  89. 
pure  compounds  of,  31. 
separation  from  sodium,  187,  196. 
sulphate,  pure,  testing  of,  35. 

use  of,  in  the  Kjeldahl  diges- 
tion, 293. 

tetroxalate,  preparation  and  stand- 
ardization of  acids  with,  260. 
pure,  preparation  of,  35. 


Potassium  tetroxalate,  standardization 
of  permanganate  solutions  by,  313, 
316. 
Potential, 

electrolytic,  separation  of  metals 

by,  199. 

of  electrical  charge  on  ions,  199. 
Precipitates,    contamination    of,    with 
soluble  salts,  22. 
digestion  of,  23. 

finely  divided,  treatment  of,  23. 
ignition  of,  56. 

transferring  to  the  filter-paper,  54. 
washing  of,  26. 

by  decantation,  26. 
without  filtration,  91. 
weighable,  properties  of,  50. 
Precipitation, 

by  change  of  solvent,  30. 
by  double  decomposition,  30 
complete,  theory  of,  21. 
excess  of  reagent  in,  22. 
methods,  theory  of  indicators  for, 

349. 

of  double  salts,  29. 
Preparation, 

of  potassium  magnesium  sulphate, 

Ex.  5,  36. 

of  pure  copper  sulphate,  Ex.  4,  36. 
of  pure  sodium  chloride,  Ex.  6,  37. 
Pressure, 

regulation  of,  in  gas  analysis,  471. 
Primary  cells,  203. 
Problems,  245,  246,  257,  277,  278,  334, 

356,  485. 
Proximate  analysis  of  coal,  390,  Ex.  70, 

392. 

Pure  salts,  preparation  of,  26. 
Pycnometers,  determination  of  specific 
gravity  of  oils  by,  449. 

use  of,  274. 
Pyrite,  analysis  of,  Ex.  36,  172. 

in  coal,  392. 
Pyrogallol,  absorption  of  oxygen  by, 

462. 

Pyrolusite,  commercial  uses  of,  322. 
determination,  of  available  oxygen 
in,  322,  Ex.  58,  325,  Ex.  65,  345. 
of  manganese  in,  by  Volhard's 
method,  323,  Ex.  59,  326. 

Quantitative  analysis, 

neatness,  necessary  for,  3. 

object  of,  1. 

patience    and    honesty   necessary 

for,  3. 

required  by  industries,  6. 
skill  and  knowledge  required  for,  2. 
utility  and  importance  of,  6. 

Reagents,  preparation  of,  505. 
Records,  care  in  keeping,  3. 


INDEX. 


549 


Recrystallization  of  salts,  27. 
Reducing,  agents,  analysis  of,  by  iodo- 
metric  methods,  342. 
solutions,  307. 
substances,  306. 

Reduction    and    oxidation,    equations, 
balancing  of,  497. 

methods,  306. 

titrations,  calculation  of,  502. 
Reichert  and  Reichert-Meissl, 
value  of  fats  and  oils,  440. 
determination  of,  440. 
table  of,  459. 
Results,  calculation  of,  4. 
Retainers  in  water  determinations,  40. 
Rider  of  balance,  8. 
Rocks, 

pulverizing  of,  153. 

taking  samples  of,   for  analysis, 

153. 

Rose  crucible,  use  of,  80. 
Rose's  metal,  analysis  of,  Ex.  27,  129. 
determination,  of  bismuth  in,  130. 
of  copper  in,  130. 
of  lead  in,  129. 
Rosin  in  shellac,  determination  of,  by 

A.  C.  Langmuir's  method,  446. 
Rosolic  acid,  use  of,  as  indicator,  251 . 
Rossetti,  table  of  density  of  water,  527. 
Rotation  of  anode  or  cathode,  209,  217, 

221 ,  225. 

Roth's  ether  separation  of  iron,  160. 
Rules  for  using  balance,  20. 

Salicylic  acid,  use  of,  in  the  Kjeldahl  di- 
gestion, 283,  288. 
Salt,  common,  purification  of,  30,  Ex. 

6,  37. 
Salts, 

calculation  of  formulae  of,  493. 

drying  of,  28.  ^ 

pure,  preparation  of,  26. 

by  recrystallization,  27. 
reason  for  analysis  of,  121. 
weighing  of,  41. 
Sample, 

drying  of,  156. 

for  analysis,  selection  and  prepara- 
tion of,  152,  347. 
of  air,  collection  of,  301,  305. 
of  flue  gases,  taking  of,  484. 
average,  taking  of,  484. 
of  illuminating-gas,  collection  of, 

478. 

of  iron  and  steel,  method  of  ob- 
taining, 365. 
of  water,  taking  of,  408. 
Sand-bath,  use  of,  42. 
Sanitary  analysis  of  water,  406. 
Saponification  of  oils,  454. 

value  of  fats  and  oils,  definition 
of,  439. 


Scale-forming  ingredients  of  water,  431 . 
Schrotter  apparatus,  determination  of 
carbon  dioxide  by,  109. 

sources  of  error  in  use  of,  110. 
Selenium,  determination  of,  105. 
Sensibility  of  balance,  9. 
definition  of,  10. 
determination  of,  Ex.  1,  15. 
Separation   of  elements,   difficulty  of 

complete,  121. 
Sewage,   contamination  of  water  by, 

407. 
Sewaschen,  apparatus  for  determining 

carbon  dioxide  in  air,  301. 
Shellac,  determination  of  rosin  by  A.  C. 

Langmuir's  method,  446. 
Sieves,  brass,  155. 
Sifting  of  powdered  samples,  155. 
Silica,  determination  of,  in  chromite, 
164. 

in  dolomite,  181. 
in  feldspar,  193. 
in  ferric  oxide,  59. 
in  precipitates,  69. 
in  pyrite,  174. 
in  silicates,  193. 
in  smaltite,  169. 
in  water,  431 . 

fusion  of  nitrates  with,  107. 
precipitates,  fusion  of,  366. 
volatilization  with  hydrofluoric 

acid,  59,  181,  193,  365. 
Silicates, 

decomposition  of,  188. 

by  the  method  of  J.  Lawrence 

Smith,  190. 

determination,  of  alkalies  in,  190. 
of  iron,  aluminium,  and  titan- 
ium in,  189. 
of  silica  in,  189. 

fusion  of,  with  alkali  carbonates, 
188,  192. 

with  boric  oxide,  190. 

with  calcium  carbonate  and 

ammonium  chloride,  190. 
Silicic  acid,  dehydration  of,  189. 

separation    of,    from    phosphoric 

acid,  118. 

Silicon,  condition  in  which  present  in 
iron  and  steel,  364. 

determination  of,  in  iron  and  steel, 
365 

after  solution  of  the  iron  in 

hydrochloric  acid,  366. 
Drown's  method  for,  366. 
tetrachloride,     formation    of,     in 

solution  of  iron  and  steel,  364. 
Silver, 

bromide,  properties  of,  95. 
chloride,  as  indicator,  351. 

properties  of,  90. 
chromate,  use  of,  as  indicator,  350. 


550 


INDEX. 


Silver,  coin,  analysis  of,  Ex.  25,  123. 

determination  of  silver  in,  Ex. 

69,  355. 
division    of.    volumetrically, 

124. 
electrolytic   analysis  of,   Ex. 

43,  218. 

cyanide,  determination  of  hydro- 
cyanic acid  as,  100. 
determination,  as  chloride,  90. 
as  sulphide,  78. 
by  Volhard's  method,  355. 
in  a  coin,  Ex.  69,  355, 
in  alloys,  355. 
in  a  silver  coin,  Ex.  25,  123, 

Ex.  43,  218. 
in  silver  nitrate,  Ex.  24,  93, 

Ex.  42,  218. 

electrolytic    deposition    of,    201, 
217,  218. 

determination    of,    in    silver 

nitrate,  Ex.  42,  218. 
iodide,  properties  of,  95. 
metallic,  purity  of,  35. 
nitrate, 

determination,   of  potassium 
and  sodium  by,  353. 

of  silver  in,  Ex.  24,  93, 

Ex.  42,  218. 
indirect     determination     by 

means  of,  353. 
purity  of,  35. 

reagent,  preparation  of,  508. 

solution,  preparation  of,  421. 

.  standard  solution  of,  352,  354. 

titration      of      chlorides 

with,  350,  351. 

of     cyanides     with, 

351,  354. 

nitrite,  preparation  of,  417. 
pure  compounds  of,  31. 
separation,  from  copper,  123,  214. 
from  lead,  123. 
from  lead  and  copper,  217. 
from  mercury,  123. 
from  other  metals,  123. 
from  zinc,  145. 
volumetric  determination  of,  226, 

349. 

Smaltite,  analysis  of,  Ex.  35,  169. 
Smith  and  Riche,  method  of  determin- 
ing tin,  224. 

Smith,  J.  Lawrence,  method  of,  for  de- 
composing silicates,  190,  195. 
Soda-lime,  absorption  of  carbon  diox- 
ide by,  114. 
Soda  reagent,  preparation  and  use  of, 

269,  270. 
Sodium, 

ammonium  phosphate,  pure,  test- 
ing of,  35. 
arsenite,  solution,  336. 


Sodium,  arsenite,  standard  solution  of, 
389. 

titration,  of  bleaching  powder 
with,  347. 

of  permanganic  acid  by, 

389. 

bicarbonate  and  carbonate,  analy- 
sis of  mixtures  of,  265,  267. 
bicarbonate,  conversion  to  carbon- 
ate, 258,  204. 

carbonate,    determination    of,    in 
caustic  soda,  Ex.  49,  267. 

and    hydroxide,    analysis    of 
mixtures  of,  265,  267. 
calculation   of    titration 
of,  with  standard  acid, 
248. 

ignition  and  weighing  of,  71. 
precipitation   of    metals    by, 

68. 

pure,  preparation  of,  258. 
purification  of,  30,  259. 
solution,     determination     of 
carbon    dioxide    in    water 
by,  296. 

for  water  analysis,  412. 
standardization  of  acids  with, 

258,  264. 

testing  for  impurities,  258. 
chloride,  purification  of,  30. 

preparation  of  pure,  Ex.  6,  37. 
determination,  as  carbonate,  71. 
in  feldspar,  196. 
in  presence  of  potassium,  353, 

491. 
in  salts  of  organic  acids,  71. 

of  volatile  acids,  76. 
hydroxide,  and  carbonate  analysis 
of  mixtures  of,  265,  Ex.  49,  267. 
calculation    of    titration    of, 

with  standard  acid,  248. 
determination  of,   in  caustic 

soda,  Ex.  49,  267. 
fifth-normal  solution,  prepa- 
ration of,  Ex.  52,  282. 
precipitation  of  metals  by,  60. 
reagent,  preparation  of,  506. 
solution,     determination     of 
carbon  dioxide  in  water  by, 
296. 

standard  solutions  of,  279. 
removal  of  carbon  diox- 
ide from,  280. 

iodate,  standardization  of  sodium 
thiosulphate  solution  with,  338. 
nitrite,  standard  solution  of,  417. 
peroxide,  for  the  Parr  calorimeter, 
398. 

fusion  of  sulphur  compounds 

with,  102. 

phosphate,  pure,  testing  of,  35. 
pure  compounds  of,  31. 


INDEX. 


551 


Sodium,  separation  of,  from  potassium, 
187,  196. 

sulphide,  reagent,  preparation  of, 

503. 
sulphide  solution,  preparation  of, 

137. 

sulphite,  determination  of  sulphur 
dioxide  in,  Ex.  66,  346. 

extraction  of  sulphur  by,  79. 
thiosulphate  solution,  336,  339. 
preparation  and  standardiza- 
tion of,  Ex.  64,  339,  443. 
standardization,  by  arsenious 
oxide,  339,  341. 

by  barium  thiosulphate, 

333,  340. 
by     potassium     dichro- 

mate,  337. 

by    potassium    perman- 
ganate, 337. 
by     resublimed     iodine, 

337,  341. 

by  the  iodates  of  sodium 
and  potassium,  338, 
341. 

Solder,  soft,  analysis  of,  Ex.  26,  128. 
determination,  of  arsenic  in,  129. 
of  iron  in,  129. 
of  lead  in,  128. 
of  tin  in,  128. 
of  zinc  in,  129. 
Solids,  total,  in  water,  determination  of, 

422. 
Solutions, 

calculation  of  standardization  of, 

499. 

standard,  226,  247. 

Sources  of  error,  in  water  determina- 
tions, 40. 

Specific  gravity,  definition  of,  272. 
determination  of.  272. 

by    means   of,    hydrometers, 
275. 

pycnometers,  274. 

the    Westphal    balance, 

275. 

of  fats  and  oils,  448. 
standardization     of      acids      by, 

261. 

standard  temperatures  for,  273. 
Specific  heat,  of  metals,  401. 

of  water,  396. 

Specific   temperature,   reaction  of  oils 
and  fats,  449. 

table  of,  461. 
Standard, 

acid,  most  desirable  strength  of, 
256. 

percentage  given  by  the  num- 
ber   of    cubic    centimeters 
used,  249,  502. 
acids,  258. 


Standard  alcoholic  caustic-potash  solu- 
tions, 281,  437. 
alkalies,  279. 
alkaline  solutions  not  permanent, 

279. 

ammonia  solutions,  282. 
barium-hydroxide  solutions,  281. 
for  set  of  weights,  18. 
hydrochloric  acid,  preparation  of, 

Ex.  48,  263. 
liters  in  use,  232. 
sodium-hydroxide    solution,    279, 
282. 

preparation  of,  Ex.  52,  282. 
removal    of    carbon    dioxide 

from,  280. 
solutions,  226,  247. 

calculation  of  standardization 

of,  499. 
steels  for  carbon  determinations, 

383. 

temperature   for  specific  gravity, 
273. 

for  volumetric  work,  231. 
Stands,  electrolytic,  205,  207. 
Stannic  oxide,  impurities  present  in. 
125. 

purification  of,  125,  128. 
separation  of  tin  as,  125. 
Stannic  phosphate,  decomposition  of, 

150. 

Stannous  chloride,  analysis  of,  by  iodo- 
metric  methods,  342. 

determination  of  tin  in,  327, 

Ex.  60,  328. 

reagent,  preparation  of,  507. 
reduction  of  iron  by,  319. 
Starch  iodide  paper,  as  indicator,  348. 

preparation  of,  348. 
Starch  solution,  as  indicator,  308,  339. 

method  of  preparing,  339. 
Steel, 

analysis  of,  364. 

determination  of  manganese   in, 
385. 

by     Deshay's     method, 

388. 

by  Ford's  method,  385. 
by     Volhard's    method, 

387. 

of  phosphorus  in,  372. 
of  silicon  in,  365. 
of  sulphur  in,  367. 

by     evolution     method. 

369. 
by  solution  in  nitric  acid, 

367. 

of  total  carbon  in,  374. 
method  of  obtaining  sample  of,  for 

analysis,  365. 

standard,   for  carbon  determina- 
tions, 383. 


552 


INDEX. 


Steel,  solution  of,  for  carbon  determi- 
nations, 384. 

in  potassium  cupric-chloride 

solution,  376. 

Sterilization  of  culture  medium  for  bac- 
terial count,  Ex.  72,  426. 
Stirrups  of  balance,  7. 
Stoichiometry,  486. 
Stoking,  effect  of,  on  flue  gases,  484. 
Storage,  batteries,  204. 

cells,  203. 
Strontium, 

carbonate,  determination  of  stron- 
tium in,  Ex.  14,  66. 
ignition  of,  65,  71 . 
percentage   of   strontium  in, 

66. 

pure,  preparation  of,  33. 
weighing  of,  66. 
determination,  as  sulphate,  74. 
in  salts  of  organic  acid,  71. 
in  salts  of  volatile  acids,  76. 
in  strontium  carbonate,  Ex. 

14,  60. 

precipitation,  by  ammonium  car- 
bonate, 64. 

separation,    as   nitrate   from   cal- 
cium, 179. 

as  sulphate  from  magnesium 

and  the  alkalies,  177. 
from  barium  and  calcium,  176. 
from  barium  as  chromate,  178. 
from  the  alkali  metals,  176. 
sulphate,  properties  of,  74. 
pure  compounds  of,  31. 
Suction,  use  of,  in  filtration,  24. 
Sulphanilic  acid,  solution  of,  for  deter- 
mination of  nitrites,  417. 
Sulphates,  determination  of,  in  water, 
432. 

insoluble,  decomposition  of,  103. 
solubility  of,  121. 

Sulphides,  analysis  of,  by  iodometric 
methods,  342. 

determination  of  sulphur  in,  101, 

104. 

drying  of,  79. 

ignition  of,  in  hydrogen,  80. 
Sulphur, 

absorption  of,  by  cadmium  solu- 
tion, 372. 

by  lead  solution,  371 . 
available  for  sulphuric-acid  manu- 
facture, determination  of,  172. 
condition  in  which  present,  in  iron 
and  steel,  364. 
in  coal,  392. 
crude,  determination  of  sulphur  in, 

104. 
determination  of,  101. 

by  fusion  with  alkali  nitrates 
and  carbonates,  101,  173. 


Sulphur,  determination  of,  by  fusion 
with  sodium  peroxide,  102. 
by  oxidation,  with  aqua  regia, 
171,  172. 

with  chlorine,  104. 
with  fuming  nitric  acid, 

102. 
with  liquid  bromine,  103, 

104. 
with  potassium  chlorate, 

103,  104. 
in  coal,  392,  393. 

by   means  of   the    Parr 

calorimeter,  394. 
Eschka's     method     for, 

392,  393. 
in  iron  and  steel,  367. 

after    solution    in    nitric 

acid,  367. 
in  pig  iron,  368. 
in  pyrite,  172. 
in  reagents,  368. 
in  smaltite,  171. 
in  steel  by  evolution  method, 
369. 

apparatus  for,  370. 
solution  of  iron  for,  371. 
dioxide,  absorption  of,  by  lead  per- 
oxide, 379. 

determination  of,  in  sodium 

sulphite,  Ex.  66,  346. 
evolution  methods   for,  in  steel, 

369. 

iodometric  determination  of,  369. 
removal  from  sulphides,  79. 
weighing    as    cadmium    sulphide, 

372. 

Sulphuric    acid,    concentrated,    drying 
properties  of,  44,  46. 

method     of    filling    U-tubes 

with,  46. 

fuming,  absorption  of  illuminants 
•       by,  466. 
normal,  247. 
pure  compounds  of,  32. 
reagent,  preparation  of,  505. 
specific  gravity,  table  of,  523. 
standard,    fifth-normal,    prepara- 
tion of,  Ex.  51,  271. 

titration  of  barium  hydroxide 

with,  303,  305. 
Sulphurous  acid,  oxidation  of,  303. 

reduction  of  iron  by,  320. 
Synthetic  equations,  balancing  of,  496. 

Tafole  of  Polenske  values  of  butter-fat, 

459. 

Tagliabue's  oil  tester,  453. 
Technical  analysis,  364. 
Tellurium,  determination  of,  105. 
Temperature, 

regulation  of,  in  gas  analysis,  471. 


INDEX. 


553 


Temperature,  standard,  for  volumetric 

work,  231. 
Tetroxalate  of  potassium, 

preparation  of,  260. 
standardization     of     acids    with, 

260. 

Thallous  iodide,  properties  of.  97. 
Theoretical  percentage,  calculation  of, 

491. 

Thermal  units,  definition  of,  395. 
Thompson's  glycerine  raej  hod  01  titrat- 
ing boric  acid,  29b. 

Thomson,  R.  T.,  classification  of  indi- 
cators, 250. 

table  of  basicity  of  acids  with  indi- 
cators, 252. 

Time,  economizing,  124. 
Tin, 

deposits  on  platinum,  dissolving 

off,  221. 

determination    of,     in    brass    or 
bronze,  146. 

in  Britannia  metal,  144. 
in  salts  of  volatile  acids,  71. 
in  smaltite,  169. 
in  soft  solder,  128. 
in  type-metal,  142. 
in  Wood's  metal,  131. 
electrolytic,  determination  of,  221. 
by      ammonium-oxalate 
method  of  Classen,  221 . 
separation     from     antimony 

137. 

precipitation  as  sulphide,  78. 
pure  compounds  of,  31. 
separation,     as     stannic     oxide, 
125. 

from  antimony,  by  H.  Rose's 
method,  136. 

by    means    of    metallic 

iron,  136,  141. 

from  arsenic  and  antimony, 
134. 

F.   W.   Clarke's  method 

for,  134,  144. 
from  copper,  215. 
from  zinc,  145. 

volumetric  determination  of,  327. 
weighing  as  oxide,  80. 
Titanium, 

determination  in  feldspar,  194. 

in  silicates,  189. 

separation  of,  from  silica  and  phos- 
phorus, 373. 

Turbidity,  determination  of,  in  water, 
408. 

numerical  standards  for,  409. 
Turmeric,  use  as  indicator,  251. 
Twaddell  scale,  276. 
Type-metal,  analysis  of,  Ex.  29,  140. 

Units  of  volume,  definition  of,  232. 


Uranium, 

method    of    titrating    phosphoric 
acid,  357. 

indicator  for,  357. 
solution,   titration  of  phosphates 
with,  359. 

standardization  of,  with  cal- 
cium phosphate,  359. 
standard  solution,  preparation  of, 

358. 
U-tube, 

making    impervious    connections 

with,  47. 
Marshand,  46. 

method   of   filling,   with   calcium 
chloride,  46. 

with    phosphorus    pent- 
oxide,  47. 
with  sulphuric  acid,  46. 

Vacuo,  reducing  weights  to,  11,  234. 
Valence,  charge  on  ions  proportional  to, 

198. 

Valve,  Bunsen,  311. 
Vapor  tension,  of  water,  526. 
Viscosity  of  oils,  determination  of,  453. 
Volatile  combustible  matter,  determina- 
tion of,  in  coal,  391,  393. 
Volhard's  method  of  determining  man- 
ganese, 323,  327,  387. 

calculation  of  results  of,  324. 
determination  of  amount  of 

error  in,  323. 
interfering  metals,  324. 
method    of    adding    perman- 
ganate solution  in,  324. 
method  of  determining  silver,  355. 
Voltage  of  current, 
reduction  of,  203. 
separation  of  metals  by,  199. 
Volt-meter,  205. 
Volume,  units  of,  232. 
Volumetric  apparatus,  227 
calibration  of,  233. 
cleaning  mixture  for,  240. 
standard  temperature  for,  231. 
determinations,  calculation  of .  499. 
methods,  226 
.     advantage  of,  2 
definition  of,  1,  226. 

Wash-bottle,  hot-water,  55 
Washing  precipitates,  26. 

by  decantation,  26. 

without  filtration,  91 
Water, 

albuminoid  ammonia  in,  determi- 
nation of,  411,  416. 

alkalies  in   determination  of,  432. 

ammonia-free,  preparation  of,  412. 

analysis  of,  406. 

calculation  of  results  of,  433. 


554 


INDEX. 


Water,  analysis  of,  for  use  in  boilers, 

430. 

literature  of,  435. 
sanitary,  interpretation  of  re- 
sults of,  428. 
bacteria  present  in,  423. 
bacterial  count  in,  424,  Ex.  72, 425. 
bacteriological     examination     of, 

423. 

B.  typhus  in,  424. 
calcium  in,  determination  of,  432. 
calibration,  by  weighing  of,  233, 

236. 

chemical  examination  of,  410. 
chlorides  in,  determination  of,  421. 
color,  determination  of,  409. 
conditions  in  which  it  is  held,  39. 
confining  liquid  for  gases,  471. 
density  of,  table  of,  527. 
determination,  39. 

in  barium  chloride,  Ex.  8,  44. 
in  copper  sulphate,  Ex.  7,  41. 
in  magnesium  sulphate,  Ex. 

9,  47. 
of  carbon  dioxide  in,,  296. 

by  Pettenkofer's  method, 

298. 
in     bottled     carbonated 

waters,  300. 

of  free  mineral  acid  in,  297. 
sources  of  error  in,  40. 
direct  weighing  of,  47. 
expansion  of,  232. 
free  ammonia  in,  determination  of, 

411,  413. 

free  mineral  acids  in,  determina- 
tion of,  432. 

ground,  characteristics  of,  430. 
hardness,  determination  of,   268, 

431,  Ex.  50,  269. 
hygroscopic,  determination  of,  40. 
iron  and  aluminium  in,  determina- 
tion of,  432. 
magnesium  in,  determination  of, 

432. 

motor,  use  of,  for  rotation  of  elec- 
trodes, 210. 

nitrates  in,  determination  of,  418. 
nitrites  in,  determination  of,  416. 
nitrogen  in,  determination  of,  411. 
normal  chlorine  of,  429. 
numerical  standards  for  odor  of, 

410. 

odor,  determination  of,  410. 
of  constitution,  determination  of, 

40. 
of    crystallization,    determination 

of,  40. 
organic  matter  in,  determination 

of,  419,  421. 

oxygen,    required,    determination 
of,  419. 


Water,  Penfield's  method  of  determin- 
ing, 43. 

physical  examination  of,  408. 

sanitary  analysis  of,  406. 

scale-forming  ingredients  of,  431. 

silica  in,  determination  of,  431. 

source  of  impurities  in,  407. 

specific  heat  of,  396. 

sulphates  in,  determination  c  1 ,  439 

surface,  characteristics  of,  429. 

taking  sample  of,  408. 

temperature' '6f,  408. 

test   for  B.  cbli  in,  424,  Ex.  73, 
428. 

total  solids  in,  determination  of, 
422. 

turbidity,  determination  of,  408. 

vapor  tension  of,  526. 

weighing    substances    containing, 

19. 
Weighable  precipitates,   properties  of, 

50. 
Weighing, 

a  salt,  41. 

bottle,  19. 

errors  in,  14. 

hygroscopic  substances,  19. 

independent  of  force  of 'gravity,  9. 

limit  of  error  in,  5. 

method  of  substitution  for,  12. 

methods  of,  7,  12. 

.substances  containing  water,  19. 

tube,  49. 
Weights, 

calibration  of,  Ex.  3,  16. 

constant,  41. 

factor,  method  of  obtaining,  492. 

fractional,  8. 

reducing  to  vacuo,  11. 

standard  for,  18. 

Westphal  balance,  use  of,  271,  275. 
Whipple,  Geo.  C.,  characteristics  of  pol- 
luted surface-waters,  429. 
Wijs'     iodine-monochloride     solution, 

443. 
Wilfarth's  modification  of  the  Kjeldahl 

method,  287. 

Winkler  modified  gas  burette,  472. 
Wood's   metal,    analysis   of,    Ex.    28, 
131. 

Zeiss    butyro-refractometer,     use    of, 

449. 

Zero-point  of  balance,  10. 
determination  of,  15. 
variations  in,  11. 
Zinc, 

absorption  of  hydrogen  sulphide 

by  alkaline  solution  of,  369. 
ammonium  phosphate,  ignition  of, 
86. 

precipitation  of,  84. 


INDEX. 


555 


Zinc,  ammonium  sulphate,  determina- 
tion of  zinc  in,  Ex.  16,  70. 

percentage  of  zinc  in,  70. 
carbonate,  properties  and  ignition 

of,  69. 
deposits  on  platinum,  difficulty  of 

dissolving,  224. 

determination,  as  phosphate,  84. 
as  sulphide,  81. 
in  brass  or  bronze,  148. 
in  German  silver,  148. 
in  ores,  225. 
in  salts  of  volatile  and  organic 

acids,  71. 
in  soft  solder,  129. 
in  Wood's  metal,  132. 
in  zinc  ammonium  sulphate, 

Ex.  16,  70. 

electrolytic,      determination     of, 
224. 


Zinc,    electrolytic,    separation,    from 
copper,  214. 

from  lead,  222. 

oxide,  neutralization  of  manganese 
solution  with,  388. 
properties  of,  70. 
purification  of,  388. 
precipitation  with  sodium  carbon- 
ate, 68,  69. 

pure  compounds  of,  31. 
reduction  of  iron  solutions  by,  311, 

319. 

separation,  as  sulphide,  145. 
as  zincate,  145. 
from  iron,  132. 
from  iron  and  aluminium,  159. 
from    iron,    aluminium,    and 

chromium,  156,  157. 
sulphate,  pure,  preparation  of,  35. 
titration  of  iron  in,  312. 


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OLSSON,  A.     Motor   Control   as   Used   in    Connection  with   Turret 

Turning  and  Gun  Elevating.  (The  Ward  Leonard  System.)  Illustrated. 
8vo,  Pamphlet,  27  pp.  (U.  S.  Navy  Electrical  Series,  No.  1.) net,  $.50 

OUDIN,  MAURICE  A.     Standard  Polyphase  Apparatus  and  Systems. 

With  many  diagrams  and  figures.     Sixth  edition,  thoroughly  revised. 

Fully  illustrated.     8vo,  cloth $3.00 


PALAZ,  A.,  Sc.D.  A  Treatise  on  Industrial  Photometry,  with  special 
application  to  Electric  Lighting.  Authorized  translation  from  the  French 
by  George  W.  Patterson,  Jr.  Second  edition,  revised.  8vo,  cloth. 
Illustrated...  ..$4.00 


PARSHALL,  H.  F.,  and  HOBART,  H.  M.     Armature  Windings   of 

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of  descriptive  letter-press.    Second  edition.     4to,  cloth $7.50 

-  Electric  Railway  Engineering.     With  numerous  tables,  figures, 

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PAULDING,  CHAS,  P.     Practical  Laws  and  Data  on  Condensation 

of  Steam  in  Covered  and  Bare  Pipes.  12mo,  cloth.  Illustrated.  102 
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PERRINE,  F.  A.  C.,  A.M.,  D.Sc.  Conductors  for  Electrical  Dis- 
tribution; Their  Manufacture  and  Materials,  the  Calculation  of  the  Cir- 
cuits, Pole  Line  Construction,  Underground  Working  and  other  Uses. 
With  diagrams  and  engravings.  Second  edition,  revised.  8vo, 
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PERRY,   JOHN.     Applied  Mechanics.     A   Treatise   for   the   Use   of 

Students  who  have  time  to  work  experimental,  numerical,  and  graphical 
exercises  illustrating  the  subject.  New  edition,  revised  and  enlarged. 

650  pages.     8vo,  cloth net,  $2.50 

PLATTNER.     Manual  of  Qualitative  and  Quantitative  Analysis  with 

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by  Prof.  Th.  Richter,  of  the  Royal  Saxon  Mining  Academy.  Translated 
by  Prof.  H.  B.  Cornwall,  assisted  by  John  H.  Caswell.  Illustrated  with 
78  woodcuts.  Eighth  edition,  revised.  463  pages.  8 vo,  cloth,  .net,  $4.00 

POPE,  F.  L.  Modern  Practice  of  the  Electric  Telegraph.  A  Tech- 
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STANDARD    TEXT   BOOKS.  11 

PRELINI,  CHARLES.  Tunneling.  A  Practical  Treatise  containing 
149  Working  Drawings  and  Figures.  With  additions  by  Charles  S.  Hill, 
C.E.,  Associate  Editor  "Engineering  News."  Fourth  edition,  thor- 
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and  Engineering  Students.  Second  edition,  revised.  8vo,  cloth. 
Illustrated.  350  pp net,  $3.00 

—  Graphical  Determination  of  Earth  Slopes.     Retaining  Walls, 

and  Dams.     8vo,  cloth,  illustrated,  136  pp net,  $2.00 

PRESCOTT,  A.  B.,  Prof.  Organic  Analysis.  A  Manual  of  the 
Descriptive  and  Analytical  Chemistry  of  Certain  Carbon  Compounds  in 
Common  Use;  a  Guide  in  the  Qualitative  and  Quantitative  Analysis  of 
Organic  Materials  in  Commercial  and  Pharmaceutical  Assays,  in  the  Esti- 
mation of  Impurities  under  Authorized  Standards,  and  in  Forensic  Exami- 
nations for  Poisons,  with  Directions  for  Elementary  Organic  Analysis. 
Sixth  edition.£,8vo,  cloth. $5.00 

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Eleventh  edition.     12mo,  cloth net,  $1.50 

and  OTIS  COE  JOHNSON.     Qualitative  Chemical  Analysis.     A 

Guide  iri  the  Practical  Study  of  Chemistry  and  in  the  Work  of  Analysis. 
Sixth  revised  and  enlarged  edition,  entirely  rewritten,  with  an 
Appendix  by  H.  H.  Willard  containing  a  few  improved  methods  of  Analysis. 
8vo,  cloth net,  $3.50 

RANKINE,  W.  J.  MACQUORN,  C.E.,  LL.D.,  F.R.S.     Machinery  and 

Mill-work.  Comprising  the  Geometry,  Motions,  Work,  Strength,  Con- 
struction, and  Objects  of  Machines,  etc.  Illustrated  with  nearly  300, 
woodcuts.  Seventh  edition.  Thoroughly  revised  by  W.  J.  Millar.  8vo, 
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-  The  Steam-Engine  and  Other  Prime  Movers.  With  diagram  of 
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Millar.  8vo,  cloth $5.00 

-Useful  Rules  and  Tables  for  Engineers  and  Others.  With 
appendix,  tables,  tests,  and  formulae  for  the  use  of  Electrical  Engineers. 
Comprising  Submarine  Electrical  Engineering,  Electric  Lighting,  and 
Transmission  of  Power.  By  Andrew  Jamieson,  C.E.,  F.R.S.E.  Seventh 
edition,  thoroughly  revised  by  W.  J.  Millar.  8vo,  cloth $4.00 

—  A  Mechanical  Text-Book.      By  Prof.  Macquorn  Rankine  and  E.  E. 
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RANKINE,  W.  J.  MACQUORN,  C.E.,  LL.D.,  F.R,S.  Applied  Me- 
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of  Structures,  Mechanics,  and  Machines.  With  numerous  diagrams. 
Eighteenth  edition.  Thoroughly  revised  by  W.  J.  Millar.  8vo,  cloth  $5.00 

—  Civil  Engineering."^  Comprising^  Engineering,    Surveys,    Earthwork, 
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Rivers,   Water-Works,    Harbors,    etc.     With   numerous   tables   and    illus- 
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12  STANDARD    TEXT    BOOKS. 

RATEAU,    A.     Experimental   Researches    on   the   Flow    of    Steam 

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RAUTENSTRAUCH,    W.,    Prof.,  and    WILLIAMS,    J.    T.     Machine 

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to  Machine  Design.  Part  I.  Machine  Drafting.  Illustrated,  70  pp., 

8vo,  cloth net,  $1.25 

Complete  in  Two  Parts.     Part  II  in  preparation. 

RAYMOND,  E.  B.  Alternating  Current  Engineering  Practically 
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chapter  on  "The  Rotary  Converter."  12mo,  cloth.  Illustrated.  232  pages. 

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REINHARDT,  CHAS.  W.     Lettering  for  Draughtsmen,  Engineers  and 

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ings. New  and  revised  edition.  Thirty-seventh  thousand.  Oblong  boards. 

$1.00 

RICE,  J.  M.,  Prof.,  and  JOHNSON,  W.  W.,  Prof.  On  a  New  Method 
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Newtonian  Conception  of  Rates  of  Velocities.  12mo,  paper $0.50 

RIPPER,  WILLIAM.     A  Course  of  Instruction  in  Machine  Drawing 

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ROBINSON,  J.  B.  Architectural  Composition.  An  attempt  to  order 
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of  designers.  233  pp.,  173  illustrations.  8vo,  cloth net,  $2.50 

ROGERS,   ALLEN.     A   Laboratory   Guide   of  Industrial   Chemistry. 

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ROTH,  W.  A.  Exercises  in  Physical  Chemistry.  Translated  by 
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SCHMALL,  C.  N.  First  Course  in  Analytic  Geometry,  Plane  and 
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SCOTT,    W.    W.     Qualitative    Chemical    Analysis;  a    Laboratory 

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and  Naval  Architects,  Designers,  Draughtsmen,  Superintendents  and  all 
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STANDARD    TEXT    BOOKS.  13 

SEIDELL,  A.     (Bureau  of  Chemistry,  Wash.,  D.  C.).     Solubilities  of 

Inorganic  and  Organic  Substances.  A  handbook  of  the  most  reliable 
Quantitative  Solubility  Determinations.  Second  printing,  with  cor- 
rections. 8vo,  cloth,  367  pp net,  $3.00 

SENTEB,  G.  Outlines  of  Physical  Chemistry.  Second  edition, 
revised.  12mo,  cloth.  Illustrated $1.75 

SEVER,  Prof.  G.  F.     Electrical  Engineering  Experiments  and  Tests 

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edition,  thoroughly  revised  and  enlarged.  8vo,  pamphlet.  Illus- 
trated  net,  $1.00 

and  TOWNSEND,  F.  Laboratory  and  Factory  Tests  in  Elec- 
trical Engineering.  Second  Edition,  thoroughly  revised  and  enlarged. 

8vo,  cloth.     Illustrated.     236  pages net,  $2.50 

SEWALL,  C.  H.  Lessons  in  Telegraphy.  For  use  as  a  textbook  in 
schools  and  colleges  or  for  individual  students.  12mo,  cloth.  Illustrated. 

$1.00 

SHELDON,  S.,  Prof.,  MASON,  H.,  and  HATJSMANN,  E.     Dynamo 

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Machines.  Eighth  edition,  completely  revised.  12mo,  cloth.  Illustrated. 

net,  $2.50 

SHELDON,  S.,  MASON,  H.,  and  HAUSMANN,  E.    Alternating  Current 

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Machinery;  its  Construction,  Design,  and  Operation."  With  many  dia- 
grams and  figures.  (Binding  uniform  with  volume  I.)  Ninth  edition, 
completely  rewritten.  12mo,  cloth.  Illustrated net,  $2.50 

SHIELDS,  J.  E.  Notes  on  Engineering  Construction.  Embracing 
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employed  in  Tunneling,  Bridging,  Canal  and  Road  Building,  etc.  12mo, 
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SHUNK,  W.  F.     The  Field  Engineer.     A  Handy  Book  of  Practice 

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standard  and  Narrow  Gauge,  and  prepared  with  special  reference  to  the 
wants  of  the  young  engineer.  Nineteenth  edition,  revised  and  enlarged. 
With  addenda.  12mo,  morocco,  tucks $2.50 

SMITH,  C.  A  M.  Handbook  of  Testing:  MATERIALS.  8 vo,  cloth. 
Illustrated $2.50 

SMITH,   F.    E.     Handbook   of   General   Instruction   for   Mechanics. 

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SOTHERN,  J.  W.  The  Marine  Steam  Turbine.  A  practical  descrip- 
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14  STANDARD   TEXT  BOOKS. 

SPEYERS,    C.   L.     Textbook     of    Physical    Chemistry.     8vo,    cloth. 

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STAHL,  A.  W.,  and  WOODS,  A.  T.  Elementary  Mechanism.  A  Text- 
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larged. 12mo,  cloth $2.00 

STALEY,  CADY,  and  PIERSON,  GEO.  S.     The  Separate  System  of 

Sewerage;  its  Theory  and  Construction.  With  maps,  plates,  and  illus- 
trations. Third  edition,  revised  and  enlarged,  with  a  chapter  on 
" Sewage  Disposal."  8vo,  cloth $3.00 

STANSBIE,  J.  H.     Iron  and  Steel.     8vo,  cloth.    Illustrated,     net,  $2.00 

STODOLA,  Dr.  A.  The  Steam-Turbine,  With  an  appendix  on  Gas 
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stein.  8vo,  cloth.  Illustrated.  434  pages net,  $4.50 

SUDBOROUGH,  J.  J.,  and  JAMES,  T.  C.  Practical  Organic  Chem- 
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SWOOPE,   C.   WALTON.     Practical  Lessons  in  Electricity.     Princi- 

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TITHERLEY,  A.  W.,  Prof.    Laboratory  Course  of  Organic  Chemistry, 

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THURSO,  JOHN  W.  Modern  Turbine  Practice  and  Water-Power 
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net,  $4.00 

TOWNSEND,  F.    Short  Course  in  Alternating  Current  Testing.      8vo 

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TURRILL,  S.  M.  Elementary  Course  in  Perspective.  12mo, 
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UNDERBILL,  C.     Solenoids,  Electromagnets,  and  Electromagnetic 

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URQUHART,  J.  W.  Dynamo  Construction.  A  practical  handbook 
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Framework  Building,  Field  Magnet  and  Armature  Winding  and  Group- 
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VAN  NOSTRAND'S  Chemical  Annual,  based  on  Biederman's  "  Chiem- 

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Eminent  Chemists.  Revised  and  enlarged.  Second  issue  1909.  12mo, 
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STANDARD   TEXT  BOOKS.  15 

VEGA,  Von  (Baron).  Logarithmic  Tables  of  Numbers  and  Trig- 
onometrical Functions.  Translated  from  the  40th,  or  Dr.  Bremiker's 
thoroughly  revised  and  enlarged  edition,  by  W.  L.  F.  Fischer,  M.A.,  F.R.S. 
Eighty-first  edition.  8vo,  half  morocco $2.50 

WEISBACH,  JULIUS.  '  A  Manual  of  Theoretical  Mechanics.  Ninth 
American  edition.  Translated  from  the  fourth  augmented  and  im- 
proved German  edition,  with  an  Introduction  to  the  Calculus  by  Eckley  B. 
Coxe,  A.M.,  Mining  Engineer.  1100  pages,  and  902  woodcut  illustrations. 

8vo,  cloth $6.00 

Sheep $7.50 

and  HERRMANN,  G.  Mechanics  of  Air  Machinery.  Author- 
ized translation  with  an  appendix  on  American  practice  by  Prof.  A. 
Trowbridge.  8vo,  cloth,  206  pages.  Illustrated net,  $3.75 

WESTON,    EDMUND    B.     Tables    Showing   Loss    of   Head   Due    to 

Friction  of  Water  in  Pipes.     Fourth  edition.     12mo,  full  leather.  .  .$1.50 

WILLSON,  F.  N.  Theoretical  and  Practical  Graphics.  An  Educational 
Course  on  the  Theory  and  Practical  Applications  of  Descriptive  Geometry 
and  Mechanical  Drawing.  Prepared  for  students  in  General  Science, 
Engraving,  or  Architecture.  Third  edition,  revised.  4to,  cloth, 
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Higher  Plane  Curves,  and  the  Helix.     4to,  cloth,  illustrated net,  $3.00 

WILSON,  GEO.  Inorganic  Chemistry,  with  New  Notation.  Revised 
and  enlarged  by  H.  G.  Madan.  New  edition.  12mo,  cloth $2.00 

WIMPERIS,  H.  E.  The  Internal  Combustion  Engine.  A  textbook 
on  Gas,  Oil,  and  Petrol  Engines  for  the  use  of  Students  and  Engineers.  8vo, 
cloth.  Illustrated net,  $3.00 

WINCHELL,   N.   H.,  and   A.   N.     Elements   of  Optical  Mineralogy. 

An  introduction  to  microscopic  petrography,  with  descriptions  of  all 
minerals  whose  optical  elements  are  known  and  tables  arranged  for  their 
determination  microscopically.  354  illustrations.  525  pages.  8vo, 
cloth '. net,  $3.50 

WRIGHT,  T.  W.,  Prof.  Elements  of  Mechanics,  including  Kinematics , 
Kinetics,  and  Statics.  Seventh  edition,  revised.  8vo,  cloth $2.50 

-and  HAYFORD,  J.   F.     Adjustment  of  Observations  by  the 

Method  of  Least  Squares,  with  applications  to  Geodetic  Work.  Second 
edition,  rewritten.  8vo,  cloth,  illustrated .net,  $3.00 

ZEUNER,  A.,  Dr.  Technical  Thermodynamics.  Translated  from  the 
Fifth,  completely  revised  German  edition  of  Dr.  Zeuner's  original  treatise 
on  Thermodynamics,  by  Prof.  J.  F.  Klein,  Lehigh  University.  8vo,  cloth, 
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